p elementhr elementpre elementblockquote elementol elementul elementmenu elementli elementdl elementdt elementdd elementfigure elementfigcaption elementmain elementdiv elementa elementem elementstrong elementsmall elements elementcite elementq elementdfn elementabbr elementruby elementrt elementrp elementdata elementtime elementcode elementvar elementsamp elementkbd elementsub and sup elementsi elementb elementu elementmark elementbdi elementbdo elementspan elementbr elementwbr elementa and area elementsa and area elementsalternate"author" 链接类型bookmark" 链接类型canonical"dns-prefetch" 链接类型external" 链接类型help" 链接类型icon"license" 链接类型manifest"modulepreload"nofollow" 链接类型noopener"noreferrer"opener"pingback" 链接类型preconnect 链接类型"prefetch" 链接类型preload"prerender" 链接类型search"stylesheet"tag" 链接类型picture elementsource elementimg elementsource,
img, and link elementsiframe elementembed elementobject elementparam elementvideo 元素audio elementtrack elementTrackEvent interfacemap elementarea elementtable elementcaption elementcolgroup elementcol elementtbody elementthead elementtfoot elementtr elementtd elementth elementtd and th elementsform elementlabel elementinput elementtype 属性的状态type=hidden)type=text) state and Search state (type=search)type=tel)type=url)type=email)type=password)type=date)type=month)type=week)type=time)type=datetime-local)type=number)type=range)type=color)type=checkbox)type=radio)type=file)type=submit)type=image)type=reset)type=button)input 元素属性input element APIsbutton elementselect elementdatalist elementoptgroup elementoption elementtextarea elementoutput elementprogress elementmeter elementfieldset elementlegend elementname attributedirname attributemaxlength attributeminlength attributedisabled attributescript elementnoscript elementtemplate elementslot elementcanvas elementPath2D objectsImageBitmap 渲染上下文OffscreenCanvas interfacecanvas 元素的安全问题hidden 属性contenteditable content attributedesignMode getter and setterinputmode attributeenterkeyhint
attributeWindow,
WindowProxy, 和 Location 对象的安全基础设施 Window objectWindowProxy exotic objectHistory interfaceLocation interfacemultipart/x-mixed-replace 资源的媒体的页面加载处理模型X-Frame-Options` headerWindowOrWorkerGlobalScope mixinbutton 元素details 和 summary 元素input 元素作为文本输入微件input element as domain-specific widgetsinput element as a range controlinput element as a color
wellinput element as a checkbox and radio button widgetsinput element as a file upload controlinput element as a buttonmarquee elementmeter elementprogress elementselect elementtextarea elementtext/htmlmultipart/x-mixed-replaceapplication/xhtml+xmltext/cache-manifesttext/pingapplication/microdata+jsontext/event-streamCross-Origin-Embedder-Policy`Cross-Origin-Embedder-Policy-Report-Only`Cross-Origin-Opener-Policy`Origin-Isolation`Ping-From`Ping-To`Refresh`Last-Event-ID`X-Frame-Options`web+ scheme prefix本标准详细地定义了 Web 平台的很大一部分内容。Web 平台的标准栈中，它相对于其他标准的定位可以总结为：
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简单地说：是的。
详细地说，广义的 "HTML5" 是指一系列的现代 Web 技术。 这些技术中很多都由 WHATWG 开发，本文档就是其中之一。 其他的可以在这里访问： the WHATWG Standards overview。
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HTML 是万维网的核心标记语言。最初 HTML 主要设计为在语义级别描述科学文档的语言。 然而它的总体设计使它在接下来的几年中能够适配一些其他类型的文档，甚至包括应用程序。
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本标准是为以下读者提供的：使用了本标准定义的特性的文档和脚本的作者， 用于操作那些使用了本标准定义的特性的页面的工具的实现者， 希望根据本标准的要求确认文档或实现的正确性的个人。
本文档对还没有整体熟悉 Web 技术的读者可能不合适, 因为在有些地方为精确性而省略了具体阐述，为完整性而较为简洁概括。 易懂的教程和编写指南可以为这个话题提供更加温和的介绍。
具体地，为了完整地理解本标准中那些更加技术的部分，对 DOM 基础的熟悉是必要的。 对 Web IDL，HTTP，XML，Unicode，字符编码，JavaScript 和 CSS 的理解在有些地方也有帮助，但并非完全必要。
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本标准只限于为编写可访问的 Web 页面（从静态文档到动态应用） 提供语义级别的标记语言，以及相应的语义级别的脚本 API。
本标准的范围不包括提供具体媒体的自定义呈现机制（尽管 Web 浏览器的默认渲染规则在本标准末尾有所涵盖， 一些挂钩到 CSS 的机制也作为语言的一部分加以提供）。
本标准的范围并非要描述整个操作系统。具体地，硬件配置软件、图像操作工具、 以及用户在高端工作站日常使用的应用程序都在范围之外。 对于应用软件，本标准只针对用户偶尔使用的，或定期使用但在不同地点的， 且 CPU 要求较低的特定应用。这样的应用例如在线购物系统、搜索系统、游戏（尤其是多人在线游戏）、 公开电话簿或地址簿、通信软件（邮件客户端、即时通信客户端、讨论软件）， 文档编辑软件等。
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在最初的5年中（1990-1995），HTML经历了若干次修订和扩展。最初由 CERN 主要托管，随后是 IETF。
随着 W3C 的诞生，HTML 的开发再次易主。1995 年第一次扩展 HTML 的尝试（HTML 3.0）以失败告终，随后转变为更加务实的 HTML 3.2，在 1997 年完成。 就在同年很快开始了 HTML4 的开发。
随后一年 W3C 成员决定停止 HTML 的演变，并开始开发基于 XML 的替代品 XHTML。 该 工作首先将 HTML4 重新规划为 XML，也就是著名的 XHTML 1.0。 唯一增加的特性就是新的序列化，该工作在 2000 年完成。 XHTML 1.0 之后，在 XHTML 模块化的口号下 W3C 的主要精力转向让其他工作组更容易地扩展 XHTML。 与此同时，W3C 致力于一门新的、与此前的 HTML 和 XHTML 都不兼容的语言：XHTML2。
大概在 1998 年 HTML 停止演化的时候，浏览器厂商开发的部分 HTML API 被标准化和发布在 DOM Level 1（1998） 和 DOM Level 2 Core，以及 DOM Level 2 HTML（2000年开始2003年完成）中。 在 2004 年发布了一些 DOM Level 3 标准，但是 Level 3 草案尚未全部完成工作组就关闭了。 这些工作也最终不了了之。
2003 年 XForms （一项定位于下一代 Web 表单的技术）的发布 重新激起了对 HTML 演化的兴趣，而不是像从前那样寻求新的替代品。 这时人们发现 XML 作为 Web 技术的部署只局限于全新的技术（比如 RSS 和后来的 Atom）， 而不是取代已经部署的技术（比如 HTML）。
概念验证显示，在不要求浏览器实现与现存 HTML Web 页面不兼容的渲染引擎的情况下， 也可能扩展 HTML4 的表单来提供 XForms 1.0 引入的很多特性。 这一概念验证是对 HTML 重新燃起的兴趣产生的第一项成果。 在这早期阶段，虽然本草案已经公开可用且已广泛征求建议，该标准只在 Opera Software 版权下发布。
应该重新开启 HTML 演化的想法在 W3C 工作组得到了测试。 Mozilla 和 Opera 共同向 W3C 工作组提交了提议，包括 HTML5 工作背后的一些原则（见下文），以及前述的只涉及表单特性的早期草案。 该提议因为与此前选择的 Web 演化方向冲突而被拒绝， W3C 职员和会员投票支持继续开发基于 XML 的替代品。
此后 Apple，Mozilla 和 Opera 共同声明他们将继续在 WHATWG（一个新的组织）团体下投入工作。 他们为此创建了一个公开的邮件列表，草案也移交到了 WHATWG 网站。随后版权也修改为这三家共同拥有， 同时允许该标准的重用。
WHATWG 基于若干核心原则，具体地：技术需要向后兼容，标准和实现需要相符（即使这意味着更改标准而不是实现）， 标准需要足够详细来使得一个实现在不需对其他实现进行逆向工程的情况下，就能可达到完全的互操作性。
其中后一个原则，要求 HTML5 标准的范围应包括先前在 HTML4，XHTML1，和 DOM2 HTML 这三篇独立的文档中标准化的内容。 同时意味着相比于此前的考虑，需要显著地引入更多的细节。
2006 年，W3C 暗示了参与 HTML5 开发的兴趣，并于 2007 年组建了工作组来与 WHATWG 共同开发 HTML5 标准。 Apple，Mozilla，和 Opera 允许 W3C 在 W3C 版权下发布该标准， 只要保留一版 WHATWG 网站的那份较少限制的许可协议。
数年中各方一同工作，然而在 2011 年，这些工作组最终发现他们有着不同的目标： W3C 希望发布一个"完成的" HTML5 版本，而 WHATWG 希望持续地维护一个 HTML Living Standard， 持续地维护该标准而不是锁定在一个带着已知问题的状态， 同时按照需求增加新的特性来发展整个平台。
从此 WHATWG 一直在（与其他组织一同）开发该标准， W3C 则复制 WHATWG 的修复工作到他们所在的文档分支（也有其他的一些改动）。
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必须承认 HTML 的很多方面乍一看毫无意义或者不太一致。
HTML，它的 DOM API，以及很多其他的支持技术是由互不相识的有着不同目的的人，经数十年开发完成。
因此特性的出现有着不同的来源，也未经专门的一致的方式设计。 而且由于 Web 独有的特点，实现的 Bug 常常会变成事实上的惯例，以及现在合法的、标准的行为。 因为编写的内容常常会无意地依赖于这些未能及时修复的实现。
尽管这样，人们还是为坚持某些设计目标而付出努力。在接下来的几个小节中描述了这些努力。
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为了避免让 Web 作者处理多线程的复杂性，HTML 和 DOM API 的设计使得脚本无法检测同时运行的其他脚本。 甚至在使用 workers 时，设计意图中实现的行为可以被认为是 在所有 浏览环境 中串行地执行所有脚本。
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本标准和多种多样的其他有相互影响和依赖。很不幸在某些情况下，相互冲突的需求使得本标准违反了其他标准的要求。 当这种情况发生时，每一项冲突都会标记为 "willful violation"，而且会说明该冲突的原因。
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HTML 有广泛的可扩展性机制，可用来安全地添加语义：
作者可以使用 class 属性来扩展元素，
在使用最接近的 "真实" HTML 元素的同时，也等效于创建了他们自己的元素，
因此不知道该扩展的浏览器和其他工具仍然可以在某种程度上很好地支持。
比如微格式就是使用的这种策略。
作者可以使用data-*=""属性来包含数据，
在客户端脚本或服务器端的站点脚本中加以处理。可以保证浏览器永远不会碰这些数据，
使得脚本可以在 HTML 元素中引入数据，并在后续脚本中寻找或者处理。
通过<meta name="" content=""> 机制，
作者可以为预定义的元数据名称的集合注册扩展
来引入页面级别的元数据。
通过rel="" 机制，
作者可以通过为预定义的链接类型注册扩展，
来给链接标注特定的含义。微格式也使用了这种机制。
作者可以使用 <script type=""> 来内嵌自定义类型的原始数据，
用于在内联脚本或服务器端脚本中做进一步处理。
作者可以使用 JavaScript 原型机制来扩展 API。这在脚本库中有广泛的使用。
作者可以使用微数据特性（itemscope="" 和 itemprop=""属性）
来嵌入嵌套的键值对数据，以供其他应用和站点共享。
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本标准定义了一个描述文档和应用的抽象语言，以及一些 API 来与使用该语言的资源的内存表示进行交互。
这一内存表示又称为 "DOM HTML"，或简称为 "DOM"。
对于传输使用了该抽象语言的资源，有很多不同的具体语法。本标准中定义了其中两种。
第一个这样的具体语法是 HTML 语法。这是对多数作者推荐的格式，它与多数遗留的 Web 浏览器相兼容。
如果一个文档以text/html MIME type进行传输，
然后它将会被 Web 浏览器作为 HTML 文档处理。
本标准定义了最新的 HTML 语法，简单地称为 "HTML"。
第二个具体的语法是 XML。当一篇文档以XML MIME type（比如application/xhtml+xml）
传输时，它将会被 Web 浏览器当做 XML 文档处理，被 XML 处理器解析。
作者需要注意 XML 和 HTML 的处理存在差别；具体地，即使很小的语法错误都将阻止 XML 文档的完整渲染，
然而它们在 HTML 语法中将会被忽略。
给 HTML 用的 XML 语法此前被称为 "XHTML"，但本标准不适用该术语 （原因之一是 MathML 和 SVG 的 HTML 语法中未使用该术语）。
DOM，HTML 语法，XML 语法不能用来表示同样的内容。
例如 HTML 语法不能表示命名空间，但 DOM 和 XML 语法却支持。
类似地，HTML 语法可以表示使用 noscript 特性的文档，
但 DOM 和 XML 语法却不能。
包含 "-->" 的注释只能在 DOM 中表示，HTML 和 XML 语法却不行。
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本标准分为以下主要的部分：
EventSource，
以及一个为脚本提供的双向全双工套接字协议，称为 Web Sockets。
还有一些附录，列出了 废弃的特性 和 IANA 考虑，以及一些索引。
阅读本标准的方式与其他标准类似。首先应该完整地阅读多次，然后至少倒着读一次， 然后应该从目录中随机选入章节并跟着所有交叉引用读一次。
如下面一致性要求部分所述，本标准描述了各一致性等级的一致性标准。 特别地，有些一致性要求适用于生产者（例如作者和他们创建的文档）， 也有一些适用于消费者（例如 Web 浏览器）。 他们的要求是不同的：对生产者的要求声明了什么是允许的，而对消费者的要求声明了软件的行为。
对生产者的要求不会对消费者产生任何影响。
继续上述例子，“属性值必须是合法的整数”刻意地没有暗示对消费者的要求。 事实上消费者可能被要求把该属性当做透明的字符串，完全不关心它的值是否满足要求。 也可能被要求以指定的规则来解析它的值（就像上述例子中那样），这些规则就定义了如何处理不合法的值（在本例子中就是非数字的值）。
这是定义、要求，或者解释。
这是注意事项。
这是翻译者的注释。
这是例子。
这是未解决的问题。
这是警告
[Exposed=Window]
interface Example {
// 这是一个 IDL 定义（接口描述语言）
};method( [ optionalArgument ] )这是给作者看的，描述了接口的使用。
/* 这是一个 CSS 片段 */术语的定义标记为 这样。 该术语的使用标记为 这样 或 这样。
元素、属性或 API 的定义标记为这样。
该元素、属性或 API 的引用标记为 这样。
其他代码片段标记为这样。
变量标记为这样。
在算法中，同步部分的步骤标记为 ⌛。
有些情况下，要求以列表的形式给出，列表项包括条件和对应的要求。 在该情况下，适用于某项条件的要求紧跟着条件出现，这些要求的条件可能有很多个。例如：
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一个基本的 HTML 文档看起来像这样：
<!DOCTYPE html> <html lang="en"> <head> <title>Sample page</title> </head> <body> <h1>Sample page</h1> <p>This is a <a href="demo.html">simple</a> sample.</p> <!-- this is a comment --> </body> </html>
HTML 文档由树状的元素和文本组成。源码中每个元素由
一个 开始标签（例如 "<body>"）
和一个 结束标签（例如 "</body>"）表示。
（某些开始标签和结束标签在某些情况下可以 省略。）
标签的嵌套必须使所有标签都完全在其他标签内部，不应出现重叠：
<p>This is <em>very <strong>wrong</em>!</strong></p>
<p>This <em>is <strong>correct</strong>.</em></p>
本标准定义了 HTML 中使用的一些元素，以及它们的嵌套规则。
元素可以有属性来控制元素的行为。在下面的例子中是一个 超链接，
由一个a元素和它的 href 属性组成：
<a href="demo.html">simple</a>
属性 应放置在开始标签中，由
属性名 和 属性值构成，
以 "=" 分隔。
如果不包含空格" ' ` = < 或
>， 属性值可以保持 没有引号 ；
否则必须使用单引号或双引号。
如果属性值是空字符串，属性值以及 "=" 可以一起省略。
<!-- empty attributes --> <input name=address disabled> <input name=address disabled=""> <!-- attributes with a value --> <input name=address maxlength=200> <input name=address maxlength='200'> <input name=address maxlength="200">
HTML 用户代理（比如 Web 浏览器）解析这些标记，把它转化为 DOM（Document Object Model）树。 DOM 树是文档在内存中的表示。
DOM 数包含若干种节点，具体地：DocumentType 节点、
Element节点、Text 节点、Comment 节点，
以及有时会出现的 ProcessingInstruction 节点。
本章顶部的代码片段将会转化为下列 DOM 树：
树中的document 元素 是 html 元素，
在 HTML 文档的这一位置总是应该有这样一个元素。它包含两个元素，
head 和 body，以及它们之间的一个 Text 节点。
DOM 树中的 Text 节点比你预想的要多，因为源码包含一些空格（表示为"␣"）和换行（"⏎"），
这些都会变成 DOM 中的 Text 节点。然而因为历史原因，并非所有这些源码中的空格和换行都出现在 DOM 中。
具体地，所有head开始标签前的空白都会被悄悄地丢掉，所有body结束标签后的空白都会出现在body的尾部。
head 元素包含一个 title 元素，其中包含一个
内容为 "Sample page" 的 Text 节点。类似地，body 元素包含一个
h1 元素，一个p 元素，和一个注释。
页面中的脚本可以操作该 DOM 树。脚本（比如 JavaScript）是可以使用
script标签或 事件处理内容属性内嵌的小程序。
比如下面是一个表单，其中的脚本设置了表单中output元素的内容并输出 "Hello World"：
<form name="main"> Result: <output name="result"></output> <script> document.forms.main.elements.result.value = 'Hello World'; </script> </form>
DOM 树中的每个元素表示为一个对象，这些对象提供 API 以供操作。
例如，一个链接（比如上述树中的a元素）的
"href"属性可以通过多种方式改变：
var a = document.links[0]; // obtain the first link in the document
a.href = 'sample.html'; // change the destination URL of the link
a.protocol = 'https'; // change just the scheme part of the URL
a.setAttribute('href', 'https://example.com/'); // change the content attribute directly
由于 DOM 树是实现者（尤其是像 Web 浏览器这样的交互式实现）用来处理和显示 HTML 文档的， 本标准更多地是按照 DOM 树来介绍的，而不是按照上面描述的标记代码。
HTML 文档表示了交互式内容的一种媒体无关的描述。HTML 文档可能会渲染在屏幕、语音合成器、或者盲文点触设备。 为了精确地控制渲染行为，作者可以使用一个像 CSS 这样的样式语言。
在下列例子中，使用 CSS 将页面设置为蓝底白字。
<!DOCTYPE html>
<html lang="en">
<head>
<title>Sample styled page</title>
<style>
body { background: navy; color: yellow; }
</style>
</head>
<body>
<h1>Sample styled page</h1>
<p>This page is just a demo.</p>
</body>
</html>
关于更多 HTML 的使用细节，鼓励作者参考教程和指南。 本标准中的一些示例可能也有用，但新手作者需要注意本标准必须非常详细地定义该语言， 以至于一开始可能很难理解。
This section is non-normative.
当使用 HTML 创建交互式网站时，需要注意避免引入漏洞，使得攻击者通过这些漏洞危害网站本身或网站用户信息的完整性。
对这一问题的全面研究超出了本文档的范围，我们强烈建议网站作者去更详细地研究这一问题。 即便如此，本节尝试对 HTML 应用程序开发中的一些常见陷阱作简单介绍。
Web 的安全模型基于“域”的概念，因此 Web 上许多潜在的攻击涉及跨域操作。[ORIGIN]
当接受不可信输入时（例如文本评论这样的用户生成内容，URL 参数中的值，来自第三方网站的消息等）， 必须在使用前验证数据，并在显示时正确转义。 否则就会允许恶意的用户执行各种攻击， 这些攻击可能是轻微的，例如提供诸如负的年龄这样的假的用户信息； 也可能是严重的，例如在每次用户查看包含该信息的页面时执行脚本； 也可能是传播性的攻击导致灾难性后果，比如删除服务器中的所有数据。
当编写过滤器验证用户输入时，过滤器应当是基于白名单的，允许已知的安全构造并禁止所有其他输入。 基于黑名单的过滤器（禁止已知的错误输入并允许其他所有输入）是不安全的， 因为并不是所有错误输入都是已知的（因为它可能会在未来发明）。
例如，一个页面查看它的 URL 查询字符串来决定显示的内容， 然后网站将用户重定向到显示该消息的页面。比如：
<ul> <li><a href="message.cgi?say=Hello">Say Hello</a> <li><a href="message.cgi?say=Welcome">Say Welcome</a> <li><a href="message.cgi?say=Kittens">Say Kittens</a> </ul>
如果这一消息没有转义就显示给用户，恶意的攻击者可能会构造一个包含脚本元素的 URL：
https://example.com/message.cgi?say=%3Cscript%3Ealert%28%27Oh%20no%21%27%29%3C/script%3E
如果攻击者随后说服受害者访问此页面，攻击者选择的脚本将在页面上运行。 这样的脚本可以执行任意的恶意操作，只要是站点提供的。 例如站点是电子商务商店，这样的脚本可以导致在用户不知情的情况下进行任意的购买。
这称为跨站脚本攻击。
有很多构造可以欺骗网站执行代码。以下是网站作者在编写白名单过滤器时应考虑的事项：
img这样看起来无害的元素时，重要的是同时为所有提供的属性给出白名单。
比如，如果一个元素允许所有属性，攻击者可能会使用onload属性来运行任意脚本。javascript:”，
用户代理也可能会实现其他的一些scheme，由于历史原因确实已经实现了不少。base 元素意味着页面中的任何相对链接的 script 元素都可能被劫持，
类似地任何表单提交也可能重定向到恶意站点。如果网站允许用户提交的表单产生用户相关的影响，例如使用名字在论坛上发布消息， 进行购买或申请护照，那么很有必要核实该请求确实是用户有意发起的， 而不是由另一个站点欺骗用户不知情地发起请求。
该问题的存在是因为 HTML 表单可能被提交到其他域名。
站点可以通过为表单填充用户相关的隐藏标记或检查所有请求的`Origin`头字段，
来防止这样的攻击。
如果页面为用户提供的界面中包含用户可能不希望执行的操作时， 该页面应当经过特殊设计来避免用户被欺骗以激活该界面的可能性。
一种用户被欺骗的可能是，如果恶意站点将受害站点放在一个小的 iframe 中然后让用户去点击，
例如让用户玩一个反应类游戏。一旦用户开始玩这个游戏，恶意站点可以在用户正要点击时快速地将这个 iframe 放到鼠标光标下。
以此来欺骗用户点击受害站点的界面。
为了避免这种情况，鼓励不期望在 iframe 下使用的站点只有检测（例如通过比较window对象和top属性的值）
到它们不在 iframe 下时才激活它们的界面。
This section is non-normative.
HTML 中的脚本有着"运行到完成"的语义，这意味着浏览器通常在执行任何其他操作 （诸如触发进一步的事件或继续解析文档）之前会不中断地执行脚本。
另一方面，HTML 文件的解析是逐步发生的，这意味着解析器可以在任何地方暂停来让脚本运行。 这通常是一件好事，但确实意味着作者需要小心地避免在事件可能已经触发之后才绑定事件处理程序。
有两项技术能够可靠地完成这件事情：使用 事件处理程序内容属性， 或者在同一脚本中创建元素和添加事件处理程序。后一种更为安全，因为如前所述， 脚本会在进一步的事件发生前一直运行到完成。
一个可以表示此种情形的例子是img元素和load事件。
该事件在元素解析完成时就会触发，特别是当图片已经被缓存之后（这很常见）。
这里，作者在img标签上使用 onload 事件处理程序
来捕捉 load 事件：
<img src="games.png" alt="Games" onload="gamesLogoHasLoaded(event)">
如果使用脚本添加元素，那么只要在同一脚本中添加事件处理函数，该事件就不会被错过：
<script>
var img = new Image();
img.src = 'games.png';
img.alt = 'Games';
img.onload = gamesLogoHasLoaded;
// img.addEventListener('load', gamesLogoHasLoaded, false); // would work also
</script>
然而如果作者首先创建了img元素，然后在另一个单独的脚本中添加了事件监听器，
那么有可能load事件会在两者之间触发，导致它被错过：
<!-- Do not use this style, it has a race condition! -->
<img id="games" src="games.png" alt="Games">
<!-- the 'load' event might fire here while the parser is taking a
break, in which case you will not see it! -->
<script>
var img = document.getElementById('games');
img.onload = gamesLogoHasLoaded; // might never fire!
</script>
This section is non-normative.
鼓励作者使用一致性检查器（又称验证器）来发现常见的错误。 WHATWG 维护着这种工具的一个列表：https://validator.whatwg.org/
This section is non-normative.
与以前版本的 HTML 规范不同，此规范不仅定义了对有效文档的处理， 也详细定义了对无效文档的处理过程。
然而，即使无效内容的处理在大多数情况下是良好定义的，文档的一致性仍然很重要： 在实践中，互操作性（所有实现以可靠和相同或等同的方式处理特定内容的情况）并不是文档一致性要求的唯一目标。 本节详细地介绍了合法文档和错误文档仍然有所区别的一些常见原因。
This section is non-normative.
先前 HTML 版本的大多数表示性特性不再被允许。一般来说，表示性的标记有这样一些问题：
尽管使用显示性的标记也可以为使用辅助技术（Assistive Technologies，AT）的用户 提供可接受的体验（比如使用 ARIA），但这样做显然比使用适当的语义标记更加困难。 此外，即使使用这样的技术也不能帮助那些既没有 AT 又没有图形的用户（比如使用文本模式浏览器的用户）访问页面。
另一方面，使用媒体无关的标记为编写更多用户（比如文本浏览器的用户）可用的文档提供了简单的方式。
使用样式无关的标记编写的站点，显然更容易维护。比如，对于到处都在使用 <font color=""> 的站点，更改颜色需要更改整个站点，
然而对基于 CSS 的站点做类似的改动，只需更改一个文件。
表示性标记往往会增加冗余，因此导致更大的文档大小。
出于这些原因，在本版本的 HTML 中移除了表示性的标记。 这一改动并不突然，HTML4 在很多年前就已经不推荐使用表示性标记， 并提供了一种模式（HTML4 Transitional） 来帮助作者从表示性标记进行迁移； 后来 XHTML 1.1 更进一步地一起废弃了那些特性。
HTML 中唯一保留下来的表示性标记特性是 style 属性和 style 元素。
生产环境中使用 style 属性也在某种程度上不再推荐，
但在创建原型（这些规则之后可以直接移动到单独的样式表中）
以及在特殊情况下（当单独的样式表不方便时）提供特定样式很有用。
类似地，style 元素在引入外部样式内容或者提供页面特定样式时很有用。
但一般来说，当样式适用于多个页面时，外部样式表可能更加方便。
值得注意的是有些先前的表示性元素在本标准中已经被重新定义为媒体无关的：
b, i,
hr, s, small, and u.
This section is non-normative.
限制 HTML 的语法是为了避免各种各样的问题。
某些不合法的语法构造在解析后会导致很不直观的 DOM 树。
为了允许在受控环境中使用的用户代理不必实现更奇怪和复杂的错误处理规则， 允许用户代理在遇到解析错误时失败。
一些错误处理行为（比如上面提到的<table><hr>...）
与流式用户代理（不存储状态且通过一次遍历来处理 HTML 文件的用户代理）不兼容。
为了避免与这样的用户代理的互操作性问题，任何导致该行为的语法都是无效的。
当一个基于 XML 的用户代理连接到一个 HTML 解析器时， 某些 XML 强制的不变式（比如元素或属性名不可包含多个冒号）可能被 HTML 文件违反。 处理这种情况可能要求解析器将 HTML DOM 信息强加给 XML 兼容的信息集。 要求这一处理的多数语法构造都是不合法的。 （包含两个连续的间隔符的注释，或以一个间隔符结尾的注释是特例，这在 HTML 语法中是允许的）。
这一规则意味着基于 XML 的用户代理只需连接一个 HTML 解析器（Parser）即可拥有理解 HTML 的能力。所以 HTML 与 XML 只是语法有所不同， 表达语义的能力应当是等价的。
某些语法构造可能导致很差的性能。为了阻止这些构造的使用，通常把它们归为不合规范的。
由于历史原因有一些语法构造相对脆弱。为了避免用户意外地掉到坑里，它们也被归为不合规范的。
例如，即使省略了关闭分号，对属性中命名的字符引用的解析也会发生。
可以安全地引入一个&以及紧随其后的不构成特殊命名的字符引用，
但如果这些字母改成构成命名的字符引用，它们将会被解释为那个特殊字符。
在这个代码片段中，属性值是“?bill&ted”：
<a href="?bill&ted">Bill and Ted</a>
在下面的代码片段中，属性值实际上会变成“?art©”，
而不是 期望的 “?art©”，
因为即使没有最后的分号“©”也会一样地被处理为
“©”最终被解释为“©”：
<a href="?art©">Art and Copy</a>
为了避免这一问题，要求所有命名的字符引用以分号结束， 所有使用命名字符引用但未添加分号的都被标记为错误。
因此正确地表达上述情况的方式如下：
<a href="?bill&ted">Bill and Ted</a> <!-- &ted is ok, since it's not a named character reference -->
<a href="?art&copy">Art and Copy</a> <!-- the & has to be escaped, since © is a named character reference -->
某些语法构造在旧的用户代理中会造成一些已知的细微或严重的问题。 因此他们被标记为不合规范的来帮助作者避免这些问题。
例如 U+0060 重音符（`）不允许出现在没有引号的属性中。 因为在某些旧用户代理中，它有时会被当做一个引号。
某些限制纯粹是为了避免已知的安全问题。
例如，对使用 UTF-7 的限制纯粹是为了避免作者成为使用 UTF-7 的已知跨站脚本攻击的的目标。[UTF7]
作者意图不清楚的标记通常被划为不规范的。 尽早修复这些错误会使得后续维护更为容易。
当用户发生了拼写错误，提早捕获该错误是有帮助的，因为这可以节省作者很多调试时间。 因此本标准通常认为使用与本标准中定义的名字不匹配的元素名、属性名等是错误的。
例如，如果作者键入<capton> 而不是 <caption>，这将被标记为错误，作者可以立即更正错字。
为了允许将来扩展语言语法，某些本来无害的特性也是不允许的。
例如，在结束标签中的“属性”目前被忽略，但它们是非法的。 以防未来使用该语法特性的语言变更与已部署（但是合法的！）内容产生冲突。
一些作者发现，在实践中总是用引号包含所有的属性并且总是包含所有的可选标签很有帮助， 相比于利用 HTML 语法的灵活性而带来的一点简洁，更偏好这一做法带来的一致性。 为了协助这样的作者，一致性检查器可以提供执行这一惯例的运行模式。
This section is non-normative.
除了语言的语法，本标准还对如何指定元素和属性做出了限制。 这些限制是出于类似的原因：
为了避免误用定义了含义的元素， 定义内容模型时，限制了可疑的元素嵌套方式。
类似地，为了让作者对元素使用中的错误引起注意， 表达出的语义存在明显的矛盾也被认为是一致性错误。
例如下面的片段中的语义是毫无意义的： 分隔符不能同时是单元格，单选按钮也不能是进度条。
<hr role="cell">
<input type=radio role=progressbar>
另一个例子是对ul元素的内容模型的限制，
该限制只允许 li 子元素。根据定义列表只可包含一些列表项。
所以如果 ul 元素包含一些除了 li 之外的元素，
它的含义就不清楚了。
某些元素有一些默认样式和行为，它们的某些组合可能会令人困惑。 如果存在没有这一问题的等效替代，就不允许这样令人困惑的组合。
例如，div 元素会被渲染为 block boxes，span 元素则为 inline boxes。将 block box 放到
inline box 中就会导致不必要的困惑；因为只嵌套 div
元素，或只嵌套 span 元素，或在div元素中嵌套
span 元素都可以与在span元素中嵌套div元素
达到同样的目的。但只有后者涉及到在 inline box 中嵌套 block box，
所以后者的组合方式是不允许的。
另一个可能的例子是 交互内容
不可嵌套。例如： button 元素不可包含 textarea 元素。
这是因为这样嵌套交互元素可能对用户造成困惑。可以并排放置这些元素。
有时，某些禁止是因为允许它将可能会让作者困惑。
例如，设置 disabled
属性为 "false" 是不允许的，因为表面上看起来它的含义是元素可用，
但实际上含义是元素被 禁用
（实现中起作用的是该属性是否存在，而非它的值）。
一些一致性错误简化了作者需要学习的语言。
例如，area 元素的 shape 属性，在实践中虽然同时接受 circ 和 circle
取值并作为同义词，但禁止
circ 值的使用，
以此来简化教程和其他学习材料。同时允许它们并没有好处，
但它们可能在学习语言时造成额外的困惑。
某些元素的解析有点奇怪（通常是由于历史原因）， 它们的内容模型限制旨在避免作者碰到这些问题。
例如，phrasing content 中不允许
form 元素，因为当解析 HTML 时，
form 元素的开始标签将会产生一个
p 元素的关闭标签。因此下面的标记会产生两个
段落（而不是一个）：
<p>Welcome. <form><label>Name:</label> <input></form>
与下面代码的解析结果完全一样：
<p>Welcome. </p><form><label>Name:</label> <input></form>
有些错误旨在避免难以调试的脚本问题。
比如，这就是为什么有两个等值的 id 属性是不合法的。
重复的 ID 导致选择到错误的元素，有时会产生难以确定原因的灾难性后果。
某些构造被禁止是因为历史上它们曾浪费大量的编写时间。 鼓励作者避免使用这些构造，将来可以节省很多时间。
例如，script 元素的 src 属性会导致元素的内容被忽略。
然而这并不明显（尤其是元素内容是可执行脚本时）—
这会导致作者花费大量时间尝试调试内联脚本，却没意识到它并未执行。
为了减少这一问题，在本标准中src
属性出现时，script元素中不允许存在可执行脚本。
这意味着作者在验证他们的文档时，不太可能在这样的错误上浪费时间。
有些作者喜欢编写用 XML 和 HTML 解释可以得到类似结果的文件。 由于种种微妙的复杂性（尤其是涉及脚本、样式、或其他任何自动的序列化）， 一般并不鼓励这一做法，尽管如此，本标准也有一些限制意在在某种程度上减轻困难。 这让作者在 HTML 和 XML 语法之间迁移时，可以更容易地将其用于过渡步骤。
例如，围绕 lang 和
xml:lang 属性有一些复杂的规则，
就是为了让 HTML 和 XML 同步。
另一个例子可能是在 HTML 序列化中对 xmlns 属性值的限制，意在确保不管是 HTML 还是 XML 解析，
合法的文档中元素总在同一命名空间。
对语法的某些限制是为了允许语言的未来版本中添加新的语法， 类似地，对元素和属性取值的内容模型的一些限制是为了允许将来对 HTML 词库进行扩充。
例如，限制 target 属性的值，
以 U+005F 下划线（_）起始的只能是特定的预定义值。这允许了在将来引入新的预定义值，
同时不会与作者定义的值产生冲突。
某些限制意在支持其他标准作出的限制。
例如，使用媒体查询列表的属性只能使用 合法的 媒体查询列表，强调了遵循那一标准的一致性规则的重要性。
This section is non-normative.
本标准的读者可能对以下文档感兴趣。
这个架构风格标准定义了 World Wide Web 中可互操作的文本操作。为其他标准的作者、软件开发者，以及内容开发者提供了通用的参考。 该标准构建在 Universal Character Set 之上，同时定义在 Unicode Standard 和 ISO/IEC 10646。 涉及的话题包括：字符、编码、字符串、引用处理模型、字符编码的选择和识别、字符转义，以及字符串索引。
由于 Unicode 包含大量的字符，引入了世界上不同的书写系统， 错误的使用会将程序和系统暴露在可能的安全攻击之下。随着越来越多的产品被国际化该问题尤为重要。 该标准描述了程序员、系统分析师、标准开发者，以及用户应当考虑的安全事项，为减少风险提供了明确的建议。
Web Content Accessibility Guidelines (WCAG) 2.0 对于提高 Web 内容的可访问性给出了广泛的建议。 遵循这些指导可以使更多的残障人士（包括失明和弱视、失聪和听觉损耗、学习和认知障碍、行动不便、语言障碍、光过敏等）可访问您的内容。 遵循这些指导也通常会使您的 Web 内容对普通用户更加可用。
该标准为设计 Web 内容写作工具提供了指导意见，使得残障人士也可使用这些工具。 遵从这些指导的写作工具将会通过为残障作者提供可访问的用户界面以提升可访问性，以及为所有作者提供、支持和提升产出 Web 内容的可访问性。
该文档为设计降低残障人士 Web 访问门槛的用户代理提供了指导意见。 用户代理包括浏览器和其他获取和渲染 Web 内容的软件。 遵循这些指导的用户代理通过它自己的用户界面以及其他内部工具 （包括与其他技术通信的能力，尤其是辅助性技术）提升可访问性。 此外，除残障人士之外的所有用户都会发现遵循该标准用户代理更加可用。
该标准依赖于 WHATWG Infra 标准。 [INFRA]
该标准所说的属性，指的是 HTML，XML 和 IDL 属性，通常在同一个上下文中。当未具体提及所指时，HTML 和 XML 属性指的是 content attributes，而 IDL attributes指的是在 IDL 接口中定义的属性。同样，“属性”这个词也同时指 Javascript 对象属性和 CSS 属性。在可能产生混淆时， 该标准使用 object properties 和 CSS properties 来区别。
一般而言，当该标准申明一个特性可以在 HTML 语法 或 XML 语法 中应用，则在另一个中也可用。 当一个特性仅在两个语言之一中应用时，该标准将显式申明它不能在另一个语言中使用， 如：“可在 HTML 中使用，…… （该语法不可用于 XML）”。
该标准使用术语 文档指任何 HTML，从短小精悍的文档到长篇累牍的论文，
亦或是多媒体报告、完整的交互式应用程序。该术语同时指 Document 对象和他们的后裔 DOM 树。至于序列化的字节流，
则根据上下文使用 HTML 语法 或 XML 语法 来表示。
在 DOM 结构的上下文中，使用的术语 HTML 文档 和
XML 文档 定义在 DOM 标准中，
并且特指 Document 对象所处的两种不同模式。 [DOM] （这样的使用都会超链接到它们的定义）
在字节流的上下文中，术语 HTML 文档 指标记为 text/html 的资源，
术语 XML 文档 指标记为 XML MIME type 的资源。
简单起见，像 shown, displayed, 以及 visible 这样的术语可能会用来指明文档渲染给用户的方式， 这些术语并不是指应用于视觉媒介；必须考虑将它们以等价的方式应用于其他的媒介。
某个元素是visible的并不仅仅指该元素在视觉上可见，比如屏幕阅读器也应将该元素阅读给用户。
To run steps in parallel means those steps are to be run, one after another, at the same time as other logic in the standard (e.g., at the same time as the event loop). This standard does not define the precise mechanism by which this is achieved, be it time-sharing cooperative multitasking, fibers, threads, processes, using different hyperthreads, cores, CPUs, machines, etc. By contrast, an operation that is to run immediately must interrupt the currently running task, run itself, and then resume the previously running task.
For guidance on writing specifications that leverage parallelism, see Dealing with the event loop from other specifications.
To avoid race conditions between different in parallel algorithms that operate on the same data, a parallel queue can be used.
A parallel queue represents a queue of algorithm steps that must be run in series.
A parallel queue has an algorithm queue (a queue), initially empty.
To enqueue steps to a parallel queue, enqueue the algorithm steps to the parallel queue's algorithm queue.
To start a new parallel queue, run the following steps:
Let parallelQueue be a new parallel queue.
Run the following steps in parallel:
While true:
Let steps be the result of dequeueing from parallelQueue's algorithm queue.
If steps is not nothing, then run steps.
Assert: running steps did not throw an exception, as steps running in parallel are not allowed to throw.
Implementations are not expected to implement this as a continuously running loop. Algorithms in standards are to be easy to understand and are not necessarily great for battery life or performance.
Return parallelQueue.
Steps running in parallel can themselves run other steps in in parallel. E.g., inside a parallel queue it can be useful to run a series of steps in parallel with the queue.
Imagine a standard defined nameList (a list), along with a method to add a name to nameList, unless nameList already contains name, in which case it rejects.
The following solution suffers from race conditions:
Let p be a new promise.
Run the following steps in parallel:
Return p.
Two invocations of the above could run simultaneously, meaning name isn't in nameList during step 2.1, but it might be added before step 2.3 runs, meaning name ends up in nameList twice.
Parallel queues solve this. The standard would let nameListQueue be the result of starting a new parallel queue, then:
Let p be a new promise.
Enqueue the following steps to nameListQueue:
Return p.
The steps would now queue and the race is avoided.
本标准中术语 支持 是指用户代理是否实现了对外部资源的语义的解码能力。 支持 某种格式或类型是指这一实现可以处理该格式或类型的外部资源，且处理过程不会忽略关键方面。 是否 支持 某种特定资源取决于该资源类型有哪些在用的特性。
例如，如果可以解码和渲染 PNG 图片的像素数据，就可以认为支持 PNG 图片格式。 即使这一实现不知道该图片还包含了动画数据。
如果不支持使用的压缩格式，即使实现可以从文件的元数据确定电影的尺寸，MPEG-4 视频文件也不会被视为支持的格式。
有些标准中（特别是 HTTP 标准）中称为 表示（representation） 的在本标准中称为 资源（resource）。 [HTTP]
资源的 关键子资源 是那些需要被正确处理的资源。 哪些资源被认为是关键的，由定义该资源格式的标准来定义。
为了方便从 HTML 迁移到 XML，遵循本标准的用户代理会把 HTML 中的元素放在
http://www.w3.org/1999/xhtml 命名空间，
至少是为了 DOM 和 CSS 的目的。
本标准中使用的术语 "HTML 元素"，是指所有在那个命名空间中的元素，
包括 XML 文档中的。
除非另有声明，所有本标准中定义和提到的元素均位于
HTML
（"http：//www.w3.org/1999/xhtml"），
本标准中所有定义和提到的属性（Attribute）没有命名空间。
术语 元素类型 用于指代给定命名空间和局部名的那些元素。
例如，button 元素的元素类型为 button，意味着它们的局部名为
"button" 且（如上述定义地）命名空间为
HTML。
如果属性名匹配 XML 中定义的 Name 生成式且不包含
U+003A COLON 字符（：），那么它就是 XML 兼容的 [XML]
当声明 忽略 某些元素或属性，或当作其他值处理，或当作其他东西处理时， 都是指节点在 DOM 中之后的处理。在这些情形下用户代理禁止改动 DOM。
只有内容属性的新值和原值不同时，才说内容属性的值发生了 改变； 将内容属性设置到它已有的值不会让它发生改变。
术语 空 用于属性值、文本 节点、或字符串时，
表示文本的长度是零（即不包含控制字符 或 U+0020 SPACE）。
插入节点 A 到节点 B 是指以 A 作为参数调用 插入步骤， 然后 A 新的父节点就是 B。类似地，从节点 B 移除节点 A 是指以 A 作为 removedNode 参数，以 B 作为 oldParent 参数调用 移除步骤。
插入节点到文档 是指把它作为参数调用 插入步骤， 然后它就 在文档树中 了。类似地， 从文档中移除节点 是指把它作为参数调用 移除步骤， 然后它就不 在文档树中 了。
把节点作为参数调用 插入步骤 后， 节点就 变成已连接的。 类似地，把它作为参数调用 移除步骤 后， 节点就 变成了分离的。
如果节点是 已连接的 且它的 包含 Shadow 的根 有 浏览环境，那么它是 连接到浏览环境的。 把节点作为参数调用 插入步骤 后， 它就 连接到了浏览环境。 把节点作为参数调用移除步骤， 或者它的 包含 Shadow 的根 不再拥有 浏览环境 后， 它就 与浏览环境分离了。
有时会用构造 "一个 Foo 对象"
（其中 Foo 其实是一个接口），
来表示 "一个实现了 Foo 接口的对象"。
获取 IDL 属性的值称为 获取（例如在作者的脚本中）， 将新的值赋值给 IDL 属性则称为 设置。
如果 DOM 对象是 活的，该对象上的属性和方法 必须 在真正的底层数据（而不是数据快照）上进行操作。
术语 插件 是指一些用户代理定义的内容处理程序，用户代理用它们
参与 Document 对象的渲染，但不会作为 Document 的
子浏览环境，也不会给
Document 的 DOM 引入任何 Node 对象。
通常，这样的内容处理程序由第三方提供，尽管用户代理也可以将内置的内容处理程序指定为插件。
用户代理不得将 text/plain 和 application/octet-stream 类型视为注册有
插件。
插件的一个例子是当用户导航到PDF文件时在 浏览环境 中实例化的PDF查看器。 无论执行PDF查看器组件的一方是否与实现用户代理本身的方相同，这将被视为插件。 但是根据定义，与用户代理（而不是使用相同的接口）分开启动的PDF查看器应用程序不是插件。
该规范没有定义与插件交互的机制，因为插件预期就是用户代理和平台特定的。 UA 可以选择支持某种插件机制，比如 Netscape Plugin API； 也可以选择对某些类型使用远程内容转换器或提供内置支持。 实际上，本标准根本没有要求用户代理支持插件。[NPAPI]
安全 插件应该遵循
sandbox 属性的语义。
例如，在沙盒 iframe 中初始化的
安全插件应该阻止其中的内容创建弹出窗口。
在与 插件 的外部内容交互时，浏览器应该格外小心。 当第三方软件以与用户代理本身相同的权限运行时，第三方软件中的漏洞变得与用户代理中的漏洞同样危险。
因为不同的用户有不同的 插件，这提供了唯一识别用户的指纹向量， 推荐用户代理对每个用户都支持同样的 plugins。
字符编码 或着没有歧义时说的 编码，是一种字节流与 Unicode 字符串的转换， 定义在 Encoding 中。编码 包括一个 编码名称 和一个或更多的 编码标签，编码的 名称 和 标签 定义在 Encoding 标准中。 [ENCODING]
本标准描述了 用户代理（实现者相关）和 文档 （作者和编写工具的实现者相关） 的符合性标准。
符合规范的文档 是符合所有符合性要求的文档。 为了提高可读性，有些符合性要求是对作者提出的；这些是对文档的隐性要求： 按照定义所有文档都有对应的作者。（有些情况下，作者本身可能是一个用户代理 — 这些用户代理受其他规则的约束，见下文。）
例如，如果一项要求声明 "作者 禁止使用
foobar 元素"，意味着文档不允许包含名为 foobar 的元素。
文档的符合性要求与实现的符合性要求没有隐含的关系。 用户代理不能随意处理不合规范的文档；不论输入的文档是否合规，本标准中描述的处理模型都适用。
按照不同的符合性要求，用户代理分为（有重合的）几类。
支持 XML 语法 的 Web 浏览器必须按照本标准的描述 处理 XML 文档中 HTML 命名空间 的元素和属性， 以便用户与之交互，除非那些元素的语义已经被其他标准覆盖。
在 XML 文档中查找 script 元素时，符合规范的 Web
浏览器会执行该元素包含的脚本。然而，如果该元素处于 XSLT 变换中（假设用户代理也支持 XSLT），
那么处理器会将 script 元素作为组成这一变换的不透明元素处理。
支持 HTML 语法 的 Web 浏览器必须按照本标准的描述 处理标记为 HTML MIME 类型 的文档，以便用户与它们交互。
支持脚本的用户代理也必须一致地实现本标准中的 IDL 片段，IDL 片段定义在 Web IDL 标准中。 [WEBIDL]
除非明确声明，覆盖 HTML 元素语义的标准不覆盖对（表示这些元素的） DOM 对象的要求。
例如，上面例子中的 script 元素仍然实现 HTMLScriptElement 接口。
处理 HTML 和 XML 文档并纯粹地渲染它们的非交互式版本的用户代理必须遵循与 Web 浏览器同样的符合性规范，除了它们可以免除用户交互的要求。
非交互式用户代理的典型例子就是打印机（静态 UA）和投影仪（动态 UA）。 多数静态的非交互式用户代理 不支持脚本。
非交互式的动态 UA 仍然会执行脚本，以便动态提交表单。 但是因为没有获得焦点的概念，这些 UA 可能不需要实现焦点相关的 DOM API。
无论是交互式还是非交互式用户代理，都可以被指定（比如通过作为用户选项）本标准定义的建议默认渲染。
这并非强制要求，特别是即使用户代理实现了建议默认渲染，也建议为此提供设置来提升用户体验， 例如改变色彩对比度，使用不同的焦点样式，或者其他提升可访问性和可用性的方式。
如果指定了建议默认渲染，支持建议默认渲染的用户代理必须根据 渲染部分 定义的规则实现 用户代理 期望 实现的行为。
不支持脚本的实现（或者完全禁用了其脚本特性的用户代理）可以不支持本标准中提到的事件和 DOM 接口。 但对于本标准中定义的事件模型和 DOM，这些用户代理仍然必须表现地像支持事件和 DOM 一样。
脚本可能是构成应用的一部分。不支持脚本或禁用了脚本的 Web 浏览器可能无法完全表达作者的意图。
符合性检查器必须验证文档是否符合
符合性检查程序必须验证文档是否符合本规范中描述的适用的一致性标准。
自动化的符合检查程序可以不必检测需要解释作者意图的错误（例如，
blockquote 的内容不是引用文档就不符合标准，如果符合性检查程序在运行时没有人为判断的输入，
就不必检查 blockquote 元素是否只包含引用的内容）。
符合性检查器必须检查没有 浏览环境 （意味着没有脚本运行，并且禁用了解析器的 脚本标志 ）时解析输入文档符合规范， 还应检查在会执行脚本的 浏览环境 下解析输入文档符合规范，并且脚本的执行永远不会导致不符合规范的状态（脚本执行过程中除外）出现。 （这只是一个“应该”而不是“必须”的要求，因为这已被证明是不可能的。[COMPUTABLE]）
"HTML 校验器" 可以指代遵循本标准中的适用要求的符合性检查器。
XML DTD 无法表达本标准的所有符合性要求。因此 XML 校验器加 DTD 不能代替符合性检查器。 此外本标准中定义的这两种写作格式都不属于 SGML，因此 SGML 系统也不能代替符合性检查器。
换句话说，有三种符合性规则：
符合性检查器可以检查前面两种。简单的基于 DTD 的校验器只能检查第一类错误， 因此根据本标准，它不是一种遵循标准的符合性检查器。
不是出于渲染文档或检查文档是否符合规范目的的，处理 HTML 和 XML 文档的应用程序和工具， 其行为应当与其处理的文档的语义保持一致。
只增加每个段落（paragraph）的嵌套层级但不增加每个章节（section）的嵌套层级的 文档大纲 生成工具是不合规范的。
只增加 <p> 的层级而不增加
<section> 的层级会破坏 DOM 原有的语义，比如让段落变成了子段落。
编写工具和标记生成器必须生成 符合规范的文档。 在适当的地方，适用于作者的符合性规则也适用于编写工具。
如果编写工具还不能决定作者的意图，可以免除“只能按规定的用途使用元素”的严格要求。 但是编写工具禁止自动地错误使用元素或鼓励它们的用户这样做。
例如，将 address 元素用于任意的联系信息是不合规范的；
该元素只能用于标记文档或章节作者的联系信息。因为编写工具不能区分此意图，所以可以免除这项要求。
但这不意味着（比如）编写工具可以为了斜体字而任意地使用 address 元素；
只意味着编写工具可以在用户给章节插入联系信息时不验证它，
也不需保证用户没用它做别的事或插入别的东西。
在符合性检查方面，文本编辑器必须像符合性检查器一样输出符合规范的文档。
当使用编写工具编辑不符合规范的文档时，可以保持未编辑部分的符合性错误。 （即允许错误的内容经过编辑工具而不发生改变）。 但如果保持了错误，禁止编写工具声明其输出是符合规范的。
编写工具大致分为两类：在结构化或语义数据上进行编辑， 或者在所见即所得的（WYSIWYG）特定媒体上进行编辑。
前者是编写 HTML 工具的理想机制，因为源码信息中的结构可用于更好地选择 HTML 元素和属性。
但是 WYSIWYG 工具也是合理的。WYSIWYG 工具应该使用它们认为合适的元素，
不应该使用它们不确定是否合适的元素。这可能在特定极端情况下意味着对能够使用的流式元素做出限制，
比如 div，b，i，以及 span，以及使用大量的
style 属性。
不论是不是 WYSIWYG，所有编写工具都应该尽量让用户创建良构的、语义丰富的、媒体无关的内容。
为了防范诸如 DoS 攻击、内存耗尽，或解决平台相关的限制， 用户代理可能会对本来不受约束的输入强加实现相关的限制。
为了兼容既有内容与标准，本标准描述了两种写作格式： 一种基于 XML， 另一种基于 SGML 启发的 自定义格式 （称为 HTML 语法）。 本标准鼓励实现同时支持以上两种格式，但至少支持其中一种。
一些符合性要求称为元素、属性、方法或对象的要求。 这些要求分为两类：描述内容模型限制的，和描述实现行为的。 前者是对文档和编写工具的要求，后者是对用户代理的要求。 类似地，另一些符合性要求称为对作者的要求；这些要求应解释为对作者产出的文档的符合性要求。 （换句话说，本标准不区分对作者和对文档的符合性要求）
This specification relies on several other underlying specifications.
The following terms are defined in Infra: [INFRA]
The Unicode character set is used to represent textual data, and Encoding defines requirements around character encodings. [UNICODE]
This specification introduces terminology based on the terms defined in those specifications, as described earlier.
The following terms are used as defined in Encoding: [ENCODING]
Implementations that support the XML syntax for HTML must support some version of XML, as well as its corresponding namespaces specification, because that syntax uses an XML serialization with namespaces. [XML] [XMLNS]
Data mining tools and other user agents that perform operations on content without running scripts, evaluating CSS or XPath expressions, or otherwise exposing the resulting DOM to arbitrary content, may "support namespaces" by just asserting that their DOM node analogues are in certain namespaces, without actually exposing the namespace strings.
In the HTML syntax, namespace prefixes and namespace declarations do not have the same effect as in XML. For instance, the colon has no special meaning in HTML element names.
The attribute with the name space in the XML namespace is defined by
Extensible Markup Language (XML). [XML]
The Name production is defined in XML. [XML]
This specification also references the <?xml-stylesheet?>
processing instruction, defined in Associating Style Sheets with XML documents.
[XMLSSPI]
This specification also non-normatively mentions the XSLTProcessor
interface and its transformToFragment() and transformToDocument() methods. [XSLTP]
The following terms are defined in URL: [URL]
application/x-www-form-urlencoded formatapplication/x-www-form-urlencoded serializerA number of schemes and protocols are referenced by this specification also:
about: scheme [ABOUT]blob: scheme [FILEAPI]data: scheme [RFC2397]http: scheme [HTTP]https: scheme [HTTP]mailto: scheme [MAILTO]sms: scheme [SMS]urn: scheme [URN]Media fragment syntax is defined in Media Fragments URI. [MEDIAFRAG]
The following terms are defined in the HTTP specifications: [HTTP]
Accept` headerAccept-Language` headerCache-Control` headerContent-Disposition` headerContent-Language` headerLast-Modified` headerReferer` headerThe following terms are defined in HTTP State Management Mechanism: [COOKIES]
Cookie` headerThe following term is defined in Web Linking: [WEBLINK]
Link` headerThe following terms are defined in Structured Field Values for HTTP: [STRUCTURED-FIELDS]
The following terms are defined in MIME Sniffing: [MIMESNIFF]
The following terms are defined in Fetch: [FETCH]
about:blankUser-Agent` valueOrigin` headerCross-Origin-Resource-Policy` headerRequestCredentials enumerationRequestDestination enumerationfetch() methodThe following terms are defined in Referrer Policy: [REFERRERPOLICY]
Referrer-Policy` HTTP headerReferrer-Policy` header algorithmno-referrer",
"no-referrer-when-downgrade",
"origin-when-cross-origin", and
"unsafe-url" referrer policiesThe following terms are defined in Mixed Content: [MIX]
The following terms are defined in Paint Timing: [PAINTTIMING]
The following terms are defined in Long Tasks: [LONGTASKS]
The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in Web IDL. [WEBIDL]
The following terms are defined in Web IDL:
[LegacyFactoryFunction][LegacyLenientThis][LegacyNullToEmptyString][LegacyOverrideBuiltIns][LegacyTreatNonObjectAsNull][LegacyUnenumerableNamedProperties][LegacyUnforgeable]The Web IDL also defines the following types that are used in Web IDL fragments in this specification:
ArrayBufferArrayBufferViewbooleanDOMStringdoubleErrorFunctionlongobjectUint8ClampedArrayunrestricted doubleunsigned longUSVStringVoidFunctionThe term throw in this
specification is used as defined in Web IDL. The DOMException
type and the following exception names are defined by Web IDL and used by this
specification:
IndexSizeError"HierarchyRequestError"InvalidCharacterError"NotFoundError"NotSupportedError"InvalidStateError"SyntaxError"InvalidAccessError"SecurityError"NetworkError"AbortError"QuotaExceededError"DataCloneError"EncodingError"NotAllowedError"When this specification requires a user agent to create a Date object
representing a particular time (which could be the special value Not-a-Number), the milliseconds
component of that time, if any, must be truncated to an integer, and the time value of the newly
created Date object must represent the resulting truncated time.
For instance, given the time 23045 millionths of a second after 01:00 UTC on
January 1st 2000, i.e. the time 2000-01-01T00:00:00.023045Z, then the Date object
created representing that time would represent the same time as that created representing the
time 2000-01-01T00:00:00.023Z, 45 millionths earlier. If the given time is NaN, then the result
is a Date object that represents a time value NaN (indicating that the object does
not represent a specific instant of time).
Some parts of the language described by this specification only support JavaScript as the underlying scripting language. [JAVASCRIPT]
The term "JavaScript" is used to refer to ECMA-262, rather than the official
term ECMAScript, since the term JavaScript is more widely known. Similarly, the MIME
type used to refer to JavaScript in this specification is text/javascript, since that is the most commonly used type, despite it being an officially obsoleted type according to RFC
4329. [RFC4329]
The following terms are defined in the JavaScript specification and used in this specification:
Atomics objectDate classRegExp classSharedArrayBuffer classTypeError classRangeError classeval() functionimport()import.metatypeof operatordelete operatorUsers agents that support JavaScript must also implement ECMAScript Internationalization API. [JSINTL]
The following term is defined in WebAssembly JavaScript Interface: [WASMJS]
The Document Object Model (DOM) is a representation — a model — of a document and its content. The DOM is not just an API; the conformance criteria of HTML implementations are defined, in this specification, in terms of operations on the DOM. [DOM]
Implementations must support DOM and the events defined in UI Events, because this specification is defined in terms of the DOM, and some of the features are defined as extensions to the DOM interfaces. [DOM] [UIEVENTS]
In particular, the following features are defined in DOM: [DOM]
Attr interfaceComment interfaceDOMImplementation interfaceDocument interfaceDocumentOrShadowRoot interfaceDocumentFragment interfaceDocumentType interfaceChildNode interfaceElement interfaceattachShadow() method.Node interfaceNodeList interfaceProcessingInstruction interfaceShadowRoot interfaceText interfaceHTMLCollection interface, its
length attribute, and its
item() and
namedItem() methodsDOMTokenList interface, and its
value attributecreateDocument() methodcreateHTMLDocument() methodcreateElement() methodcreateElementNS() methodgetElementById() methodgetElementsByClassName() methodappendChild() methodcloneNode() methodimportNode() methodpreventDefault() methodid attributesetAttribute() methodtextContent attributeEvent interfaceEvent and derived interfaces constructor behaviorEventTarget interfaceEventInit dictionary typetype attributetarget attributecurrentTarget attributebubbles attributecancelable attributecomposed attributeisTrusted attributeinitEvent() methodaddEventListener() methodEventListener callback interfaceDocumentNode, and the concept of
cloning steps used by that algorithmis valueMutationObserver interface and mutation observers in generalThe following features are defined in UI Events: [UIEVENTS]
MouseEvent interfaceMouseEvent interface's relatedTarget attributeMouseEventInit dictionary typeFocusEvent interfaceFocusEvent interface's relatedTarget attributeUIEvent interfaceUIEvent interface's view attributeauxclick eventclick eventdblclick eventmousedown eventmouseenter eventmouseleave eventmousemove eventmouseout eventmouseover eventmouseup eventwheel eventkeydown eventkeypress eventkeyup eventThe following features are defined in Touch Events: [TOUCH]
Touch interfacetouchend eventThe following features are defined in Pointer Events: [POINTEREVENTS]
pointerup eventThis specification sometimes uses the term name to refer to the event's
type; as in, "an event named click" or "if the event name is keypress". The terms
"name" and "type" for events are synonymous.
The following features are defined in DOM Parsing and Serialization: [DOMPARSING]
The following features are defined in Selection API: [SELECTION]
User agents are encouraged to implement the features described in execCommand. [EXECCOMMAND]
The following parts of Fullscreen API are referenced from this
specification, in part to define the rendering of dialog elements, and also to
define how the Fullscreen API interacts with HTML: [FULLSCREEN]
requestFullscreen()High Resolution Time provides the current high
resolution time and the DOMHighResTimeStamp
typedef. [HRT]
This specification uses the following features defined in File API: [FILEAPI]
Blob interface and its
type attributeFile interface and its
name and
lastModified attributesFileList interfaceBlob's snapshot stateThis specification uses cleanup Indexed Database transactions defined by Indexed Database API. [INDEXEDDB]
The following terms are defined in Media Source Extensions: [MEDIASOURCE]
The following terms are defined in Media Capture and Streams: [MEDIASTREAM]
MediaStream interfaceThe following terms are defined in Reporting: [REPORTING]
The following features and terms are defined in XMLHttpRequest: [XHR]
XMLHttpRequest interface, and its
responseXML attributeProgressEvent interface, and its
lengthComputable,
loaded, and
total attributesFormData interfaceFormDataEntryValue typeThe following features are defined in Battery Status API: [BATTERY]
getBattery() methodImplementations must support Media Queries. The <media-condition> feature is defined therein. [MQ]
While support for CSS as a whole is not required of implementations of this specification (though it is encouraged, at least for web browsers), some features are defined in terms of specific CSS requirements.
When this specification requires that something be parsed according to a particular CSS grammar, the relevant algorithm in CSS Syntax must be followed, including error handling rules. [CSSSYNTAX]
For example, user agents are required to close all open constructs upon
finding the end of a style sheet unexpectedly. Thus, when parsing the string "rgb(0,0,0" (with a missing close-parenthesis) for a color value, the close
parenthesis is implied by this error handling rule, and a value is obtained (the color 'black').
However, the similar construct "rgb(0,0," (with both a missing
parenthesis and a missing "blue" value) cannot be parsed, as closing the open construct does not
result in a viable value.
To parse a CSS <color> value, given a string input with an optional element element, run these steps:
Let color be the result of parsing input as a CSS <color>. [CSSCOLOR]
If color is failure, then return failure.
If color is 'currentcolor', then:
If element is not given, then set color to opaque black.
Otherwise, set color to the computed value of the 'color' property of element.
Return color.
The following terms and features are defined in Cascading Style Sheets (CSS): [CSS]
CSS also defines the following border properties: [CSS]
| Top | Bottom | Left | Right | |
|---|---|---|---|---|
| Width | 'border-top-width' | 'border-bottom-width' | 'border-left-width' | 'border-right-width' |
| Style | 'border-top-style' | 'border-bottom-style' | 'border-left-style' | 'border-right-style' |
| Color | 'border-top-color' | 'border-bottom-color' | 'border-left-color' | 'border-right-color' |
The basic version of the 'display' property is defined in CSS, and the property is extended by other CSS modules. [CSS] [CSSRUBY] [CSSTABLE]
The following terms and features are defined in CSS Logical Properties: [CSSLOGICAL]
The following terms and features are defined in CSS Color: [CSSCOLOR]
The following terms are defined in CSS Images: [CSSIMAGES]
The term paint source is used as defined in CSS Images Level 4 to define the interaction of certain HTML elements with the CSS 'element()' function. [CSSIMAGES4]
The following features are defined in CSS Backgrounds and Borders: [CSSBG]
The following features are defined in CSS Box Alignment: [CSSALIGN]
The following terms and features are defined in CSS Display: [CSSDISPLAY]
The following features are defined in CSS Flexible Box Layout: [CSSFLEXBOX]
The following terms and features are defined in CSS Fonts: [CSSFONTS]
The following features are defined in CSS Grid Layout: [CSSGRID]
The following terms are defined in CSS Inline Layout: [CSSINLINE]
The following terms and features are defined in CSS Intrinsic & Extrinsic Sizing: [CSSSIZING]
The 'list-style-type' property is defined in CSS Lists and Counters. [CSSLISTS]
The following features are defined in CSS Overflow. [CSSOVERFLOW]
The following features are defined in CSS Positioned Layout: [CSSPOSITION]
The following features are defined in CSS Multi-column Layout. [CSSMULTICOL]
The 'ruby-base' value of the 'display' property is defined in CSS Ruby Layout. [CSSRUBY]
The following features are defined in CSS Table: [CSSTABLE]
The following features are defined in CSS Text: [CSSTEXT]
The following features are defined in CSS Writing Modes: [CSSWM]
The following features are defined in CSS Basic User Interface: [CSSUI]
The algorithm to update animations and send events is defined in Web Animations. [WEBANIMATIONS].
Implementations that support scripting must support the CSS Object Model. The following features and terms are defined in the CSSOM specifications: [CSSOM] [CSSOMVIEW]
Screen interfaceLinkStyle interfaceCSSStyleDeclaration interfacestyle IDL attributecssText attribute of CSSStyleDeclarationStyleSheet interfaceresize eventscroll eventThe following features and terms are defined in CSS Syntax: [CSSSYNTAX]
The following terms are defined in Selectors: [SELECTORS]
The following features are defined in CSS Values and Units: [CSSVALUES]
The term style attribute is defined in CSS Style Attributes. [CSSATTR]
The following terms are defined in the CSS Cascading and Inheritance: [CSSCASCADE]
The CanvasRenderingContext2D object's use of fonts depends on the features
described in the CSS Fonts and Font Loading specifications, including
in particular FontFace objects and the font source concept.
[CSSFONTS] [CSSFONTLOAD]
The following interfaces and terms are defined in Geometry Interfaces: [GEOMETRY]
DOMMatrix interface, and associated
m11 element,
m12 element,
m21 element,
m22 element,
m41 element, and
m42 elementDOMMatrix2DInit and
DOMMatrixInit dictionariesDOMMatrix from a dictionary
and create a DOMMatrix from a 2D dictionary
algorithms for DOMMatrix2DInit or DOMMatrixInitThe following terms are defined in the CSS Scoping: [CSSSCOPING]
The following terms and features are defined in CSS Color Adjustment: [CSSCOLORADJUST]
The following term is defined in Intersection Observer: [INTERSECTIONOBSERVER]
The following interfaces are defined in the WebGL specifications: [WEBGL]
WebGLRenderingContext interfaceWebGL2RenderingContext interfaceWebGLContextAttributes dictionaryImplementations may support WebVTT as a text track format for subtitles, captions, metadata, etc., for media resources. [WEBVTT]
The following terms, used in this specification, are defined in WebVTT:
The following terms are defined in Fetch: [FETCH]
The following terms are defined in The WebSocket protocol: [WSP]
Sec-WebSocket-Protocol fieldThe role attribute is defined in
Accessible Rich Internet Applications (ARIA), as are the following
roles: [ARIA]
In addition, the following aria-* content
attributes are defined in ARIA: [ARIA]
Finally, the following terms are defined ARIA: [ARIA]
ARIAMixin interface, with its associated
ARIAMixin getter steps and
ARIAMixin setter steps hooksThe following terms are defined in Content Security Policy: [CSP]
report-uri directiveframe-ancestors directivesandbox directiveThe following terms are defined in Service Workers: [SW]
The following algorithms are defined in Secure Contexts: [SECURE-CONTEXTS]
The following terms are defined in Permissions Policy: [PERMISSIONSPOLICY]
The following feature is defined in Payment Request API: [PAYMENTREQUEST]
PaymentRequest interfaceWhile support for MathML as a whole is not required by this specification (though it is encouraged, at least for web browsers), certain features depend upon small parts of MathML being implemented. [MATHML]
The following features are defined in Mathematical Markup Language (MathML):
annotation-xml elementmath elementmerror elementmi elementmn elementmo elementms elementmtext elementWhile support for SVG as a whole is not required by this specification (though it is encouraged, at least for web browsers), certain features depend upon parts of SVG being implemented.
User agents that implement SVG must implement the SVG 2 specification, and not any earlier revisions.
The following features are defined in the SVG 2 specification: [SVG]
SVGElement interfaceSVGImageElement interfaceSVGScriptElement interfaceSVGSVGElement interfacea elementdesc elementforeignObject elementimage elementscript elementsvg elementtitle elementuse elementThe following feature is defined in Filter Effects: [FILTERS]
The following features are defined in Cooperative Scheduling of Background Tasks: [REQUESTIDLECALLBACK]
The following terms are defined in Storage: [STORAGE]
The following features are defined in Web App Manifest: [MANIFEST]
This specification does not require support of any particular network protocol, style sheet language, scripting language, or any of the DOM specifications beyond those required in the list above. However, the language described by this specification is biased towards CSS as the styling language, JavaScript as the scripting language, and HTTP as the network protocol, and several features assume that those languages and protocols are in use.
A user agent that implements the HTTP protocol must implement HTTP State Management Mechanism (Cookies) as well. [HTTP] [COOKIES]
This specification might have certain additional requirements on character encodings, image formats, audio formats, and video formats in the respective sections.
强烈不鼓励本规范的厂商专有的用户代理扩展。 文档不能使用这样的扩展，因为这样做减少了互操作性并且分割了用户群，只允许特定用户代理的用户访问相关内容。
所有扩展的定义必须使得，扩展的使用既不会违反本标准，也不会导致本标准定义的功能的不一致。
例如（强烈不鼓励这样做）实现上可以为控件添加一个新的 IDL 属性
"typeTime" 返回（比如）用户选择控件的当前值所花费的时间。
另一方面，在表单的 elements 数组中定义一个新的控件也会违反上述要求，
因为它将违反本标准给出的 elements 定义。
当本标准需要厂商中立的扩展时，或者相应地更新本标准，或者编写一个扩展标准并重写本标准中的要求。 如果有人在他的活动中使用了本标准，并决定识别该扩展标准的要求，这一标准就变成了本标准中符合性要求的 适用规范。
有人可能会编写一个规范来定义任意的字节流为合规的，然后声明他的随机产生的垃圾为符合规范的。 但是这并不意味着他的随机垃圾符合所有人的目的：如果有人认为该标准不适用于他的工作， 他就可以很合理地说上述随机垃圾就是垃圾，根本不符合规范。 就符合性而言，（某一特定）社区内 同意 适用才是最重要的。
用户代理必须将它们不理解的原始和属性当作语义中立的来处理； （DOM 处理器）把它们留在 DOM 中，（CSS 处理器）根据 CSS 设置它们的样式， 但不要推测它们的语义。
当功能的支持被禁用时（例如作为减轻安全问题的紧急措施或协助开发或出于性能原因）， 用户代理的表现必须像不支持该功能一样，像本标准没提到该功能一样。 例如，如果某个特性是通过 Web IDL 接口的属性来访问的，实现该接口的对象将会忽略掉这个属性 — 把属性留在对象上，让它返回 null 或抛出异常是不够的。
操作以本标准的方式解析或创建的 HTML 文档
（例如作为 document.evaluate() API 的一部分）
的 XPath 1.0 的实现必须在 XPath 1.0 标准的基础上遵循以下改动。
首先，移除这一段话：
节点测试中的 QName 使用表达式上下文的命名空间声明展开为 expanded-name， 展开的方式与开始和结束标签的元素类型名称一样，但是不使用以
xmlns声明的默认命名空间：如果 QName 没有前缀，那么命名空间 URI 就是 null （这与属性名的展开方式相同）。 如果 QName 的命名空间前缀 在表达式上下文中没有声明将会引发错误。
然后，在同样的位置插入下面一段：
节点测试中的 QName 使用表达式上下文的命名空间声明展开为 expanded-name。 如果 QName 有前缀，则该前缀必须在表达式上下文中有一个 URI 与之关联的命名空间声明， 且相应的。 如果 QName 的命名空间前缀 在表达式上下文中没有声明将会引发错误。
如果 QName 没有前缀且轴的主节点类型为元素，那么使用默认的元素命名空间。 否则，如果 QName 没有前缀，命名空间 URI 就是 null。 默认的元素命名空间是 XPath 表达式的上下文中的成员。 按照 DOM3 XPath API 执行 XPath 表达式时，默认的元素命名空间的值由以下方式决定：
- 如果上下文节点来自 HTML DOM，默认元素命名空间是 "http://www.w3.org/1999/xhtml"。
- 否则，默认元素命名空间 URI is null。
这相当于将 XPath 2.0 的默认元素命名空间特性添加到 XPath 1.0， 并将 HTML 命名空间用作 HTML 文档的默认元素命名空间。这样做是希望实现兼容于遗留的 HTML 内容的同时， 支持本标准在 HTML 元素命名空间方面引入的变更，以及希望使用 XPath 1.0 而不是 XPath 2.0。
这一变更的 有意的违反 XPath 1.0 标准， 这样做是希望实现兼容于遗留内容的同时支持本标准在 HTML 元素命名空间方面引入的变更。 [XPATH10]
当输出方法为 "html" 时（显式地设置或采用了 XSLT 1.0 的默认规则）， XSLT 1.0 处理器的 DOM 输出受一下规则的影响：
如果转换程序输出没有命名空间的元素， 处理器必须在构造相应的 DOM 元素节点之前将元素的命名空间更改为 HTML 命名空间， 将局部名称 改为 ASCII 小写， 并且把元素上无命名空间的属性名 改为 ASCII 小写。
本要求是 XSLT 1.0 标准的 故意违反， 本标准为了兼容基于 DOM 的 XSLT 转换，而改变了 HTML 的命名空间和大小写敏感性规则。 （不影响序列化该输出的处理器） [XSLT10]
本标准不指定 XSLT 处理具体如何与 HTML 解析器 基础设施交互
（例如，XSLT 处理器是否将所有元素放到 打开元素栈 中）。
但是当成功地完成后，XSLT 处理器必须 停止解析，
也必须首先将 当前文档就绪状态 设置为
"interactive"，在退出后再设置为 "complete"。
本标准不指定 XSLT 如何与 导航 算法进行交互， 如何与 事件循环 相匹配， 也不指定错误页面如何处理（例如 XSLT 错误是替代 XSLT 增量输出，还是内联呈现等等）。
在 script 元素部分，有一些关于 XSLT 与 HTML 之间交互的非规范性注释，
在 template 元素部分 有一些关于 XSLT，XPath，和 HTML 之间交互的非规范性注释。
A string pattern is a prefix match for a string s when pattern is not longer than s and truncating s to pattern's length leaves the two strings as matches of each other.
This document defines the following policy-controlled features:
Headers/Feature-Policy/autoplay
Headers/Feature-Policy/document-domain
autoplay", which has a default allowlist of 'self'.cross-origin-isolated", which has a default allowlist of 'self'.document-domain", which
has a default allowlist of *.HTML 中有很多地方接受特定的数据类型，比如日期和数字。 这一部分描述了那些类型的内容应符合的标准，以及如何解析它们。
强烈要求实现者仔细地检查考虑用来实现如下语法解析的第三方库。 例如，日期库实现的错误处理行为可能与本标准所要求的不同。 因为在描述类似本标准使用的日期语法的其他标准中，通常没有给出错误处理行为， 所以实现的错误处理可能很不一样。
空格字符 是
在 Unicode PropList.txt 数据文件中
Unicode 属性为 "White_Space" 的字符。[UNICODE]
不要与 Unicode.txt 数据文件中
"Bidi_Class" 属性的 "White_Space" 值（缩写为 "WS"）相混淆。
有些下面描述的微解析器遵循这样的模式：维护一个 input 变量保存待解析字符串，以及一个 position 变量指向 input 中下一个要解析的字符。
有些属性是 布尔属性。 布尔属性出现在元素上表示值为 true，不出现表示表示值为 false。
如果该属性出现，它的值必须为空字符串或者匹配属性的标准名称的 ASCII 大小写不敏感 字符串， 并且前后没有空格。
布尔属性上不允许 "true" 和 "false" 值。 为了表示 false 值，应该一并忽略该属性。
这里有一个例子，表示复选框被选中和禁用。
checked 和
disabled 属性就是布尔属性。
<label><input type=checkbox checked name=cheese disabled> Cheese</label>
这可以等价地写成：
<label><input type=checkbox checked=checked name=cheese disabled=disabled> Cheese</label>
你也可以混合两种风格；下面的写法仍然是等价的：
<label><input type='checkbox' checked name=cheese disabled=""> Cheese</label>
有些属性定义为取值只能是有限的关键字集合中的一个。 这样的属性称为 枚举属性。 这些关键字都定义为映射到特定的状态 state （可能多个关键字映射到同一个状态， 此时这些关键字互为同义词；此外有些关键字其实是不合规范的，它们在还在这里完全是历史原因）。 除此之外可以给出两个默认状态：第一个是 非法值的默认状态， 第二个是 缺失值的默认状态。
如果指定了枚举属性，属性值必须是一个匹配其中一个关键字（不包括不合规范）的 ASCII 大小写不敏感 字符串，并且前后没有空格。
指定属性时，如果它的值是匹配其中一个关键字的 ASCII 大小写不敏感 字符串 那么该属性表示的状态就是该关键字的状态。如果属性值不匹配任何给定的关键字，但该属性有一个 非法值的默认状态，那么该属性表示那个状态。 否则如果该属性的值不匹配任何关键字但定义有一个 缺失值的默认状态， 那么该属性表示的状态就是 那个状态。否则，没有默认状态的情况， 非法值不表示任何状态。
当 没有 指定属性时，如果定义有 缺失值的默认状态， 那么这个（缺失）属性表示的状态就是这个状态。否则， 缺失的属性不表示任何状态。
空字符串可以是一个合法的关键字。
由一个或更多 ASCII 数字 组成的字符串就是 合法的整数。 可以有一个 U+002D HYPHEN-MINUS 字符 （-） 作为前缀。
没有 U+002D HYPHEN-MINUS 字符 （-） 作为前缀的 合法整数 表示 该数字字符串以 10 为基所表示的数字。 有着 U+002D HYPHEN-MINUS 字符 （-） 作为前缀的 合法的整数 表示零减去该字符串以 10 为基所表示的数字。
解析整数的规则 由下列算法给出。 被调用时，必须以给定的步骤执行，在第一个有返回值的步骤中止。该算法会返回一个整数或者一个错误。
令 input 为被解析的字符串。
令 position 为 input 里面的指针，初始指向字符串的开始。
令 sign 拥有值 "positive"。
跳过 input 中的 ASCII 空格，得到 position。
如果 position 越过了 input 的末尾，返回一个错误。
如果 position 处的（第一个）字符是一个 U+002D HYPHEN-MINUS 字符 （-）:
否则，如果 position 处的（第一个）字符是一个 U+002B PLUS SIGN 字符 （+）:
+" 会被忽略但并不符合规范。）如果 position 处的字符不是 ASCII 数字，则返回一个错误。
从 input 的给定 position 收集 是 ASCII 数字 的代码点序列，将得到的序列解释为以10为基的整数。 令 value 为该整数。
如果 sign 为 "positive"，返回 value，否则返回0减去 value 的结果。
由一个或更多 ASCII 数字 组成的字符串就是 合法的非负整数 。
合法的非负整数 表示该数字字符串以10为基所表示的数字。
解析非负整数的规则 由以下算法给出，被调用时该算法必须按顺序执行以下步骤，在第一个有返回值的步骤中止。 该算法将返回 0，正整数或者错误。
令 input 为被解析的字符串。
令 value 为使用 解析整数的规则 解析 input 的结果。
如果 value 是一个错误, 返回一个错误。
如果 value 小于零, 返回一个错误。
返回 value。
合法的浮点数 这样构成：
合法的浮点数 表示将有效数乘以10的指数。 其中有效数是第一个数字，按十进制解析（包括小数点和小数点后的数字，如果后面有的话） 如果整个字符串以 U+002D HYPHEN-MINUS character （-）字符开始且该数字不为零的话， 将有效数解析为负数。 其中指数是 E 后面的数字（如果有的话），如果在 E 和该数字之间有一个 U+002D HYPHEN-MINUS 字符（-） 且该数字不为零的话，解析为负数，如果在 E 和该数字之间有一个 U+002B PLUS SIGN 字符（+）的话忽略该字符。 如果没有 E，指数当做零来处理。
Infinity 和 Not-a-Number （NaN）值不是 合法的浮点数 。
典型地，
合法的浮点数
的概念只用于限制作者，对用户代理的要求采用下面的
解析浮点数值的规则
（例如，progress 元素的 max 属性）。
然而，在某些情况下要求用户代理检查字符串是否是
合法的浮点数（例如，
Number 状态的 input 元素的
值清理算法，或者
解析 srcset 属性 算法）。
浮点数的最佳表示 n 是 运行 ToString（n）得到的字符串。 ToString 抽象方法不是唯一确定的。如果在特定的值上运行 ToString 可能得到多个字符串，用户代理必须对那个值总是返回同样的字符串 （虽然它可能和其他用户代理使用的值不同）。
解析浮点数值的规则 由以下算法给出。 该算法必须在第一个有返回值的步骤中止。 该算法将会返回一个数字或者错误。
令input 为被解析的字符串。
令position 为 input 内的指针，初始指向字符串的开始。
令value 的值为 1。
令divisor 的值为 1。
令exponent 的值为 1。
跳过 input 中的 ASCII 空格，得到 position。
如果 position 越过了 input 的末尾，返回一个错误。
如果 position 处的（第一个）字符是一个 U+002D HYPHEN-MINUS 字符 （-）:
否则，如果 position 处的（第一个）字符是一个 U+002B PLUS SIGN 字符 （+）：
+" 会被忽略但并不符合规范。）如果 position 处的字符是 U+002E FULL STOP （.），且不是 input 的最后一个字符， 而且 position 表示的字符的后一个字符是 ASCII 数字， 则将 value 设置为零并跳到 fraction 标记的步骤。
如果 position 处的数字不是 ASCII 数字， 则返回一个错误。
从 input 的给定 position 收集 是 ASCII 数字 的代码点序列，将得到的序列解释为以10为基的整数。 将 value 乘以该整数。
Fraction：如果 position 处的数字是 U+002E FULL STOP （.），运行以下子步骤：
将 position 前进到下一个字符。
如果 position 越过了 input 的末尾或者不是 ASCII 数字、 U+0065 LATIN SMALL LETTER E （e），或 U+0045 LATIN CAPITAL LETTER E （E）， 跳到 conversion 标记的步骤。
如果 position 处的字符是 U+0065 LATIN SMALL LETTER E character （e）或 U+0045 LATIN CAPITAL LETTER E 字符（E），跳过下面的子步骤。
Fraction loop： 将 divisor 乘以 10。
将 position 前进到下一个字符。
如果 position 越过了 input 的末尾， 跳到 conversion 标记的步骤。
如果 position 处的字符是 ASCII 数字， 跳回到 fraction loop 标记的子步骤。
如果 position 处的字符是 U+0065 （e）或 U+0045 （E），则：
将 position 前进到下一个字符。
如果 position 越过了 input 的末尾，跳到标记为 conversion 的步骤。
如果 position 处的字符是 U+002D HYPHEN-MINUS 字符（-）：
如果 position 越过了 input 的末尾，跳到标记为 conversion 的步骤。
否则，如果 position 处的字符是 U+002B PLUS SIGN 字符（+）：
如果 position 越过了 input 的末尾，跳到标记为 conversion 的步骤。
如果 position 处的字符不是 ASCII 数字，则跳到标记为 conversion 的步骤。
从 input 的给定 position 收集 是 ASCII 数字 的代码点序列，将得到的序列解释为以 10 为基的整数。 将 exponent 乘以该整数。
将 value 乘以 10 的 exponent 指数。
Conversion： 令S 为 IEEE 754 双精度浮点数值的有限集， −0 除外，但新增 21024 和 −21024 两个值。
令rounded-value 为 S 最接近 value 的数字， 如果有两个同样接近的数值，则选择有效数为奇数的数字。 （在这个处理中，21024 和 −21024 这两个特殊值应视为有奇数的有效数。）
如果 rounded-value 是 21024 或 −21024，返回错误。
返回 rounded-value。
解析维度值的规则 由下面算法给出。被调用时必须按给定顺序执行这些步骤，并在第一个有返回值的步骤中止。 本算法将会返回一个大于等于 0.0 的数字，或者一个错误；如果返回了数字， 那么它可能是百分数或者长度。
令input 为被解析的字符串。
令position 为 input 中的指针，初始指向字符串的开始。
跳过 input 中的 ASCII 空格，得到 position。
如果 position 越过了 input 的末尾，返回一个错误。
如果 position 处的字符是一个 U+002B PLUS SIGN 字符 （+）， 将 position 前进到下一个字符
。如果 position 越过了 input 的末尾，返回一个错误。
如果 position 处的字符不是 ASCII 数字，则返回一个错误。
从 input 的给定 position 收集 是 ASCII 数字 的代码点序列，将得到的序列解释为以 10 为基的整数。 令 value 为该整数。
如果 position 越过了 input 的末尾，将 value 作为长度返回。
如果 position 处的字符是 U+002E FULL STOP character （.）：
将 position 前进到下一个字符。
如果 position 越过了 input 的末尾，或者 position 处的字符不是 ASCII 数字，则将 value 作为长度返回。
令divisor 值为 1。
Fraction loop：将 divisor 乘以 10。
将 position 前进到下一个字符。
如果 position 越过了 input 的末尾，则将 value 作为长度返回。
如果 position 处的字符是一个 ASCII 数字， 返回到标记为 fraction loop 的子步骤。
如果 position 越过了 input 的末尾，则将 value 作为长度返回。
如果 position 处的字符是 U+0025 PERCENT SIGN character（%）, 将 value 作为百分数返回。
将 value 作为长度返回。
解析非零维度值的规则 由下列算法给出。被调用时必须按给定顺序执行这些步骤，并在第一个有返回值的步骤中止。 本算法将会返回一个大于 0.0 的数字或者一个错误；如果返回了数字， 那么它可能是百分数或者长度。
令input 为被解析的字符串。
令value 为使用 解析维度值的规则 解析 input 的结果。
如果 value 是一个错误，返回一个错误。
如果 value 为 0，返回一个错误。
如果 value 为百分数，将 value 作为百分数返回。
将 value 作为长度返回。
合法的浮点数列表 是以 U+002C COMMA 字符分隔的 合法浮点数 , 且没有其他字符（比如 ASCII 空格）。 此外，可能会有浮点数个数限制，或者取值范围限制。
解析浮点数列表的规则 如下：
令input 为被解析的字符串。
令position 为 input 中的指针，初始指向字符串的开始。
令numbers 为空的浮点数列表。该列表将会是本算法的结果。
从 input 的给定 position 收集 是 ASCII 空格, U+002C COMMA，或者 U+003B SEMICOLON 字符的代码点序列。 本步骤跳过了所有前导分隔符。
当 position 没有越过 input 的末尾时：
从 input 的给定 position 收集 不是 ASCII 空格, U+002C COMMA，U+003B SEMICOLON， ASCII 数字， U+002E FULL STOP, 或 U+002D HYPHEN-MINUS 字符的代码点序列。 本步骤跳过了前导的无用字符。
从 input 的给定 position 收集 不是 ASCII 空格, U+002C COMMA，或者 U+003B SEMICOLON 字符的代码点序列，令 unparsed number 为收集的结果。
令number 为使用 解析浮点数值的规则 解析 unparsed number 的结果。
如果 number 是一个错误，将 number 设置为 0。
将 number 添加到 numbers。
从 input 的给定 position 收集 是 ASCII 空格, U+002C COMMA，或者 U+003B SEMICOLON 字符的代码点序列。 本步骤跳过了分隔符。
返回 numbers。
解析维度值列表的规则 如下。 这些规则返回一个由数字或单位组成的偶对列表，包含0个或更多偶对。 其中的单位是 percentage, relative，和 absolute 之一。
令raw input 为被解析的字符串。
如果 raw input 的最后一个字符是 U+002C COMMA character（,）, 则从 raw input 中移除该字符。
用逗号分割 raw input 字符串。 令raw tokens 为得到的标记列表。
令result 为 number/unit 偶对的空列表。
对 raw tokens 中的每一个标记，运行以下子步骤：
令input 为该标记。
令position 为 input 中的指针，初始指向字符串的开始。
令value 为数字 0。
令unit 为 absolute。
如果 position 越过了 input 的末尾，将 unit 设为 relative 并跳到最后一个子步骤。
如果 position 处的字符是 ASCII 数字， 从 input 的给定 position 收集 是 ASCII 数字 的代码点序列，将得到的序列解释为以10为基的整数， 将 value 递增这个整数。
如果在 position 处的字符是 U+002E（.），则：
从给定的 position 跳过 input 中的 ASCII 空白字符。
如果 position 处的字符是 U+0025 PERCENT SIGN 字符（%）, 将 unit 设为 percentage。
否则，如果 position 处的字符是 U+002A ASTERISK 字符（*）， 则将 unit 设为 relative。
在 result 中增加一项，包含由 value 给定的数字， 以及由 unit 给定的单位。
返回 result 列表。
当本部分定义的日期与时间语法中使用了 ASCII 数字 时, 这些数字表示10进制数。
虽然这里描述的格式旨在成为相应的 ISO8601 格式的子集，但是本规范比 ISO8601 更详细地定义了解析规则。因此，在使用实现者之前，请仔细检查任何日期解析库，然后再使用它们来实现下面描述的解析规则; ISO8601 库可能不会以完全相同的方式解析日期和时间。[ISO8601]
本标准中的 外推格里历 是指现代格里历向前外推到第 1 年。 外推格里历 中的日期 （有时直接称 外推格里历日期）， 的指用外推格里历描述的日期，即使当时还没有颁布格里历。 [GREGORIAN]
格里历在本标准中作为连线协议使用并非出于决策者的文化偏好。参考：
对表单中的 date，time，和 number 格式 的讨论 （对作者而言），
关于表单控件本地化的实现说明，，
以及 time 元素。
月份 由特定的 外推格里历日期 组成， 不包含时区信息，也不包含年和月以上的日的信息。[GREGORIAN]
表示 year 年 month 月的 合法的月字符串 包含下列部分（有序）：
解析月字符串 的规则如下。将返回年和月，或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
令input 为待解析字符串。
令position 为 input 内的指针初始指向字符串开始。
解析月的部分 来获得 year 和 month。 如果它没有返回，则失败。
如果 position 没有 超出 input 的结尾，则失败。
返回 year 和 month。
给定 input 字符串和位置 position， 解析月的部分 的规则如下。 将返回年和月，或者什么都不返回。 如果算法的任何一个步说它 "失败了"，这意味着它在那一步退出并什么都不返回。
从 input 中给定的位置 position 收集 ASCII 数字 组成的代码点序列。 如果收集到的序列不够 4 个字符则失败。 否则将结果序列解释为 10 进制整数。令那个数为 year。
如果 year 不是大于0的数字则失败。。
如果 position 超出了 input 的末尾或者 position 处的字符不是 U+002D HYPHEN-MINUS 字符，则失败。 否则将 position 前进一个字符。
从 input 中给定的位置 position 收集 ASCII 数字 组成的代码点序列。 如果收集到的序列不是恰好 2 个字符则失败。 否则将结果序列解释为 10 进制整数。令那个数为 month。
如果 month 不是 1 ≤ month ≤ 12 范围内的数字就失败。
返回 year 和 month。
日期 由特定的 外推格里历日期 组成， 不包含时区信息，由年、月、日组成。[GREGORIAN]
表示 year 年，month 月，day 日的 合法的日期字符串 由下列几个有序的组成部分：
解析日期字符串 的规则如下。将会返回一个日期或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
令input 为待解析字符串。
令position 为 input 内的指针初始指向字符串开始。
解析日期部分 来获得 year，month 和 day。 如果它没有返回，则失败。
如果 position 没有 超出 input 的结尾，则失败。
令date 的年月日分别为 year，month，day。
返回 date。
给定 input 字符串和位置 position， 解析日期部分 的规则如下。 将返回年、月、日，或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
解析月的部分 来获得 year 和 month 如果它没有返回，则失败。
令maxday 为 year 年 month 月的天数。
如果 position 超出了 input 的末尾或者 position 处的字符不是 U+002D HYPHEN-MINUS 字符，则失败。 否则将 position 前进一个字符。
从 input 中给定的位置 position 收集 ASCII 数字 组成的代码点序列。 如果收集到的序列不是恰好 2 个字符则失败。 否则将结果序列解释为 10 进制整数。令那个数为 day。
如果 day 不是 1 ≤ day ≤ maxday 范围内的数字就失败。
返回 year，month 和 day。
一个 无年的日期 由一个格里历月和该月内的一天组成，但没有关联的年份。[GREGORIAN]
表示 month 月 day 日的 合法的无年日期 包含如下有序的部分：
换句话说，如果 month 为 "02"（二月），
那么可以是 29 日，把这一年当做闰年。
解析无年的日期字符串 的规则如下。 将会返回月和日，或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
令input 为待解析字符串。
令position 为 input 内的指针初始指向字符串开始。
解析无年的日期部分 来获得 month 和 day。 如果它没有返回，则失败。
如果 position 没有 超出 input 的结尾，则失败。
返回 month 和 day。
给定 input 字符串和位置 position， 解析无年的日期部分 的规则如下。 将返回月和日，或者什么都不返回。 如果算法的任何一个步说它 "失败了"，这意味着它在那一步退出并什么都不返回。
从 input 中给定的位置 position 收集 U+002D HYPHEN-MINUS 字符 （-）组成的代码点序列。 如果收集到的序列长度不是 0 或 2，就失败。
从 input 中给定的位置 position 收集 ASCII 数字 组成的代码点序列。 如果收集到的序列长度不是 2 就失败。 否则将结果序列解释为 10 进制整数。令那个数为 month。
如果 month 不是 1 ≤ month ≤ 12 范围内的数字就失败。
令 maxday 为 任意闰年（例如 4 或 2000）的 month 月的天数。
如果 position 超出了 input 的末尾或者 position 处的字符不是 U+002D HYPHEN-MINUS 字符，则失败。 否则将 position 前进一个字符。
从 input 中给定的位置 position 收集 ASCII 数字 组成的代码点序列。 如果收集到的序列不是恰好 2 个字符则失败。 否则将结果序列解释为 10 进制整数。令那个数为 day。
如果 day 不是 1 ≤ day ≤ maxday 范围内的数字就失败。
返回 month 和 day。
时间 由一个没有时区信息的特定时间组成。 包含小时、分钟、秒，以及秒的小数。
hour 时 minute 分 second 秒的 合法的时间字符串 由以下有序的几部分组成：
second 部分不可以是 60 或 61；闰秒无法表示。
解析时间字符串 的规则如下。将会返回一个时间或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
令input 为待解析字符串。
令position 为 input 内的指针初始指向字符串开始。
解析时间部分 来获得 hour，minute 和 second。 如果它没有返回，则失败。
如果 position 没有 超出 input 的结尾，则失败。
令time 的时分秒分别为 hour，minute，second。
返回 time。
给定 input 字符串和位置 position， 解析时间部分 的规则如下。 将返回时、分、秒，或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
从 input 中给定的位置 position 收集 ASCII 数字 组成的代码点序列。 如果收集到的序列不是恰好 2 个字符则失败。 否则将结果序列解释为 10 进制整数。令那个数为 hour。
如果 hour 不是 0 ≤ hour ≤ 23 范围内的数字就失败。
如果 position 超出了 input 的末尾或者 position 处的字符不是 U+003A COLON 字符，则失败。 否则将 position 前进一个字符。
从 input 中给定的位置 position 收集 ASCII 数字 组成的代码点序列。 如果收集到的序列不是恰好 2 个字符则失败。 否则将结果序列解释为 10 进制整数。令那个数为 minute。
如果 minute 不是 0 ≤ minute ≤ 59 范围内的数字就失败。
令second 为 0。
如果 position 没有超出 input 的末尾并且 position 处的字符是 U+003A（:），则：
将 position 前进一个字符。
如果 position 恰好位于或者超出了 input 的末尾， 或者 input 中从 position 开始的 两个 字符不都是 ASCII 数字，则失败。
从 input 中给定的位置 position 收集 ASCII 数字 或 U+002E FULL STOP 字符 组成的代码点序列。 如果收集到的序列长度为 3， 或者长度大于 3 且第 3 个字符不是 U+002E FULL STOP 字符， 或者有多于一个的 U+002E FULL STOP 字符，则失败。 否则将结果序列解释为 10 进制数（可能有小数部分）。 将 second 设置为那个数字。
如果 second 不是 0 ≤ second < 60 范围内的数字就失败。
返回 hour，minute 和 second。
本地日期和时间 由一个特定的 外推格里历日期（包含年、月、日） 和时间（包含时、分、秒，以及秒的小数），但没有时区。[GREGORIAN]
表示日期和时间的 合法的本地日期和时间字符串 由以下有序的部分组成：
表示日期和时间的 合法的正则化的本地日期和时间字符串 由以下有序的几部分组成：
解析本地日期和时间字符串 的规则如下。 将会返回一个日期或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
令input 为待解析字符串。
令position 为 input 内的指针初始指向字符串开始。
解析日期部分 来获得 year，month 和 day。 如果它没有返回，则失败。
如果 position 超出了 input 的末尾或者 position 处的字符是 U+0054 LATIN CAPITAL LETTER T 字符（T） 或 U+0020 SPACE 字符，则失败。否则将 position 前进一个字符。
解析时间部分 来获得 hour，minute 和 second。 如果它没有返回，则失败。
如果 position 没有 超出 input 的结尾，则失败。
令date 的年月日分别为 year，month，day。
令time 的时分秒分别为 hour，minute，second。
返回 date 和 time。
时区 由一个小时和分钟的有符号数字组成。
表示时区的 合法的时区字符串 包含下列之一：
一个 U+005A LATIN CAPITAL LETTER Z 字符 （Z），只有时区为 UTC 时才允许
或者下列有序的几部分：
此格式允许从-23:59到+23:59的时区偏移。 现在，实际时区的偏移范围是-12:00到+14:00，分钟分量始终是 00，30 或 45。 但不能保证情况永远是这样的，因为时区被用作政治足球，受到异想天开的政策的支配。
另请参阅下面的 全球日期和时间 部分的使用说明和示例，具体了解如何在正式的时区形成之前的历史时间上使用时区偏移。
解析时区偏移字符串 的规则如下。 将会返回一个时区偏移或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
令input 为待解析字符串。
令position 为 input 内的指针初始指向字符串开始。
解析时区偏移部分 来获得 timezonehours 和 timezoneminutes。 如果它没有返回，则失败。
如果 position 没有 超出 input 的结尾，则失败。
返回距离 UTC timezonehours 小时 timezoneminutes 分钟的时区偏移。
给定 input 字符串和位置 position， 解析时区偏移部分 的规则如下。 将会返回一个时区偏移的小时和分钟，或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
如果 position 处的字符是一个 U+005A LATIN CAPITAL LETTER Z 字符（Z），则：
令timezonehours 为 0。
令timezoneminutes 为 0。
将 position 前进到 input 的下一个字符。
否则，如果 position 是 U+002B PLUS SIGN （+）或 U+002D HYPHEN-MINUS （-），则：
如果 position 处的字符是一个 U+002B PLUS SIGN （+），令 sign 为 "正"。 如果它是一个 U+002D HYPHEN-MINUS （-），则令 sign 为 "负"。
将 position 前进到 input 的下一个字符。
如果 s 恰好是两个字符长度，则：
将 s 解释为10进制整数，并赋值给 timezonehours。
如果 position 超出了 input 的末尾或者 position 处的字符不是 U+003A COLON 则失败。 否则将 position 前进一个字符。
从 input 中给定的位置 position 收集 ASCII 数字 组成的代码点序列。 如果收集到的序列不是恰好 2 个字符则失败。 否则将结果序列解释为 10 进制整数。令那个数为 timezoneminutes。
如果 s 恰好是 4 个字符长度，则：
将 s 的前两个字解释为 10 进制整数，并赋值给 timezonehours。
将 s 的后两个字解释为 10 进制整数，并赋值给 timezoneminutes。
其他情况失败。
其他情况失败。
返回 timezonehours 和 timezoneminutes。
全球日期和时间 由一个特定的 外推格里历日期（包含年、月、日） 和时间（包含时、分、秒，以及秒的小数），以及附加的时区信息，一个包括小时和分钟的有符号数。[GREGORIAN]
表示日期、时间和时区的的 合法的全球日期和时间字符串 由以下有序的部分组成：
在 20 世纪中叶 UTC 形成之前的时间必须按照 UT1（0° 经度处的太阳时）而不是 UTC（ UT1 的近似，按照 SI 制秒增长）表示和解析。时区形成前的时间必须按照 UT1 时间表示和解析， 且带有明确的时区信息，以此来近似适当的本地时间和伦敦格林尼治观测到的时间差。
下面是按照 合法的全球日期和时间字符串 编写的日期示例。
0037-12-13 00:00Z"1979-10-14T12:00:00.001-04:00"8592-01-01T02:09+02:09"关于这些日期，值得一提的是：
T" 只能被一个空格代替。
字符串 "2001-12-21 12:00Z"（两部分之间有两个空格）
不能成功解析。解析全球日期和时间字符串 的规则如下。 将会返回一个带有时区信息（为了能够往返转换或显示等用途）的 UTC 时间，或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
令input 为待解析字符串。
令position 为 input 内的指针初始指向字符串开始。
解析日期部分 来获得 year，month 和 day。 如果它没有返回，则失败。
如果 position 超出了 input 的末尾或者 position 处的字符是 U+0054 LATIN CAPITAL LETTER T 字符（T） 或 U+0020 SPACE 字符，则失败。否则将 position 前进一个字符。
解析时间部分 来获得 hour，minute 和 second。 如果它没有返回，则失败。
如果 position 超出了 input 的结尾，则失败。
解析时区偏移部分 来获得 timezonehours 和 timezoneminutes。 如果它没有返回，则失败。
如果 position 没有 越过 input 的结尾，则失败。
令时刻 time 的年月日时分秒分别为 year，month，day， hour，minute，second，减去 timezonehours 小时 timezoneminutes 分钟。 该时刻的时区为 UTC 时区。
令timezone 为距 UTC timezonehours 小时 timezoneminutes 分钟的时区偏移。
返回 time 和 timezone。
星期 由一个表示年的数字（星期年）和一个表示星期的数字（周号）组成，表示从周一开始的 7 天时间段。 在这个日历系统中，每个星期年有 52 或 53 个这样的 7 天，定义如下。这样的 7 天周期从格里历 1969 年 12 月 29 日（1969-12-29）开始， 这一周被定义为 1970 年的第 1 周。后续的星期都依次进行编号。一个星期年的第一周之前是上一个星期年的最后一周，反之亦反。 [GREGORIAN]
如果编号为 year 的星期年对应的 外推格里历 年 year 第一天（1月1日）是周二，或者这一天是周三且 year 可被 400 整除或可被 4 整除但不可被 100 整除， 那么这一星期年有 53 周。其他所有星期年都有 52 周。
有 53 周的星期年的 最后一天的周号 是 53； 有 52 周的星期年的最后一天的周号是 52。
特定日期的星期年的编号可能与包含这一天的 外推格里历 年不同。 星期年 y 的第一周为包含第一个周二的格里历年 y。
为了现代目的，这里定义的 星期 等价于定义在 ISO 8601 中的 ISO 星期。 [ISO8601]
表示星期年 year 和周号 week 的 合法的星期字符串 有下列几个有序的组成部分：
解析星期字符串 的规则如下。将会返回星期年和周号两个数字，或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
令input 为待解析字符串。
令position 为 input 内的指针初始指向字符串开始。
从 input 中给定的位置 position 收集 ASCII 数字 组成的代码点序列。 如果收集到的序列长度小于 4 个字符长度则失败。 否则将结果序列解释为 10 进制整数。令那个数为 year。
如果 year 小于等于零则失败。
如果 position 超出了 input 的末尾或者 position 处的字符不是 U+002D HYPHEN-MINUS 字符则失败。 否则将 position 前进一个字符。
如果 position 超出了 input 的末尾或者 position 处的字符不是 U+0057 LATIN CAPITAL LETTER W 字符（W）则失败。 否则将 position 前进一个字符。
从 input 中给定的位置 position 收集 ASCII 数字 组成的代码点序列。 如果收集到的序列不是恰好 2 个字符长度则失败。 否则将结果序列解释为 10 进制整数。令那个数为 week。
令maxweek 为 year 年 最后一天的周号。
如果 week 不是在 1 ≤ week ≤ maxweek 范围内的数字就失败。
如果 position 没有 超出 input 末尾，则失败。
返回星期年 year 和周号 week。
时间间隔 由 秒数构成。
由于月和秒是不可比较的（一月没有精确的秒数， 它的长度取决于究竟从哪天算起），本标准中定义的 时间间隔 不允许包含 月（或者年，它等于12个月）。 时间间隔只能描述秒数。
表示 时间间隔 t 的 合法的时间间隔字符串 的组成方式可以是下列任何一种：
字面量 U+0050 LATIN CAPITAL LETTER P 字符加一个或更多下列有序的子组件， 其中 天、小时、分钟和秒的数量对应 t 中同样数量的秒：
一个或更多 ASCII 数字 加 U+0044 LATIN CAPITAL LETTER D 字符，表示天数。
一个 U+0054 LATIN CAPITAL LETTER T 字符加一个或更多下列子组件，按照给定的顺序：
和本标准中定义的其他时间日期相关的微语法一样，时间间隔也基于 ISO 8601 中定义的一种格式。 [ISO8601]
一个或更多任意顺序的 时间间隔组件， 每个都有不同的 时间间隔； 它们表示的描述之和等于 t 中的秒数。
时间间隔组件 是一个包含下列组件的字符串：
零个或更多 ASCII 空格。
如果 倍数 为 1 （即单位是秒）， 则可以有一个 U+002E FULL STOP 字符（.）接着一个、两个或三个 ASCII 数字， 表示秒的小数组件。
零个或更多 ASCII 空白。
下面的字符之一，表示 时间间隔组件 的数字部分使用的时间单位的 倍数：
零个或更多 ASCII 空格。
这并不基于 ISO 8601 中的任何格式。这是为了提出一种比 ISO 8601 时间间隔格式更加人类可读的选择。
解析时间间隔字符串 的规则如下。将会返回一个 时间间隔 或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
令input 为待解析字符串。
令position 为 input 内的指针初始指向字符串开始。
令months，seconds，和 component count 均为零。
令M-disambiguator 为 minutes。
该标志的另一个值是 months。它用来消除 ISO8601 时间间隔中 "M" 单位的歧义， 在 ISO8601 中月和分钟都使用这个单位。虽然月是不允许的，但为了向前兼容， 以及避免误解 ISO8601 时间间隔（在其他上下文中仍然是合法的）， 仍然对月进行了解析。
从 position 开始 跳过 input 中的 ASCII 空格。
如果 position 越过了 input 的结尾，则失败。
如果 position 处的字符是一个 U+0050 LATIN CAPITAL LETTER P 字符， 则将 position 前进一个字符。将 M-disambiguator 设置为 months， 并从 position 开始 跳过 input 中的 ASCII 空格。
while true:
令units 为 undefined。它的值可能是： years，months，weeks，days，hours，minutes，或 seconds。
令next character 为 undefined。用来从 input 中处理字符。
如果 position 越过了 input 的结尾，则 break。
如果 position 处的字符是一个 U+0050 LATIN CAPITAL LETTER T 字符， 则将 position 前进一个字符。将 M-disambiguator 设置为 minutes， 并从 position 开始 跳过 input 中的 ASCII 空格。
将 next character 设置为 input 中 position 处的字符。
如果 next character 为一个 U+002E FULL STOP 字符 （.），则令 N 等于 0。 （不要前进 position。下面会处理它。）
否则，如果 next character 是一个 ASCII 数字，则从 input 中给定的位置 position 收集 ASCII 数字 组成的代码点序列。将结果序列解释为 10 进制整数。令 N 为那个数字。
否则如果 next character 不是数字，则失败。
如果 position 越过了 input 的结尾，则 break。
将 next character 设置为 input 中 position 处的字符， 这次将 position 前进到下一字符。 （如果 next character 刚才是一个 U+002E FULL STOP 字符（.），它将仍然是那个字符。）
如果 next character 是 U+002E （.），则：
如果 s 是空字符串则失败。
令length 为 s 中的字符数。
令fraction 为将 s 解释为10进制数的结果， 把这个数除以 10length。
将 N 增加 fraction。
从 position 开始 跳过 input 中的 ASCII 空格。
如果 position 越过了 input 的结尾，则失败。
将 next character 设为 input 中 position 指向的字符， 并将 position 前进到下一字符。
如果 next character 既不是 U+0053 LATIN CAPITAL LETTER S 字符也不是 U+0073 LATIN SMALL LETTER S 字符，则失败。
将 units 设为 seconds。
否则：
如果 next character 是 ASCII 空格， 那么从 position 开始 跳过 input 中的 ASCII 空格。 将 next character 设为 input 中 position 指向的字符， 并将 position 前进到下一字符。
如果 next character 是一个 U+0059 LATIN CAPITAL LETTER Y 字符或 U+0079 LATIN SMALL LETTER Y 字符，将 units 设置为 years 并将 M-disambiguator 设置为 months。
如果 next character 是一个 U+004D LATIN CAPITAL LETTER M 字符或 U+006D LATIN SMALL LETTER M 字符，且 M-disambiguator 是 months，则将 units 设置为 months。
如果 next character 是一个 U+0057 LATIN CAPITAL LETTER W 或 U+0077 LATIN SMALL LETTER W 字符，将 units 设置为 weeks 并将 M-disambiguator 设置为 minutes。
如果 next character 是一个 U+0044 LATIN CAPITAL LETTER D 或 U+0064 LATIN SMALL LETTER D 字符，将 units 设置为 days 并将 M-disambiguator 设置为 minutes。
如果 next character 是一个 U+0048 LATIN CAPITAL LETTER H 或 U+0068 LATIN SMALL LETTER H 字符，将 units 设置为 hours 并将 M-disambiguator 设置为 minutes。
如果 next character 是一个 U+004D LATIN CAPITAL LETTER M 字符或 U+006D LATIN SMALL LETTER M 字符，且 M-disambiguator 是 minutes，则将 units 设置为 minutes。
如果 next character 是一个 U+0053 LATIN CAPITAL LETTER S 或 U+0073 LATIN SMALL LETTER S 字符，将 units 设置为 seconds 并将 M-disambiguator 设置为 minutes。
否则如果 next character 不是上述任何字符，则失败。
递增 component count。
令multiplier 为 1。
如果 units 为 years，将 multiplier 乘以 12 并设置 units 为 months。
如果 units 为 months，把 N 与 multiplier 的乘积加到 months 上。
否则：
如果 units 为 weeks，将 multiplier 乘以 7 并设置 units 为 days。
如果 units 为 days，将 multiplier 乘以 24 并设置 units 为 hours。
如果 units 为 hours，将 multiplier 乘以 60 并设置 units 为 minutes。
如果 units 为 minutes，将 multiplier 乘以 60 并设置 units 为 seconds。
否则 units 只能是 seconds 了。 将 N 与 multiplier 的乘积加到 seconds 上。
从 position 开始 跳过 input 中的 ASCII 空格。
如果 component count 为零，则失败。
如果 months 不为零，则失败。
返回包含 seconds 秒的 时间间隔。
合法的可选时间的日期字符串 是下列中的一种：
解析日期或时间字符串 的规则如下。 该算法将会返回一个 date，time，或 全球日期和时间 或者什么都不返回。 如果算法的任何一个点说它 "失败了"，这意味着它在那一点退出并什么都不返回。
令input 为待解析字符串。
令position 为 input 内的指针初始指向字符串开始。
将 start position 设置为 position。
将 date present 和 time present 标志设置为 true。
解析日期部分 来获得 year，month 和 day。 如果它没有返回，设置 date present 为 false。
如果 date present 为 true，且 position 没有越过 input 的末尾， 且 position 处的字符是 U+0054 LATIN CAPITAL LETTER T 字符 （T）或 U+0020 SPACE 字符， 将 position 前进到 input 的下一个字符。
否则，如果 date present 为 true，并且 position 越过了 input 的末尾或 position 处的字符既不是 U+0054 LATIN CAPITAL LETTER T 字符 （T）也不是 U+0020 SPACE 字符， 则设置 time present 为 false。
否则，如果 date present 为 false，将 position 设置回 start position。
如果 time present 为 true，则 解析时间部分 来获得 hour，minute 和 second。 如果它没有返回，则失败。
如果 date present 和 time present 标志都是 true， 但 position 越过了 input 的末尾，则失败。
如果 date present 和 time present 标志都是 true， 解析时区偏移部分 来获得 timezonehours 和 timezoneminutes。如果它没有返回则失败。
如果 position 没有 越过 input 的结尾，则失败。
如果 date present 标志为 true 且 time present 标志为 false， 则令 date 为年与日分别为 year，month，day 的日期并返回 date。
否则，如果 time present 标志为 true 且 date present 标志为 false， 则令 time 为时分秒分别为 hour，minute，second 的时间并返回 time。
否则，令 time 为 year 年 month 月 day 日 hour 时mminute 分 second 秒， 减去 timezonehours 小时 timezoneminutes分钟，这一时刻是 UTC 时区的时间； 令 timezone 距离 UTC timezonehours 小时 timezoneminutes 分钟； 返回 time 和 timezone。
简单颜色 由三个在 0..255 范围的 8 位数字组成。 分别表示 sRGB 色彩空间的颜色的红、绿、蓝分量。 [SRGB]
合法的简单颜色 字符串应恰好包含 7 个字符长度， 第一个字符是 U+0023 NUMBER SIGN 字符（#），剩下的 6 个都是 ASCII 十六进制数， 其中前两个表示红色分量，中间两个表示绿色分量，最后两个表示蓝色分量，均为 16 进制。
合法的小写简单颜色 字符串是 不使用任何 U+0041 LATIN CAPITAL LETTER A 到 U+0046 LATIN CAPITAL LETTER F 字符的 合法的简单颜色。
解析简单色值的规则 由下列算法给出。 被调用时必须依次执行这些步骤，在第一个有返回值的步骤中止。该算法将会返回一个 简单颜色 或者错误。
令input 为待解析字符串。
如果 input 不是恰好 7 个字符则返回一个错误。
如果 input 的首字符不是 U+0023 NUMBER SIGN 字符（#），则返回一个错误。
如果 input 的后面 6 个字符不都是 ASCII 十六进制数字，则返回一个错误。
令result 为一个 简单颜色。
将第2、3个字符解释为 16 进制数，令该结果为 result 的红色分量。
将第4、5个字符解释为 16 进制数，令该结果为 result 的绿色分量。
将第6、7个字符解释为 16 进制数，令该结果为 result 的蓝色分量。
返回 result。
给定 简单颜色 序列化简单色值的规则 在下列算法中给出：
令result 为包含一个 U+0023 NUMBER SIGN 字符（#）的字符串。
将红、绿、蓝分量依次转换为两位 16 进制数字，并使用 ASCII 小写十六进制数字， 需要的话填充零。将这些数字按照红绿蓝的顺序追加到 result。
返回 result，它将是一个 合法的小写简单颜色。
有些废弃的遗留属性以更复杂的方式解析颜色，它们使用 解析遗留色值的规则，由以下算法给出。 被调用时必须依次执行这些步骤，在第一个有返回值的步骤中止。该算法将会返回一个 简单颜色 或者错误。
令input 为待解析字符串。
如果 input 位空字符串则返回一个错误。
从 input 中 移除前后的 ASCII 空白。
如果 input ASCII 大小写不敏感地 匹配
字符串 "transparent"，则返回一个错误。
如果 input ASCII 大小写不敏感地 匹配 命名颜色，则返回该关键字对应的 简单颜色。 [CSSCOLOR]
没有识别 CSS2 System Colors。
如果 input 是 4 个字符长度，且 input 的第一个字符是 U+0023（#）， 且 input 后面的 3 个字符都是 ASCII 十六进制数字，则：
令result 为一个 简单颜色。
将 input 第二个字符解释为十六进制数字；令 result 的红色分量为得到的数字乘以 17。
将 input 第三个字符解释为十六进制数字；令 result 的绿色分量为得到的数字乘以 17。
将 input 第四个字符解释为十六进制数字；令 result 的蓝色分量为得到的数字乘以 17。
返回 result。
将 input 中任何代码点大于 U+FFFF 的字符（即任何不在基本多文种平面的字符）替换为两个字符的字符串 "00"。
如果 input 字符长度大于 128，把 input 截断到只有 128 个字符。
如果 input 中第一个字符是 U+0023 NUMBER SIGN 字符（#），移除它。
将 input 中任何非 ASCII 十六进制数字 的字符替换为 U+0030 DIGIT ZERO（0）。
While input 长度为零或者不是 3 的倍数，添加一个 U+0030 DIGIT ZERO （0）字符到 input。
将 input 分割成三个等长字符来获得三个分量。 令length 为这三个分量的长度（input 长度的三分之一）。
如果 length 大于 8，移除每个分量中前面的 length-8 个字符，并且令 length 为 8。
While length 大于 2 并且每个分量的第一个字符是 U+0030 DIGIT ZERO （0）字符，移除该字符并将 length 减一。
如果 length 仍然 大于 2，截断每个分量，分别只保留前两个字符。
令result 为一个 简单颜色。
将第一个分量解释为十六进制数字；令 result 的红色分量为得到的数字。
将第二个分量解释为十六进制数字；令 result 的绿色分量为得到的数字。
将第三个分量解释为十六进制数字；令 result 的蓝色分量为得到的数字。
返回 result。
2D 图形上下文 有单独的颜色语法，还处理了不透明度。
空格分隔的令牌集合 是包含零个或更多词（称为令牌）的字符串，它们以一个或更多 ASCII 空白 分隔， 其中的词是由一个或更多的除 ASCII 空白 之外的字符组成的任何字符串。
包含 空格分隔的令牌集合 的字符串前后都可能有 ASCII 空白。
空格分隔的无序不重复令牌集合 是一个没有令牌重复的 空格分隔的令牌集合。
空格分隔的有序不重复令牌集合 是一个没有令牌重复但令牌顺序有意义的 空格分隔的令牌集合。
空格分隔的令牌集合 有时定义有允许值的集合。 当定义了允许值的集合时，集合中的令牌必须都从那个允许的列表中来；其他值是不合规范的。 如果没有提供允许值的集合，那么所有值都是符合规范的。
在 set of space-separated tokens 中的令牌如何进行比较 （例如是否大小写敏感) 由各自集合定义。
逗号分隔的字符集 is a string containing zero or more tokens each separated from the next by a single U+002C COMMA character (,), where tokens consist of any string of zero or more characters, neither beginning nor ending with ASCII whitespace, nor containing any U+002C COMMA characters (,), and optionally surrounded by ASCII whitespace.
是包含零个或多个字符的字符串，它们以一个 U+002C COMMA 字符（,）分隔，其中的字符是由一个或更多的字符组成的任意字符串，首尾不包括 ASCII 空白也不能包含 U+002C COMMA 字符（,），但前后可以有 ASCII 空白。
例如字符串" a ,b,,d d "由四个字符组成："a", "b", 空字符串和 "d d"。每个字符的前后空白不计入字符，空字符串可以是一个字符。
有时逗号分隔的字符集对合法字符的构成还有额外限制。如果定义了那些限制，字符就必须完全遵守；其他值都是不符合规范的。如果没定义其他限制，所有值都是符合规范的。
type 类型的元素的 合法的哈希名引用
字符串由一个 U+0023 NUMBER SIGN 字符（#）和完全匹配
同一个 树 中的 type 元素的 name 属性的字符串构成。
给定上下文节点 scope，解析对 type 类型的元素的哈希名引用的规则 如下：
如果被解析字符串不包含 U+0023 NUMBER SIGN 字符，或者第一个这样的字符是字符串的最后一个字符， 则返回 null 并中止这些步骤。
令s 为从紧接着第一个 U+0023 NUMBER SIGN 字符之后的字符开始，直到待解析字符串结尾的字符串。
返回 scope 的 树 中，按照 树序 的第一个
id 或 name 属性的值是 s
的类型为 type 的元素。如果没有这样的元素就返回 null。
虽然解析时用到了 id 属性，
但它不用于决定一个值是否是 合法的 哈希名引用。
也就是说，一个基于 id 引用元素的哈希名引用是符合性错误。
（除非那个元素也有一个具有相同值的 name 属性）。
合法的媒体查询列表 是一个匹配
Media Queries 标准中的 <media-query-list> 生成式的字符串。[MQ]
匹配用户环境 的字符串可以是空字符串、只包含 ASCII 空白 的字符串， 或者根据 Media Queries 标准给出的定义匹配用户环境的媒体查询列表。[MQ]
一个字符串如果是合法 URL 字符串，同时是非空字符串， 那么它就是 合法的非空 URL。
如果 去掉前后 ASCII 空白 后是一个 合法 URL 字符串， 那么它就是一个 可能由空格包裹的合法 URL。
如果 去掉前后 ASCII 空白 后是一个 合法的非空 URL， 那么它就是一个 可能由空格包裹的合法的非空 URL。
本标准将 about:legacy-compat 定义为保留 URL。
因为 about: URL 不可解析，当需要兼容 XML 工具时，
用在 HTML 文档 的 DOCTYPE 中。[ABOUT]
本标准将 about:html-kind 定义为保留 URL。
因为 about: URL 不可解析，可用于媒体轨道的类型标识。[ABOUT]
本标准将 about:srcdoc 定义为保留 URL。
因为 about: URL 不可解析，
被用于 iframe srcdoc 文档
的 URL [ABOUT]
Document 对象 document 的 后备基 URL
是通过执行这些步骤获得的 URL 记录：
如果 document 是
iframe srcdoc 文档，
则返回
document 的 浏览环境 的
容器文档 的
文档基 URL。
如果 document 的 URL 为 about:blank，
且 document 的 浏览环境 的
创建者的 base URL 是非空，那么返回该
创建者的 base URL。
返回 document 的 URL。
Document 对象的 文档基 URL 是通过执行这些步骤获得的
绝对 URL：
如果 Document 中不存在拥有 href
属性的 base 元素，那么 文档基 URL 为
Document 的 后备基 URL；中止这些步骤。
否则，文档基 URL 为
Document 中按照 树序 的
第一个有 href 属性的
base 元素的 冻结的基 URL。
解析 URL 是从字符串中获取它表示的 URL 记录 的过程。 虽然这一过程定义在 WHATWG URL 标准中，但方便起见 HTML 标准也定义了一个包装过的版本。 [URL]
这一包装只用于出于历史原因当 URL 解析器的字符编码必须匹配文档或环境设置对象时。 除此之外可以直接使用 URL 解析器。
相对于一个 document 或 environment settings object 解析 URL url， 用户代理必须使用下列步骤。 解析 URL 的结果可能是失败，也可能是 结果 URL 字符串 和 结果 URL 记录。
如果指定了 document， 令 encoding 为 document 的 字符编码， 否则令它为 environment settings object 的 API URL 字符编码。
如果指定了 document， 令 baseURL 为 document 的 基 URL， 否则令它为 environment settings object 的 API 基 URL。
令 urlRecord 为 使用 baseURL 和 encoding，在 url 上应用 URL 解析器 的结果。
如果 urlRecord 解析失败，中止这些步骤并产生一个错误。
令urlString 为在 urlRecord 上应用 URL 序列化 的结果。
将 urlString 作为 结果 URL 字符串， urlRecord 作为 结果 URL 记录 返回。
当一个文档的 文档基 URL 变化时，该文档中所有元素多 受基 URL 变化的影响。
当一个元素 受基 URL 变化影响 时执行下面的 基 URL 变化步骤（定义在 DOM 标准中）：
如果该超链接标识的 URL 正在展现给用户，
或任何从该 URL 来的数据正在影响显示，那么应该相对于该元素的
节点文档
重新解析
href 属性并适当地更新 UI。
例如 CSS :link/:visited
伪类 可能不受影响。
如果该超链接有 ping 属性
且它的 URL 正在展现给用户，那么应该相对于该元素的
节点文档
重新解析
ping 属性的令牌并适当地更新 UI。
cite 属性的 q，blockquote，
ins，或 del 元素如果 cite 属性标识的 URL 正在展现给用户，
或任何从该 URL 来的数据正在影响显示，那么应该相对于该元素的
节点文档
重新解析
href 属性并适当地更新 UI。
该元素不直接受影响。
例如更改基 URL 不影响 img 元素显示的图片，
尽管脚本中后续对 src IDL 属性的访问将返回新的
绝对 URL，可能不再对应于正在显示的图片。
类型 为 "basic"，
"cors"，或 "default" 的
响应 称为 CORS-same-origin。 [FETCH]
类型 为
"opaque" 或 "opaqueredirect" 的
响应 称为 CORS-cross-origin。
响应 的 不安全响应 是它的 内部响应（如果有的话），否则就是 响应 自己。
给定 url，destination， corsAttributeState，以及可能有的 same-origin fallback 标志， 创建一个可能 CORS 的请求 应该执行以下步骤：
如果 corsAttributeState 是 No CORS，
令 mode 为 "no-cors" ，否则令 mode 为 "cors"。
如果设置了 same-origin fallback 标志 且 mode 为
"no-cors"，则将 mode 设置为 "same-origin"。
令 credentialsMode 为 "include"。
如果 corsAttributeState 为 Anonymous，
将 credentialsMode 设为 "same-origin"。
令 request 为一个新的 request， 其 url 为 url， 目标地址 为 destination， 模式 为 mode， credentials 模式 为 credentialsMode， 且设置了 use-URL-credentials 标志。
必须获取响应的 Content-Type 元数据 并按照 WHATWG MIME Sniffing 标准的要求解释。[MIMESNIFF]
必须按照 WHATWG MIME Sniffing 标准的要求找到资源的 计算后的 MIME 类型。 [MIMESNIFF]
嗅探图片 Content-Type 的规则， 区分资源是文本还是二进制的规则，以及 嗅探音频和视频的规则 也定义在 WHATWG MIME Sniffing 标准中。 这些规则返回的结果是一个 MIME 类型。 [MIMESNIFF]
WHATWG MIME Sniffing 标准中的规则必须严格遵守。 当用户代理使用与服务器所期望的内容类型检测不同的启发式方法时，可能会发生安全问题。 有关更多详细信息，请参阅 WHATWG MIME Sniffing 标准。 [MIMESNIFF]
meta 元素提取字符编码给定字符串 s，
从一个 meta 元素提起字符编码的算法 如下。
它或者返回一个字符编码，或者什么都不返回。
令 position 为 s 中的指针初始指向字符串开始。
Loop：在 s 中找到 position 后面
ASCII 大小写不敏感地 匹配 "charset"
的前 7 个字符。 如果没有找到这样的匹配，什么都不返回并中止这些步骤。
跳过所有紧接着 "charset" 后面的
ASCII 空白（也可能没有）。
如果下一个字符不是 U+003D EQUALS SIGN （=），让 position 指向下一个字符之前， 跳回到标记为 loop 的步骤。
跳过所有紧接着等号后面的 ASCII 空白（也可能没有）。
处理下一个字符：
该算法与 HTTP 规范中的算法不同 （例如，HTTP 不允许使用单引号，并且需要支持该算法不支持的反斜线转义机制）。 虽然在历史上本算法用于 HTTP 相关的上下文，但不同的实现支持的语法前段时间就不一样了。[HTTP]
CORS 设置属性 是一个 枚举属性。下表列出了关键字以及属性状态 — 每行中第一列给出了那一行的关键字映射到的状态。
| 状态 | 关键字 | 简要介绍 |
|---|---|---|
| Anonymous | anonymous
| Requests for the element will have their
mode set to "cors" and their
credentials mode set to "same-origin"。
|
| (the empty string) | ||
| 使用 Credentials | use-credentials
| 该元素的 请求 会把 模式 设置为 "cors"
把 credentials 模式 设置为
"include"。
|
属性的 非法值默认 是 Anonymous 状态，它的 缺失值默认 是 No CORS 状态。
用于 反射 时，
Anonymous 状态的
标准关键字 为 anonymous。
CORS 设置属性 控制的多数请求都是通过 创建一个可能 CORS 的请求 算法来完成的。
对于更先进的特行，比如请求的
mode 总是 "cors"，
某些 CORS 设置属性 用于其他语义，
不过还是只对 请求 的 credentials 模式 起作用。
为了执行这个转换，定义了 CORS 设置属性 credentials 模式
来通过属性状态确定 CORS 设置属性：
same-origin"include"引荐来源策略属性 是一个 枚举属性。 每个 引荐来源策略包括空字符串都是该属性的关键字， 都映射到一个同名的状态。
该属性的 非法值的默认 和 缺失值的默认 都是空字符串状态。
这些状态对各种 获取 处理模型的影响详细地定义在 本标准中，WHATWG Fetch 标准中，以及 Referrer Policy 中。 [FETCH] [REFERRERPOLICY]
对给定的 获取 使用哪个处理模型取决于多个信号； 引荐来源策略属性只是其中之一。一般来说，这些信号的处理顺序是：
首先，是否出现一个 noreferrer 超链接类型；
然后，引荐来源策略属性 的值；
最后，`Referrer-Policy` HTTP 头。
Support in all current engines.
A nonce content
attribute represents a cryptographic nonce ("number used once") which can be used by Content
Security Policy to determine whether or not a given fetch will be allowed to proceed. The
value is text. [CSP]
Elements that have a nonce content attribute ensure that the
crytographic nonce is only exposed to script (and not to side-channels like CSS attribute
selectors) by taking the value from the content attribute, moving it into an internal slot
named [[CryptographicNonce]], exposing it to script
via the HTMLOrSVGElement interface mixin, and setting the content attribute to the
empty string. Unless otherwise specified, the slot's value is the empty string.
nonceReturns the value of the element's [[CryptographicNonce]] internal slot.
Can be set, to update that slot's value.
The nonce IDL attribute must, on getting, return the
value of this element's [[CryptographicNonce]]; and on setting, set this element's
[[CryptographicNonce]] to the given value.
Note how the setter for the nonce IDL attribute does not update the corresponding
content attribute. This, as well as the below setting of the nonce content attribute to the empty string when an element
becomes browsing-context connected, is meant to prevent exfiltration of the nonce
value through mechanisms that can easily read content attributes, such as selectors. Learn more in
issue #2369, where this behavior was
introduced.
The following attribute change
steps are used for the nonce content attribute:
If element does not include HTMLOrSVGElement, then
return.
If localName is not nonce or
namespace is not null, then return.
If value is null, then set element's [[CryptographicNonce]] to the empty string.
Otherwise, set element's [[CryptographicNonce]] to value.
Whenever an element including HTMLOrSVGElement
becomes browsing-context connected, the user agent must execute the following steps
on the element:
Let CSP list be element's shadow-including root's CSP list.
If CSP list contains a header-delivered Content Security Policy, and
element has a nonce content attribute
attr whose value is not the empty string, then:
Let nonce be element's [[CryptographicNonce]].
Set an attribute value for
element using "nonce" and the empty
string.
Set element's [[CryptographicNonce]] to nonce.
If element's [[CryptographicNonce]] were not restored it would be the empty string at this point.
As each Document's CSP
list is append-only, user agents can optimize away the contains a header-delivered
Content Security Policy check by, for example, holding a flag on the Document,
set during Document
creation and initialization.
The cloning steps for elements that
include HTMLOrSVGElement must set the
[[CryptographicNonce]] slot on the copy to the value of the slot on the element being
cloned.
Support in all current engines.
A lazy loading attribute is an enumerated attribute. The following table lists the keywords and states for the attribute — the keywords in the left column map to the states in the cell in the second column on the same row as the keyword.
The attribute directs the user agent to fetch a resource immediately or to defer fetching until some conditions associated with the element are met, according to the attribute's current state.
| Keyword | State | Description |
|---|---|---|
lazy
| Lazy | Used to defer fetching a resource until some conditions are met. |
eager
| Eager | Used to fetch a resource immediately; the default state. |
The attribute's missing value default and invalid value default are both the Eager state.
The will lazy load element steps, given an element element, are as follows:
If scripting is disabled for element, then return false.
This is an anti-tracking measure, because if a user agent supported lazy loading when scripting is disabled, it would still be possible for a site to track a user's approximate scroll position throughout a session, by strategically placing images in a page's markup such that a server can track how many images are requested and when.
If element's lazy loading attribute is in the Lazy state, then return true.
Return false.
Each img and iframe element has associated lazy load resumption
steps, initially null.
For img and iframe elements that will lazy load, these steps are run from the lazy load
intersection observer's callback or when their lazy loading attribute is set
to the Eager state. This causes the element to
continue loading.
Each Document has a lazy load intersection observer, initially set to
null but can be set to an IntersectionObserver instance.
To start intersection-observing a lazy loading element element, run these steps:
Let doc be element's node document.
If doc's lazy load intersection observer is null, set it to a new
IntersectionObserver instance, initialized as follows:
The intention is to use the original value of the
IntersectionObserver constructor. However, we're forced to use the
JavaScript-exposed constructor in this specification, until Intersection Observer
exposes low-level hooks for use in specifications. See bug w3c/IntersectionObserver#427
which tracks this. [INTERSECTIONOBSERVER]
The callback is these steps, with arguments entries and observer:
For each entry in entries using a method of iteration which does not trigger developer-modifiable array accessors or iteration hooks:
Let resumptionSteps be null.
If entry.isIntersecting is true, then
set resumptionSteps to entry.target's
lazy load resumption steps.
If resumptionSteps is null, then return.
Stop intersection-observing a lazy loading element for
entry.target.
Set entry.target's lazy load resumption
steps to null.
Invoke resumptionSteps.
The intention is to use the original value of the
isIntersecting and
target getters. See w3c/IntersectionObserver#427.
[INTERSECTIONOBSERVER]
The options is an IntersectionObserverInit dictionary with the
following dictionary members: «[ "rootMargin" → lazy load root
margin ]»
This allows for fetching the image during scrolling, when it does not yet — but is about to — intersect the viewport.
The lazy load root margin suggestions imply dynamic changes to the
value, but the IntersectionObserver API does not support changing the root
margin. See issue w3c/IntersectionObserver#428.
Call doc's lazy load intersection observer's observe method with element as the
argument.
The intention is to use the original value of the observe method. See w3c/IntersectionObserver#427.
[INTERSECTIONOBSERVER]
To stop intersection-observing a lazy loading element element, run these steps:
Let doc be element's node document.
Assert: doc's lazy load intersection observer is not null.
Call doc's lazy load intersection observer unobserve method with element as
the argument.
The intention is to use the original value of the unobserve method. See w3c/IntersectionObserver#427.
[INTERSECTIONOBSERVER]
The lazy load root margin is an implementation-defined value, but with
the following suggestions to consider:
Set a minimum value that most often results in the resources being loaded before they intersect the viewport under normal usage patterns for the given device.
The typical scrolling speed: increase the value for devices with faster typical scrolling speeds.
The current scrolling speed or momentum: the UA can attempt to predict where the scrolling will likely stop, and adjust the value accordingly.
The network quality: increase the value for slow or high-latency connections.
User preferences can influence the value.
It is important for privacy that the lazy load root margin not leak additional information. For example, the typical scrolling speed on the current device could be imprecise so as to not introduce a new fingerprinting vector.
定义有些 IDL 属性是为了 反映 特定的内容属性。 这意味着在获取 IDL 属性时返回的是内容属性的当前值， 在设置 IDL 属性时将内容属性更改为给定的值。
通常，当获取 IDL 属性时如果内容属性不存在，IDL 属性必须表现为该内容属性的值为空字符串； 当设置时，如果内容属性的值不存在，则必须首先添加它。
如果反映内容属性的 IDL 属性是USVString属性，
且它的内容属性定义为包含一个 URL。那么在获取时如果内容属性不存在，那么 IDL 属性必须返回空字符串。
否则，IDL 属性必须相对于元素的节点文档
解析内容属性的值，如果解析成功则返回
结果 URL 字符串。
如果解析失败，则必须将内容属性的值转换
为 USVString 并返回。
在设置时，必须设置内容属性的值为新值。
如果反映内容属性的 IDL 属性是 DOMString属性，
且它的内容属性是 枚举属性，
且该 IDL 属性 只限于已知的一些值，
那么当获取时必须返回与属性所处状态关联的关键字值（如果有的话），或者空字符串
（如果没有关联的关键字值，或属性不在任何已定义的状态。比如属性缺失且没有 缺失默认值）。
该状态有如果有多个关键字值，则返回匹配的那个。
如果有多个匹配的关键字值，则指定一个作为 标准关键字 并选择它。
在设置时，内容属性必须设置为指定的新值。
如果反映内容属性的 IDL 属性是可为空的 DOMString 属性，
且它的内容属性是一个 枚举属性，那么当获取时，
如果对应的内容属性在 缺失默认值 状态，则 IDL 属性必须返回 null；
否则 IDL 属性必须返回属性所属状态对应的关键字值。如果有多个关键字值，返回匹配的那个。
如果有多个匹配的 If there are multiple conforming keyword values, then one will be
如果有多个匹配的关键字值，则指定一个作为 标准关键字 并选择它。
在设置时，如果新值为 null 则内容属性必须被移除；否则内容属性必须设置为指定的新值。
如果反映内容属性的 IDL 属性是 DOMString 或 USVString 属性但不归于上述类别中，
那么获取和设置必须透明地，保持大小写地进行。
如果反映内容属性的 IDL 属性是 布尔 属性，
当获取该 IDL 属性时，如果设置了该内容属性则必须返回 true，
如果不存在则返回 false。当设置时，如果该 IDL 属性被设为 false 则必须移除该内容属性。
如果该 IDL 属性被设为 true 则必须将内容属性设为空字符串。
（这与布尔内容属性的规则对应。）
如果反映内容属性的 IDL 属性是有符号整数类型 （long），
那么当获取时，必须根据解析有符号整数的规则
解析内容属性，如果解析成功且值在该 IDL 属性的类型的取值范围内，则必须返回结果值。
如果解析失败或返回了范围外的值，或者该属性不存在，则必须返回默认值，如果没有默认值则返回0。
当设置时，必须将给定的值转换为可以将该数字表示为合法整数
的可能的最短字符串，然后该字符串必须被用作新的内容属性值。
如果反映内容属性的 IDL 属性是有符号整数类型 （long）
且 仅限为非负数字，那么当获取时，
该内容属性必须根据解析非负整数的规则
解析，如果解析成功且值在该 IDL 属性类型的取值范围内，必须返回结果值。
如果解析失败或返回了范围外的值，或者该属性不存在，则必须返回默认值，
如果没有默认值则返回 −1。
当设置时，如果值为负用户代理必须抛出
"IndexSizeError" DOMException。
否则必须将给定的值转换为可以将该数字表示为
合法非负整数 的可能的最短字符串，
然后该字符串必须被用作新的内容属性值。
如果反映内容属性的 IDL 属性是 无符号 整数类型 （unsigned long） ，那么当获取时，必须根据
解析非负整数的规则
解析内容属性，如果解析成功且值在 0 到 2147483647 范围内（含），
必须返回结果值。 如果解析失败或返回了范围外的值，或者该属性不存在，必须返回默认值，
如果没有默认值则返回 0。当设置时，如果新值处于 0 到 2147483647 范围内，
则令 n 为这个新值，否则令n 为默认值（如果没有默认值则为0）；
然后必须将 n 转换为可将该数字表示为
合法非负整数
的可能的最短字符串，然后该字符串必须被用作新的内容属性值。
如果反映内容属性的 IDL 属性是无符号整数类型（unsigned long） 且
仅限于大于零的非负整数
，那么行为与上一种情形类似，但零是不允许的。
当获取时，首先必须根据 解析非负整数的规则
解析内容属性， 如果解析成功且值在 1 到 2147483647 范围内（含），则必须返回结果值。
如果解析失败或返回了范围外的值，或者该属性不存在， 则必须返回默认值（没有默认值则为1）。
当设置时，如果值为0，用户代理必须抛出 "IndexSizeError" DOMException。否则，
如果新值在 1 到 2147483647 范围内，那么令 n 为这个新值。
否则令 n 为默认值（ 如果没有默认值则为 1）；
然后必须将n 转换为可将该数字表示为
合法非负整数
的可能的最短字符串，然后该字符串必须被用作新的内容属性值。
如果反映内容属性的 IDL 属性是无符号整数类型（unsigned long） 且
仅限于有fallback 的大于0的非负整数，则行为与前一种情形类似，但不允许的值将被转换为默认值。
当获取时， 首先必须根据解析非负整数的规则
解析内容属性，如果解析成功且值在 1 到 2147483647 范围内（含），则必须返回结果值。
如果解析失败或返回了范围外的值，或者该属性不存在，则必须返回默认值。
当设置时，如果新值在 1 到 2147483647 范围内，则令 n 为这个新值，
否则令 n 为默认值；然后必须将 n 转换为可将该数字表示为
合法非负整数
的可能的最短字符串，然后该字符串必须被用作新的内容属性值。
如果反映内容属性的 IDL 属性是固定范围
到 [min, max] 的无符号整数类型（unsigned long），
当获取时，必须先根据 解析非负整数的规则
解析内容属性，如果解析成功且值在 min 和 max 之间（包含），
必须返回该结果值。如果失败，必须返回默认值。如果解析成功但值小于 min，必须返回 min。
如果解析成功但值大于 max，必须返回 max。当设置时，其行为与设置普通无符号整数一样。
如果反映内容属性的 IDL 属性是浮点数类型（double 或
unrestricted double），那么当获取时，
必须根据
解析浮点数值的规则 解析内容属性，如果解析成功则必须返回结果值。
如果解析失败或该属性不存在，则必须返回默认值（如果没有默认值则为0）。 当设置时，
必须将给定的值转换为
浮点数的最佳表示，然后该字符串必须被用作新的内容属性值。
如果反映内容属性的 IDL 属性是浮点数类型（double 或 unrestricted
double）且
仅限于大于零的数字，那么行为与上一种情形类似但不允许零和负值。当获取时，必须根据
解析浮点数值的规则解析内容属性，如果解析成功且值大于 0.0，则必须返回结果值。
否则如果解析失败或返回了范围外的值，或者该属性不存在，则必须返回默认值（如果没有默认值则为0）。
当设置时，如果新值小于等于0，那么必须忽略该值。
否则，必须将给定的值转换为
浮点数的最佳表示，然后该字符串必须被用作新的内容属性值。
当设置 Infinity 和 Not-a-Number （NaN）这些值时会抛出异常。 定义在 Web IDL 中。 [WEBIDL]
如果反映内容属性的 IDL 属性的类型为 DOMTokenList，那么当获取时，
必须返回 DOMTokenList 对象，与它关联的元素为被查询的元素，
与它关联的属性的本地名称为被查询的属性名。
如果反映内容属性的 IDL 属性为 HTMLElement 类型，
或者依赖于 HTMLElement 的接口，那么当获取时，
它必须运行下列算法（在第一个返回值的地方停机）：
document.getElementById()
方法将会找到的元素。当设置时，如果给定的元素有 id 属性，
且与被设置属性的元素有同样的 树，
且给定的元素是该 树 中 ID
为该 id 属性值的第一个元素，则必须将内容属性设置为
该 id 属性的值。否则，必须设置该内容属性为空字符串。
HTMLFormControlsCollection 和 HTMLOptionsCollection 接口是
从 HTMLCollection 接口派生出来的 集合 。
HTMLAllCollection 接口是 集合，
但它不派生自 HTMLCollection。
HTMLAllCollection interfaceThe HTMLAllCollection interface is used for the legacy document.all attribute. It operates similarly to
HTMLCollection; the main differences are that it allows a staggering variety of
different (ab)uses of its methods to all end up returning something, and that it can be called as
a function as an alternative to property access.
All HTMLAllCollection objects are rooted at a Document
and have a filter that matches all elements, so the elements represented by the
collection of an HTMLAllCollection object consist of all the descendant
elements of the root Document.
Objects that implement the HTMLAllCollection interface are legacy platform objects with an additonal [[Call]] internal
method described in the section below. They also have an
[[IsHTMLDDA]] internal slot.
Objects that implement the HTMLAllCollection interface have several unusual
behaviors, due of the fact that they have an [[IsHTMLDDA]] internal slot:
The ToBoolean abstract operation in JavaScript returns
false when given objects implementing the HTMLAllCollection interface.
The Abstract Equality Comparison algorithm,
when given objects implementing the HTMLAllCollection interface, returns true when
compared to the undefined and null values.
(Comparisons using the Strict Equality Comparison
algorithm, and Abstract Equality comparisons to other values such as strings or objects, are
unaffected.)
The typeof operator in JavaScript returns the string
"undefined" when applied to objects implementing the
HTMLAllCollection interface.
These special behaviors are motivated by a desire for compatibility with two classes of legacy
content: one that uses the presence of document.all as a
way to detect legacy user agents, and one that only supports those legacy user agents and uses
the document.all object without testing for its presence
first. [JAVASCRIPT]
[Exposed=Window,
LegacyUnenumerableNamedProperties]
interface HTMLAllCollection {
readonly attribute unsigned long length;
getter Element (unsigned long index);
getter (HTMLCollection or Element)? namedItem(DOMString name);
(HTMLCollection or Element)? item(optional DOMString nameOrIndex);
// Note: HTMLAllCollection objects have a custom [[Call]] internal method and an [[IsHTMLDDA]] internal slot.
};lengthReturns the number of elements in the collection.
item(index)Returns the item with index index from the collection (determined by tree order).
item(name)item(name)namedItem(name)namedItem(name)Returns the item with ID or name name from the collection.
If there are multiple matching items, then an HTMLCollection object containing all those elements is returned.
Only button, form, iframe,
input, map, meta, object,
select, and textarea elements can have a name for the purpose of this
method; their name is given by the value of their name attribute.
The object's supported property indices are as defined for
HTMLCollection objects.
The supported property names consist of the non-empty values of all the id attributes of all the elements represented by the
collection, and the non-empty values of all the name attributes of
all the "all"-named elements represented by the collection, in
tree order, ignoring later duplicates, with the id of
an element preceding its name if it contributes both, they differ from each
other, and neither is the duplicate of an earlier entry.
On getting, the length
attribute must return the number of nodes represented by the collection.
The indexed property getter must return the result of getting the "all"-indexed element from this
HTMLAllCollection given the passed index.
The namedItem(name)
method must return the result of getting the "all"-named
element(s) from this HTMLAllCollection given name.
The item(nameOrIndex)
method must perform the following steps:
If nameOrIndex was not provided, return null.
Return the result of getting the
"all"-indexed or named element(s) from this HTMLAllCollection, given
nameOrIndex.
The following elements are "all"-named elements:
a,
button,
embed,
form,
frame,
frameset,
iframe,
img,
input,
map,
meta,
object,
select, and
textarea
To get the "all"-indexed element from an
HTMLAllCollection collection given an index index, return the
indexth element in collection, or null if there is no such
indexth element.
To get the "all"-named element(s) from an
HTMLAllCollection collection given a name name, perform the
following steps:
If name is the empty string, return null.
Let subCollection be an HTMLCollection object rooted at the same
Document as collection, whose filter matches only elements that are
either:
"all"-named elements with a name attribute equal to
name, or,
elements with an ID equal to name.
If there is exactly one element in subCollection, then return that element.
Otherwise, if subCollection is empty, return null.
Otherwise, return subCollection.
To get the "all"-indexed or named
element(s) from an HTMLAllCollection collection given
nameOrIndex:
If nameOrIndex, converted to a
JavaScript String value, is an array index property name, return the result of getting the "all"-indexed element from this
HTMLAllCollection given the number represented by nameOrIndex.
Return the result of getting the "all"-named
element(s) from this HTMLAllCollection given
nameOrIndex.
If argumentsList's size is zero, or if argumentsList[0] is undefined, return null.
Let nameOrIndex be the result of converting argumentsList[0] to a DOMString.
Let result be the result of getting the "all"-indexed or named element(s)
from this HTMLAllCollection given nameOrIndex.
Return the result of converting result to an ECMAScript value.
The thisArgument is ignored, and thus code such as Function.prototype.call.call(document.all, null, "x") will still search for
elements. (document.all.call does not exist, since document.all does not inherit from Function.prototype.)
HTMLFormControlsCollection 接口HTMLFormControlsCollection 接口用于
form 元素中 列出的元素
的 集合。
Support in all current engines.
Support in all current engines.
interface HTMLFormControlsCollection : HTMLCollection {
// inherits length and item()
getter (RadioNodeList or Element)? namedItem(DOMString name); // shadows inherited namedItem()
};
interface RadioNodeList : NodeList {
attribute DOMString value;
};
length返回集合中元素的数目。
item(index)从集合中返回下标为 index 的项目。 这些项目以 树序 排序。
namedItem(name)HTMLFormControlsCollection/namedItem
Support in all current engines.
namedItem(name)如果有多个匹配的项目，则返回一个包含所有那些元素的 RadioNodeList 对象。
返回该对象表示的首个选中的单选按钮的值。
可以被设置，此时选中第一个值为该对象表示的值的单选按钮。
该对象 支持的属性下标
与 HTMLCollection 中定义的相同。
支持的属性名 由
该集合表示的 所有元素的所有
id 和 name 属性的非空值组成。
这些值以 树序 排列，
忽略后面的重复项，元素的 id 在 name 之前
（如果都存在的话），它们是互不相同的也互不为重复项。
namedItem(name)
方法必须根据下列算法操作：
id 属性或
name 属性等于 name，则返回该节点并停止算法。id 属性或
name 属性等于 name，则返回 null 并停止该算法。RadioNodeList 对象，
表示该 HTMLFormControlsCollection 对象的一个 实时 视图，
且 RadioNodeList 对象中只包含
id 属性或 name 属性等于
name 的节点。RadioNodeList 对象中的节点必须以
树序 排序。RadioNodeList 对象。继承自 NodeList 接口的 RadioNodeList 接口的成员
必须与 NodeList 对象上的表现一致。
Support in all current engines.
在获取RadioNodeList 对象上的
value IDL 属性时，
必须返回运行下列步骤的返回值：
令 element 为 RadioNodeList 对象表示的第一个
（树序）
type 属性处于
单选按钮 状态且
选中状态 为真的 input 元素。
如果这样的元素不存在，令它为 null。
如果 element 为 null，返回空字符串。
如果 element 为没有 value 属性的元素，
返回“on” 字符串。
当设置时，value IDL 属性必须执行下列步骤：
如果新的值是字符串“on”：令 element 为
RadioNodeList 对象表示的第一个（树序）
type 属性处于
单选按钮 状态且
value 内容属性缺失，
或存在且等于新的值（如果有新值的话）的 input 元素。
如果这样的元素不存在，令 element 为 null。
除此之外：令 element 为
RadioNodeList 对象表示的第一个（树序）
type 属性处于
单选按钮 状态且
value 内容属性存在且等于新值（如果有的话）
的 input 元素。
如果这样的元素不存在，令 element 为 null。
如果 element 不是 null， 则设置它的 选中状态 为真。
HTMLOptionsCollection 接口HTMLOptionsCollection 接口用于 option 元素的 集合。它经常以 select 元素为根，
且拥有操作其后代属性和方法。
interface HTMLOptionsCollection : HTMLCollection {
// inherits item(), namedItem()
[CEReactions] attribute unsigned long length; // shadows inherited length
[CEReactions] setter void (unsigned long index, HTMLOptionElement? option);
[CEReactions] void add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
[CEReactions] void remove(long index);
attribute long selectedIndex;
};
length [ = value ]返回集合中元素的数目。
当设置为较小的数字时，截断相应容器中 option 元素的数目。
当设置为较大的数字时，在容器中新增空白的 option 元素。
item(index)返回集合中下标为 index 的项目。项目以 树序 排序。
当 index 比集合中项目的数目大时，在相应容器中新增空白 option 元素。
当设为 null 时，从集合中移除在下标 index 处的项目。
当设为一个 option 元素时，在下标 index 处新增或替换为该元素。
namedItem(name)如果有多个匹配项则返回第一个。
add(element [, before ] )在 before 节点前插入 element。
before 参数是数字时，在该数字处的项前插入 element； 是集合中的元素时，在该元素前插入 element。
如果 before 被省略或者为 null， 或者是一个溢出的数字， 就在列表尾部插入 element。
如果 element 是被插入元素的祖先，
该方法将会抛出 "HierarchyRequestError" DOMException。
remove(index)从集合中移除下标为 index 的元素。
selectedIndex [ = value ]返回第一个项的下标（如果有的话），否则，如果没有选中项则返回 −1。
可以被设置，来改变选中项。
该对象 支持的属性下标
与 HTMLCollection 对象中定义的相同。
当获取时，length
属性必须返回 集合表示的 节点的数目。
当设置时，其行为取决于新的值等于、大于或小于
当时 集合表示的 节点的数目。
如果等于，则设置该属性不应做任何事情。如果大于，则必须将 n 个新的、
没有属性和子节点的 option 元素追加到 HTMLOptionsCollection
的根 select 元素上。其中 n 为这两个数字之差（新的值减去旧的值）。
同时必须触发 Mutation 事件，如同插入了包含这些 option 元素的
DocumentFragment 一样。如果新的值更小，则必须从父节点中移除集合中最后的
n 个节点，其中 n 为这两个数之差（旧的值减去新的值）。
设置 length
不会移除或增加任何 optgroup 元素，也不会给既有的 optgroup 元素增加子节点。
（虽然可以从中移除子节点）。
支持的属性名 由
所有 集合表示的 元素的
所有id 和 name属性的非空值构成。
这些值以 树序 排列，
忽略后面的重复项，元素的 id 在
name 之前（如果都存在的话），它们互不相同也互不为重复项。
当用户代理 设置一个新的索引属性的值 或 设置已存在的索引属性的值 时， 设属性下标为 index，新的值为 value，它必须运行下列算法：
如果 value 为 null，以 index 为参数运行 remove 方法的步骤，并终止本步骤。
令 length 为 集合表示的 节点的数目。
令 n 为 index 减 length。
如果 n 大于0，则 追加
一个由 n-1 个新的、 没有属性和子元素的 option 元素组成的
DocumentFragment 到 HTMLOptionsCollection 所在的根 select 元素。
如果 n 大于等于0，追加
value 到 select 元素。否则，以 value
替换 集合中第 index 个元素。
add(element, before) 方法必须按照下列算法执行：
如果 element 为 HTMLOptionsCollection 所在的根
select 元素的祖先节点，则抛出一个
"HierarchyRequestError" DOMException 并终止这些步骤。
如果 before 为一个元素，但它并非 HTMLOptionsCollection 所在的根
select 元素的后代，则抛出一个
"NotFoundError" DOMException 并终止这些步骤。
如果 element 和 before 为同一元素，则返回并终止这些步骤。
如果 before 为一个节点，那么令 reference 为该节点。 否则如果 before 为整数且集合中存在第 before 个节点，令 reference 为该节点。否则令 reference 为 null。
如果 reference 不为 null，令 parent 为 reference
的父节点。否则令 parent 为 HTMLOptionsCollection 所在的根
select 元素。
在reference之前插入 element 到 parent 节点中。
remove(index) 方法
必须按照下列算法执行：
如果 集合表示的 元素数目为0， 终止这些步骤。
如果 index 小于零，或小于 集合表示的 节点数目，终止这些步骤。
令 element 为集合中第 index 个元素。
将 element 从它的父节点移除。
selectedIndex IDL
属性必须像 HTMLOptionsCollection 所在的根 select 元素上的同名属性一样操作。
DOMStringList 接口DOMStringList 接口是表示字符串列表的一种毫不时髦的复古的方式。
[Exposed=(Window,Worker)]
interface DOMStringList {
readonly attribute unsigned long length;
getter DOMString? item(unsigned long index);
boolean contains(DOMString string);
};
新 API 必须使用 sequence<DOMString> 或
其他等价的接口，而不是 DOMStringList。
length返回 strings 中字符串的个数。
item(index)返回 strings 中下标为 index 的字符串。
contains(string)如果 strings 包含 string 则返回 true，否则返回 false。
每个 DOMStringList 对象有一个与之关联的 列表。
DOMStringList 对象 支持的属性下标
为 0 到关联列表的 大小 减 1。如果与它关联的列表
为空，它就没有
支持的属性下标。
length 属性的读取方法必须返回
与该 DOMStringList 对象关联的列表的 大小。
item(index) 方法被调用时，
必须返回该 DOMStringList 对象的关联列表中第 index 个项目，
如果 index 加一大于该 DOMStringList 对象的关联列表的 大小。
contains(string)
方法被调用时，如果该 DOMStringList 对象的关联列表
包含 string，则必须返回 true，
否则必须返回 false。
本节使用 JavaScript 标准的术语和排版惯例。 [JAVASCRIPT]
可序列化对象 支持被序列化以及之后反序列化， 且这一过程与给定的 JavaScript 领域 无关。 这允许它们存储在磁盘中用于后续恢复，或者在文档和 worker 的边界上克隆 （包括不同 域 的文档之间， 或者不同的 事件循环 之间）。
并非所有对象都是 可序列化对象，也并不是 可克隆的对象 的所有方面都需要在序列化时保留。
平台对象 可以是
可序列化对象
只要它们实现了用 [Serializable]
IDL 扩展属性 标注的接口。
这样的接口必须定义下面的算法：
把 value 中的数据序列化为 serialized 字段的一系列步骤。 序列化到 serialized 中的结果数据必须独立于任何 JavaScript 领域。
如果不可能序列化，这些步骤会抛出异常。
这些步骤可能会执行一个 子序列化 来序列化嵌套的数据结构。 它们不应直接调用 StructuredSerialize，因为这样做会忽略 一个重要的 memory 参数。
引入这些步骤，如果与该算法无关，则不应该提及 forStorage 参数。
把 serialized 中的数据反序列化的一系列步骤，用它适当地建立 value。 value 将会是一个新创建的相应 平台对象 类型的实例， 它的内部数据还未建立；这一工作交给这些步骤：
如果不可能反序列化这些步骤会抛出一个异常。
这些步骤可能会执行一个 子反序列化 来反序列化嵌套的数据结构。 它们不能直接调用 StructuredDeserialize，一文这样做会忽略 重要的 targetRealm 和 memory 参数。
由平台对象的定义决定哪些数据被这些步骤序列化和反序列化。通常这些步骤是非常对称的。
[Serializable] 扩展属性不允许有任何参数，
并且不能出现任何不是接口的地方。在一个接口上它必须只出现一次。它不能用在回调接口上。
如果它出现在部分接口（partial interface）上，或者一个混入接口（mixin）上，
那么它必须也出现在原始（original）或被混入（mixed-in-to）接口上，
并且部分接口或混入提供任何的 序列化步骤 和
反序列化步骤 应该理解为
增加到原始或被混入接口的对应步骤上。
加入我们在定义一个平台对象 Person，它与两部分数据相关联：
Person 实例或 null接着，我们使用 [Serializable]
扩展属性
标注 Person 接口，可以把 Person 实例定义为
可序列化对象，同时定义下列算法：
在 JavaScript 规范中定义的对象直接由 StructuredSerialize 抽象操作处理。
最初，本规范定义了“克隆对象”的概念， 可以从一个 JavaScript 领域 克隆到另一个。 但为了更好地指定某些更复杂的情况的行为，更新了模型来显示地进行序列化和反序列化。
可传输对象 支持在 事件循环 之间传输。传输其实就是有效地重新创建对象并共享对底层数据的引用，然后将被传输的对象分离。 在转让昂贵资源的所有权时很有用。并非所有对象都是 可传输对象， 也并非 可传输对象 的所有方面都会在传输时被保留。
传输是不可逆转的、不幂等的操作。对象一旦被传输就不可再次传输或使用。
平台对象 可以是
可传输对象，
只要它们实现了以 [Transferable] IDL
扩展属性 注解的接口。
这样的接口必须定义下列算法：
把 value 中的数据传输到 dataHolder 字段的一系列步骤。 dataHolder 中的结果数据必须独立于任何 JavaScript 领域。
如果不可能传输，这些步骤会抛出异常。
接收 dataHolder 中的数据并用它适当地建立 value 的一系列步骤。 value 将会是一个新创建的相应 平台对象 类型的实例， 它的内部数据还未建立；这一工作交给这些步骤：
如果不可能接收这次传输，这些步骤会抛出异常。
由平台对象的定义决定哪些数据被这些步骤传输。通常这些步骤是非常对称的。
[Transferable] 扩展属性不允许有任何参数，
并且不能出现任何不是接口的地方。在一个接口上它必须只出现一次。它不能用在回调接口上。
如果它出现在部分接口（partial interface）上，或者一个混入接口（mixin）上，
那么它必须也出现在原始（original）或被混入（mixed-in-to）接口上，
并且部分接口或混入提供任何的 传输步骤 和
接收步骤 应该理解为
增加到原始或被混入接口的对应步骤上。
是 可传输对象 的 平台对象 有一个 [[Detached]] 内部槽。 用于确保一旦平台对象已经被传输，它不能被再次传输。
JavaScript 标准中定义的对象直接交由 StructuredSerializeWithTransfer 抽象操作处理。
StructuredSerializeInternal 抽象操作接受一个 JavaScript 值 value 作为输入， 把它序列化为一个 领域 无关的形式，在这里表示为一个 记录。这一序列化的形式有着后续反序列化到不同领域的新 JavaScript 值需要的所有必要信息。
这一过程可能会抛出一个异常，例如当尝试序列化不可序列化对象时。
如果没有提供 memory，令 memory 为一个空的 映射。
memory 映射是为了避免把对象序列化两次。 最终会用于保持环状结构以及标识图中的重复对象。
如果 memory[value] 存在，则 返回 memory[value]。
令deep 为 false。
如果 Type(value) 为 Undefined，Null，Boolean， Number，BigInt 或 String，则返回 { [[Type]]: "primitive"，[[Value]]: value }。
如果 Type(value) 为 Symbol，则抛出一个
"DataCloneError" DOMException。
令serialized 为一个未初始化的值。
如果 value 有一个 [[BooleanData]] 内部槽，则把 serialized 设置为 { [[Type]]: "Boolean",[[BooleanData]]: value.[[BooleanData]] }。
否则，如果 value 有一个 [[NumberData]] 内部槽，则把 serialized 设置为 { [[Type]]: "Number",[[NumberData]]: value.[[NumberData]] }。
否则，如果 value 有一个 [[BigIntData]] 内部槽，则把 serialized 设置为 { [[Type]]: "BigInt", [[BigIntData]]: value.[[BigIntData]] }。
否则，如果 value 有一个 [[StringData]] 内部槽，则把 serialized 设置为 { [[Type]]: "String",[[StringData]]: value.[[StringData]] }。
否则，如果 value 有一个 [[DateValue]] 内部槽，则把 serialized 设置为 { [[Type]]: "Date",[[DateValue]]: value.[[DateValue]] }。
否则，如果 value 有一个 [[RegExpMatcher]] 内部槽，则把 serialized 设置为 { [[Type]]: "RegExp",[[RegExpMatcher]]: value.[[RegExpMatcher]]，[[OriginalSource]]: value.[[OriginalSource]]， [[OriginalFlags]]: value.[[OriginalFlags]] }。
否则，如果 value 有一个 [[ArrayBufferData]] 内部槽，则：
令size 为 value.[[ArrayBufferByteLength]]。
如果 ! IsSharedArrayBuffer(value) 为 true，则：
令 agentCluster 为 surrounding agent 的 agent cluster。
如果 agentCluster 的 跨域隔离 为 false，则抛出一个
"DataCloneError" DOMException。
该检查只在序列化时需要（反序列化时不需要）。因为
跨域隔离 不会随着时间改变，而且
SharedArrayBuffer 不能离开 agent cluster。
如果 forStorage 为 true，则抛出一个
"DataCloneError" DOMException。
把 serialized 设置为 { [[Type]]: "SharedArrayBuffer",[[ArrayBufferData]]: value.[[ArrayBufferData]],[[ArrayBufferByteLength]]: size, [[AgentCluster]]: agentCluster }。
否则：
如果 ! IsDetachedBuffer(value) 为 true，则抛出一个
"DataCloneError" DOMException。
令dataCopy 为 ? CreateByteDataBlock(size)。
在分配地址失败时可能会抛出一个 RangeError 异常。
执行 ! CopyDataBlockBytes(dataCopy,0,value.[[ArrayBufferData]],0,size)。
把 serialized 设置为 { [[Type]]: "ArrayBuffer",[[ArrayBufferData]]: dataCopy,[[ArrayBufferByteLength]]: size }。
否则，如果 value 有一个 [[ViewedArrayBuffer]] 内部槽，则：
令 buffer 为 value 的 [[ViewedArrayBuffer]] 内部槽的值。
令 bufferSerialized 为 ? StructuredSerializeInternal(buffer,forStorage, memory)。
断言： bufferSerialized.[[Type]] 是 "ArrayBuffer"。
如果 value 有一个 [[DataView]] 内部槽，则把 serialized 设置为 { [[Type]]: "ArrayBufferView",[[Constructor]]: "DataView",[[ArrayBufferSerialized]]: bufferSerialized,[[ByteLength]]: value.[[ByteLength]],[[ByteOffset]]: value.[[ByteOffset]] }。
否则：
断言： value 有一个 [[TypedArrayName]] 内部槽。
把 serialized 设置为 { [[Type]]: "ArrayBufferView",[[Constructor]]: value.[[TypedArrayName]],[[ArrayBufferSerialized]]: bufferSerialized, [[ByteLength]]: value.[[ByteLength]],[[ByteOffset]]: value.[[ByteOffset]],[[ArrayLength]]: value.[[ArrayLength]] }。
否则，如果 value 有 [[MapData]] 内部槽，则：
设置serialized 为 { [[Type]]: "Map",[[MapData]]: 一个新的空 列表 }。
设置deep 为 true。
否则，if value 有 [[SetData]] 内部槽，则：
设置 serialized 为 { [[Type]]: "Set",[[SetData]]: 一个新的空 列表 }。
设置 deep 为 true。
否则，如果 value 有 [[ErrorData]] 内部槽且 value 不是 平台对象，那么：
令 name 为 ? Get(value, "name")。
如果 name 不是 "Error", "EvalError", "RangeError", "ReferenceError", "SyntaxError", "TypeError" 或 "URIError"，则设置 name 为 "Error"。
令 valueMessageDesc 为 ? value.[[GetOwnProperty]]("message")。
如果 IsDataDescriptor(valueMessageDesc) 为 false， 令 message 为 undefined，否则为 ? ToString(valueMessageDesc.[[Value]])。
设置 serialized 为 { [[Type]]: "Error", [[Name]]: name, [[Message]]: message }。
用户代理应该把任何有用的还没标准化的附加数据都加到序列化表示 serialized 里，尤其是
stack 属性。
该数据正在标准化，见 Error Stacks 提案。[JSERRORSTACKS]
否则，如果 value 是一个外来 Array 对象，那么：
令valueLenDescriptor 为 ?
OrdinaryGetOwnProperty(value，"length")。
令valueLen 为 valueLenDescriptor.[[Value]]。
设置serialized 为 { [[Type]]: "Array",[[Length]]: valueLen, [[Properties]]: 一个新的空 列表 }。
设置deep 为 true。
如果 value 有一个 [[Detached]] 内部槽且值为 true，
则抛出一个 "DataCloneError" DOMException。
令typeString 为 value 的 主接口 标识。
设置serialized 为 { [[Type]]: typeString }。
设置deep 为 true。
否则，如果 value 是一个 平台对象，则抛出一个
"DataCloneError" DOMException。
否则，如果 IsCallable(value) 为 true，则抛出一个
"DataCloneError" DOMException。
否则，如果 value 有除了 [[Prototype]] 和 [[Extensible]] 之外的任何内部槽，
则抛出一个 "DataCloneError" DOMException。
例如 [[PromiseState]] 或 [[WeakMapData]] 内部槽。
否则，如果 value 是一个外来对象且 value 不是
关联到任何 JavaScript realm 的 %Object.prototype% intrinsic 对象，
则抛出一个 "DataCloneError" DOMException。
例如代理对象。
否则：
设置serialized 为 { [[Type]]: "Object",[[Properties]]: 一个新的空 列表 }。
设置deep 为 true。
%Object.prototype% 最终会被本步骤或后续步骤处理。 最终结果是它的外来属性被忽略，反序列化后结果将是一个空对象（而不是 prototype 外部对象不变量）。
设置 memory[value] 为 serialized。
如果 deep 为 true，则：
如果 value 有一个 [[MapData]] 内部槽，则：
令 copiedList 为一个新的空 列表。
对 copiedList 中的 每一个 记录 { [[Key]]，[[Value]] } entry：
令 serializedKey 为 ? StructuredSerializeInternal(entry.[[Key]],forStorage, memory)。
令 serializedValue 为 ? StructuredSerializeInternal(entry.[[Value]],forStorage, memory)。
追加 { [[Key]]: serializedKey, [[Value]]: serializedValue } 到 serialized.[[MapData]]。
否则，如果 value 有一个 [[SetData]] 内部槽，则：
否则，如果 value 是一个 可序列化的 平台对象，则给定 value，serialized，和 forStorage， 为 value 的 primary interface 执行 序列化步骤。
序列化步骤 可能需要执行一个 字序列化。 该操作接受 subValue 值作为输入，返回 StructuredSerializeInternal (subValue, forStorage,memory)。 （换句话说，子序列化 是 StructuredSerializeInternal 的与该调用一致的特化。）
否则，对 ! EnumerableOwnPropertyNames(value, key) 里的每个 key ：
如果 ! HasOwnProperty(value, key) 为 true，则：
令 inputValue 为 ? value.[[Get]](key, value)。
令 outputValue 为 ? StructuredSerializeInternal(inputValue, forStorage, memory)。
把 { [[Key]]: key, [[Value]]: outputValue } 加到 serialized.[[Properties]]。
返回 serialized。
注意 StructuredSerializeInternal 产生的 记录 可能包含其他记录的 "指针"，这产生了环装引用。例如当我们传递下面的 JavaScript 对象到 StructuredSerializeInternal 中时：
const o = {};
o.myself = o;
它产生了下面的结果：
{
[[Type]]: "Object",
[[Properties]]: «
{
[[Key]]: "myself",
[[Value]]: <a pointer to this whole structure>
}
»
}
返回 ? StructuredSerializeInternal(value, false).
返回 ? StructuredSerializeInternal(value, true).
StructuredDeserialize 抽象操作接受一个 记录 serialized 作为输入，它由之前的 StructuredSerialize 或 StructuredSerializeForStorage 产生，把它反序列化到一个新的 targetRealm 中创建的 JavaScript 值。
该过程可能抛出一个异常，例如为新的对象尝试分配内存时（尤其是 ArrayBuffer 对象）。
如果没有提供 memory，令 memory 为一个空的 映射。
memory 映射是为了避免把对象反序列化两次。 最终会用于保持环状结构以及标识图中的重复对象。
如果 memory[sirialized] 存在，则 返回 memory[serialized]。
令deep 为 false。
令value 为一个未初始化的值。
否则，如果 serialized.[[Type]] 是 "primitive"，则设置 value 为 serialized.[[Value]]。
否则，如果 serialized.[[Type]] 是 "Boolean"，则设置 value 为一个 targetRealm 中的新的 Boolean 对象，其 [[BooleanData]] 内部槽值为 serialized.[[BooleanData]]。
否则，如果 serialized.[[Type]] 是 "Number"，则设置 value 为一个 targetRealm 中的新的 Number 对象，其 [[NumberData]] 内部槽值为 serialized.[[NumberData]]。
否则，如果 serialized.[[Type]] 是 "BigInt"，则设置 value 为一个 targetRealm 中的新的 BigInt 对象，其 [[BigIntData]] 内部槽的值为 serialized.[[BigIntData]]。
否则，如果 serialized.[[Type]] 是 "String"，则设置 value 为一个 targetRealm 中的新的 String 对象，其 [[StringData]] 内部槽值为 serialized.[[StringData]]。
否则，如果 serialized.[[Type]] 是 "Date"，则设置 value 为一个 targetRealm 中的新的 Date 对象，其 [[DateValue]] 内部槽值为 serialized.[[DateValue]]。
否则，如果 serialized.[[Type]] 是 "RegExp"，则设置 value 为一个 targetRealm 中的新的 RegExp 对象，其 [[RegExpMatcher]] 内部槽值为 serialized.[[RegExpMatcher]]，其 [[OriginalSource]] 内部槽值为 serialized.[[OriginalSource]]，其 [[OriginalFlags]] 内部槽值为 serialized.[[OriginalFlags]]。
否则，如果 serialized.[[Type]] 是 "SharedArrayBuffer"，则：
如果 targetRealm 对应的 agent cluster 不是
serialized.[[AgentCluster]]，则抛出一个
"DataCloneError" DOMException。
否则，设置 value 到一个 targetRealm 中的新的 SharedArrayBuffer 对象， 其 [[ArrayBufferData]] 内部槽值为 serialized.[[ArrayBufferData]]， 其 [[ArrayBufferByteLength]] 内部槽值是 serialized.[[ArrayBufferByteLength]]。
否则，如果 serialized.[[Type]] 是 "ArrayBuffer"，则设置 value 为一个 targetRealm 中新的 ArrayBuffer 对象，其 [[ArrayBufferData]] 内部槽值为 serialized.[[ArrayBufferData]]，其 [[ArrayBufferByteLength]] 内部槽值为 serialized.[[ArrayBufferByteLength]]。
如果这抛出了一个异常，则抛出一个 "DataCloneError"
DOMException。
如果没有足够的内存来创建这样一个 ArrayBuffer 对象，该步骤可能会抛出异常。
否则，如果 serialized.[[Type]] 是 "ArrayBufferView"，则：
令deserializedArrayBuffer 为 ? StructuredDeserialize(serialized.[[ArrayBufferSerialized]], targetRealm,memory)。
如果 serialized.[[Constructor]] 是 "DataView"，则设置 value 为 targetRealm 中的一个新的 DataView 对象，其 [[ViewedArrayBuffer]] 内部槽值为 deserializedArrayBuffer，其 [[ByteLength]] 内部槽值为 serialized.[[ByteLength]]，其 [[ByteOffset]] 内部槽值为 serialized.[[ByteOffset]]。
否则，设置 value 为一个 targetRealm 中新的有类型数组对象， 使用 serialized.[[Constructor]] 给出的构造器，其 [[ViewedArrayBuffer]] 内部槽值为 deserializedArrayBuffer，其 [[TypedArrayName]] 内部槽值为 serialized.[[Constructor]]，其 [[ByteLength]] 内部槽值为 serialized.[[ByteLength]]，其 [[ByteOffset]] 内部槽值为 serialized.[[ByteOffset]]，其 [[ArrayLength]] 内部槽值为 serialized.[[ArrayLength]]。
否则，如果 serialized.[[Type]] 是 "Map"，则：
设置value 为 targetRealm 中一个新的 Map 对象，其 [[MapData]] 内部槽值为一个新的空 列表。
设置deep to true。
否则，如果 serialized.[[Type]] 是 "Set"，则：
设置value 为一个 targetRealm 中新的设置对象，其 [[SetData]] 内部槽值为一个新的空 列表。
设置deep 为 true。
否则，如果 serialized.[[Type]] 是 "Array"，则：
令outputProto 为 targetRealm.[[Intrinsics]].[[%Array.prototype%]]。
设置 value 为 ! ArrayCreate(serialized.[[Length]]， outputProto)。
设置deep 为 true。
否则，如果 serialized.[[Type]] 是 "Object"，则：
设置value 为 targetRealm 中一个新的 Object。
设置deep 为 true。
否则，如果 serialized.[[Type]] 是 "Error"，则：
令 prototype 为 %Error.prototype%。
如果 serialized.[[Name]] 是 "EvalError"，则设置 prototype 为 %EvalError.prototype%。
如果 serialized.[[Name]] 是 "RangeError"，则设置 prototype 为 %RangeError.prototype%。
如果 serialized.[[Name]] 是 "ReferenceError"，则设置 prototype 为 %ReferenceError.prototype%。
如果 serialized.[[Name]] 是 "SyntaxError"，则设置 prototype 为 %SyntaxError.prototype%。
如果 serialized.[[Name]] 是 "TypeError"，则设置 prototype 为 %TypeError.prototype%。
如果 serialized.[[Name]] 是 "URIError"，则设置 prototype 为 %URIError.prototype%。
令 message 为 serialized.[[Message]]。
设置 value 为 ! ObjectCreate(prototype, « [[ErrorData]] »)。
令 messageDesc 为 PropertyDescriptor{ [[Value]]: message, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }。
如果 message 不是 undefined，则执行 !
OrdinaryDefineOwnProperty(value, "message",
messageDesc)。
serialized 上面任何有用的附加数据都应该反序列化后再附加到value 上。
否则：
令 interfaceName 为 serialized.[[Type]]。
如果 interfaceName 标识的接口没有 暴露 在
targetRealm 中，则抛出一个 "DataCloneError"
DOMException。
设置value 为在 targetRealm 中创建的、 以 interfaceName 标识的接口的一个新的实例。
设置deep 为 true。
设置 memory[serialized] 为 value。
如果 deep 为 true，则：
如果 serialized.[[Type]] 是 "Map"，则：
对 serialized.[[MapData]] 中的 每一个 记录 { [[Key]]，[[Value]] } entry：
令deserializedKey 为 ? StructuredDeserialize(entry.[[Key]],targetRealm, memory)。
令deserializedValue 为 ? StructuredDeserialize(entry.[[Value]],targetRealm, memory)。
追加 { [[Key]]: deserializedKey, [[Value]]: deserializedValue } 到 value.[[MapData]]。
否则，如果 serialized.[[Type]] 是 "Set"，则：
对 serialized.[[SetData]] 中 每一个 entry：
令deserializedEntry 为 ? StructuredDeserialize(entry,targetRealm, memory)。
追加 deserializedEntry 到 value.[[SetData]]。
否则，如果 serialized.[[Type]] 是 "Array" 或 "Object"，则：
对 serialized.[[Properties]] 中的 每一个 记录 { [[Key]]，[[Value]] } entry：
令deserializedValue 为 ? StructuredDeserialize(entry.[[Value]],targetRealm, memory)。
令result 为 ! CreateDataProperty(value, entry.[[Key]],deserializedValue)。
断言： result 为 true。
否则：
给定 serialized 和 value，为 serialized.[[Type]] 标识的接口 执行适当的 反序列化步骤。
反序列化步骤 可能会执行一个 子反序列化。该操作接受 先前序列化后的 Record subSerialized 作为输入，返回 StructuredDeserialize(subSerialized,targetRealm, memory)。（换句话说，子反序列化 是 是 StructuredDeserialize 的与该调用一致的特化。）
返回 value。
令memory 为一个空的 映射。
除了它在 StructuredSerializeInternal 中的正常使用外， 在本算法中 memory 还用来确保 StructuredSerializeInternal 忽略了 transferList 中的项目，来让我们做自己的处理。
对 transferList 中的 每一个 transferable：
如果 transferable 既没有 [[ArrayBufferData]] 内部槽
又没有 [[Detached]] 内部槽，则抛出一个
"DataCloneError" DOMException。
如果 transferable 有一个 [[ArrayBufferData]] 内部槽且
! IsSharedArrayBuffer(transferable) 为 true 或
! IsDetachedBuffer(transferable) 为 true，则抛出一个
"DataCloneError" DOMException。
如果 transferable 有一个 [[Detached]] 内部槽且
transferable.[[Detached]] 为 true，则抛出一个
"DataCloneError" DOMException。
令placeholder 为一个用户代理定义的占位对象。
设置 memory[transferable] 为 placeholder。
令serialized 为 ? StructuredSerializeInternal(value, false,memory)。
令transferDataHolders 为一个新的空 列表。
对 transferList 中的 每一个 transferable：
令placeholder 为 memory[transferable]。
令dataHolder 为一个未初始化的值。
如果 transferable 有一个 [[ArrayBufferData]] 内部槽，则：
设置dataHolder 为 { [[TransferConsumed]]: false,[[Type]]: "ArrayBuffer", [[ArrayBufferData]]: transferable.[[ArrayBufferData]],[[ArrayBufferByteLength]]: transferable.[[ArrayBufferByteLength]] }。
执行 ! DetachArrayBuffer(transferable)。
否则：
令interfaceName 为 transferable 的 主接口 的标识符。
设置dataHolder 为 { [[TransferConsumed]]: false,[[Type]]: interfaceName }。
指定 transferable 和 dataHolder， 为 interfaceName 标识的接口执行适当的 传输步骤。
设置transferable.[[Detached]] 为 true。
在 serialized 中，用 dataHolder 替换所有的 placeholder 实例。
追加 dataHolder 到 transferDataHolders。
返回 { [[Serialized]]: serialized,[[TransferDataHolders]]: transferDataHolders }。
令 memory 为一个空的 映射。
除了它在 StructuredDeserialize 中的正常使用外， 在本算法中 memory 还用来帮助我们确定传输值的列表。
令deserialized 为 ? StructuredDeserialize(serializeWithTransferResult.[[Serialized]], targetRealm,memory)。
令transferredValues 为一个新的空 列表。
对 serializeWithTransferResult.[[TransferDataHolders]] 中的 每一个 transferDataHolder：
追加 memory[transferDataHolder] 到 transferredValues。
返回 { [[Deserialized]]: deserialized,[[TransferredValues]]: transferredValues }。
其他规范可以使用这里定义的抽象操作。下面为每个抽象操作何时比较有用提供了指导以及例子。
把一个值克隆到另一个 JavaScript Realm， 带着传输列表但提前不知道目标领域。 这一情况下序列化步骤可以立即执行，反序列化步骤延迟到目标 Realm 已知的时候执行。
messagePort.postMessage()
使用这一对抽象操作，因为目标 Realm 直到 MessagePort
发出 时才知道。
为给定的值创建一个 JavaScript Realm 无关的可以保存无限时间的快照， 然后（可能很多次）具象化回 JavaScript 值。
StructuredSerializeForStorage 可以在序列化被用于持久保存值的时候
（而不是在 Realm 之间传递）使用。当尝试序列化 SharedArrayBuffer 对象时会抛出异常，
因为保存共享内存没有意义。类似地，当指定有着定制的
序列化步骤
的 平台对象
且 forStorage 参数为 true 时，也可能抛出异常或者有不同的行为。
history.pushState() 和 history.replaceState() 在作者提供的状态对象上使用
StructuredSerializeForStorage，把它们存储为
会话历史入口 的
序列化状态。
然后 history.state 属性用了
StructuredDeserialize 来返回最初提供的状态对象的一份克隆。
broadcastChannel.postMessage() 对它的输入使用了
StructuredSerialize，然后对结果多次使用了 StructuredDeserialize
来对每一个广播目标产生新的拷贝。注意在多目标情况下传输没有意义。
持久化 JavaScript 值到文件系统的 API 可能还会对它的输入使用 StructuredSerializeForStorage 并对它的输出使用 StructuredDeserialize。
一般来说，调用点可能会传递 Web IDL 值而不是 JavaScript 值； 这将被理解为在调用这些算法之前执行到 JavaScript 值的隐式 转换。
本规范曾经定义了“结构化克隆”算法和一个 StructuredClone 抽象操作。 然而在实践中，它的所有已知用途可以更好地通过单独的序列化和反序列化步骤来实现，所以把它删除了。
如果在任意对象上进行操作，对于非用户代码同步调用到用户代理方法的调用点必须在调用 StructuredSerialize，StructuredSerializeForStorage， 或 StructuredSerializeWithTransfer 等抽象操作之前小心地 准备执行脚本 以及 准备运行 fallback。 这是必要的，因为序列化过程可能会调用作者定义的访问器作为最终的深度序列化步骤， 这些访问器调用的操作可能会依赖于 entry 和 incumbent 的概念已经建立好。
window.postMessage() 对它的参数执行
StructuredSerializeWithTransfer 操作，但是在算法中的同步部分立即执行这一操作则需要小心。
因为可能无需 准备执行脚本 和
准备执行 fallback 就能使用该算法。
作为对比，假设有个 API 直接通过 事件循环 的 任务 来定期地使用 StructuredSerialize 序列化一些作者提供的对象， 可能需要事先确保它执行了适当的准备工作。现在我们还没见过平台上有这样的 API； 通常，作为作者代码的同步结果来事先执行序列化更加容易。
HTML UA 中的每个 XML 和 HTML 文档表示为一个 Document 对象。 [DOM]
Document 对象的 URL 定义在 DOM 标准中。 It is initially set when
当创建 Document 对象时设置，但可以在 Document 的生命期内改变；
例如当用户 导航 到一个页面的一个 片段
并用一个新的 URL 调用 pushState() 方法时就会改变。[DOM]
交互式用户代理通常在其用户界面中显示 Document 对象的 URL。
这是用户用来判断当前网站是否在冒充其他网站的主要机制。
在 脚本 中使用
createDocument()
或 createHTMLDocument()
创建 Document 时，Document 立即 准备好了执行加载后的任务。
文档的 referrer 是一个字符串（表示一个 URL）。
可以在创建 Document 时设置，如果没有明确设置，它的值是空字符串。
Document 对象DOM 定义了 Document 接口，本规范对它进行了很多扩展：
enum DocumentReadyState { "loading"，"interactive"，"complete" };
typedef (HTMLScriptElement or SVGScriptElement) HTMLOrSVGScriptElement;
[LegacyOverrideBuiltIns]
partial interface Document {
// 资源元数据管理
[PutForwards=href, LegacyUnforgeable] readonly attribute Location? location;
attribute USVString domain;
readonly attribute USVString referrer;
attribute USVString cookie;
readonly attribute DOMString lastModified;
readonly attribute DocumentReadyState readyState;
// DOM 树访问器
getter object (DOMString name);
[CEReactions] attribute DOMString title;
[CEReactions] attribute DOMString dir;
[CEReactions] attribute HTMLElement? body;
readonly attribute HTMLHeadElement? head;
[SameObject] readonly attribute HTMLCollection images;
[SameObject] readonly attribute HTMLCollection embeds;
[SameObject] readonly attribute HTMLCollection plugins;
[SameObject] readonly attribute HTMLCollection links;
[SameObject] readonly attribute HTMLCollection forms;
[SameObject] readonly attribute HTMLCollection scripts;
NodeList getElementsByName(DOMString elementName);
readonly attribute HTMLOrSVGScriptElement? currentScript; // classic scripts in a document tree only
// 动态标记插入
[CEReactions] Document open(optional DOMString unused1, optional DOMString unused2); // both arguments are ignored
WindowProxy? open(USVString url,DOMString name,DOMString features);
[CEReactions] undefined close();
[CEReactions] undefined write(DOMString..。 text);
[CEReactions] undefined writeln(DOMString..。 text);
// 用户交互
readonly attribute WindowProxy? defaultView;
boolean hasFocus();
[CEReactions] attribute DOMString designMode;
[CEReactions] boolean execCommand(DOMString commandId,optional boolean showUI = false,optional DOMString value = "");
boolean queryCommandEnabled(DOMString commandId);
boolean queryCommandIndeterm(DOMString commandId);
boolean queryCommandState(DOMString commandId);
boolean queryCommandSupported(DOMString commandId);
DOMString queryCommandValue(DOMString commandId);
// 只适用于 Document 对象的特殊 事件处理器 IDL 属性
[LegacyLenientThis] attribute EventHandler onreadystatechange;
// also has obsolete members
};
Document 实现了 GlobalEventHandlers；
Document 实现了 DocumentAndElementEventHandlers；
Document 有一个 引荐来源策略 （一个 引荐来源策略），
初始为空字符串，它表示由 Document 发起的 获取
的默认 引荐来源策略。
Document 有一个 CSP 列表，它是一个在该上下文中活动的 内容安全策略 对象的列表。
该列表在没有指定时是空的。
Document 有一个 权限策略，是一个
初始为空的 权限策略。
Document 有一个 模块映射，
是一个初始为空的 模块映射。
Document 有一个 跨域 opener 策略，是一个
跨域 opener 策略，初始为 "unsafe-none"。
DocumentOrShadowRoot interfaceDOM defines the DocumentOrShadowRoot mixin, which this specification
extends.
partial interface mixin DocumentOrShadowRoot {
readonly attribute Element? activeElement;
};referrerSupport in all current engines.
返回用户导航到此文档的那个 Document 的 URL，
除非它被阻止或没有这样的文档，这时返回空字符串。
noreferrer 链接类型可用于阻止引荐来源。
referrer 属性必须返回 该文档的引荐来源。
cookie [ = value ]返回适用于该 Document 的 HTTP cookie。如果没有 Cookie 或 Cookie 不适用于该资源，
则返回空字符串。
可以被设置，用来给该元素的 HTTP Cookie 集合添加一个新的 Cookie。
如果内容 被沙盒化到一个唯一的 origin 中
（例如有 sandbox 属性的 iframe），
当读取和设置时抛出 "SecurityError" DOMException。
Support in all current engines.
cookie 属性表示由文档的
URL 标识的资源的 Cookie。
满足下列条件的 Document 对象为
cookie-averse 文档 对象：
当读取时，如果文档是一个 cookie-averse Document 对象，
那么用户代理必须返回空字符串。否则，如果 Document 的 origin 是一个
不透明 origin，用户代理必须抛出 "SecurityError"
DOMException。否则，用户代理必须作为 "非HTTP" API 返回
使用 无 BOM 的 UTF-8 解码算法 解码后的，
文档 URL 的 Cookie 字符串。[COOKIES]
当设置时，如果文档是一个 cookie-averse Document 对象，
则用户代理必须什么都不做。否则，如果 Document 的 origin 是一个
不透明 origin，用户代理必须抛出一个
"SecurityError" DOMException。
否则用户代理的行为必须像是通过 "非HTTP" API 为文档的 URL
接收 set-cookie-string 时一样。
其中，set-cookie-string 由一个新的 UTF-8 编码的 值组成。[COOKIES] [ENCODING]
因为在 frame 之间可以访问 cookie 属性，
Cookie 的路径限制只是用来帮助管理对站点的哪一部分发送哪些 Cookie 的工具，并不是任何形式的安全特性。
cookie 属性的读取方法和设定方法同步地访问共享状态。
由于没有加锁机制，在多进程用户代理中其他浏览环境可能会在脚本执行中修改 Cookie。
比如一个站点试着读取 Cookie，增加它的值然后写回去，用新的值作为会话的唯一标识；
如果该站点在不同的浏览器窗口中同时做这件事情，两个会话使用的唯一标识可能是相同的，这可能引发灾难性后果。
lastModifiedSupport in all current engines.
根据服务器的报告返回文档的（用户的本地时区的）最后修改时间，按照这样的格式："MM/DD/YYYY hh:mm:ss"。
如果不知道最后修改时间，返回当前时间。
当读取 lastModified 属性时，
必须返回 Document 的源文件的最后修改（在用户本地时区的）日期和时间，按照下列格式：
上述除了年之外的所有数字数字组件必须以两位 ASCII 数字 的形式给出， 表示十进制数，必要时需要补零。年必须按照最短可能的字符串给出，可能是 4 个或更多的 ASCII 数字，表示十进制数，必要时需要补零。
Document 的源文件的最后修改日期和时间必须从使用的网络协议的相关特性中获取。
比如文档的 HTTP Last-Modified 头部，或者（对于本地文件）文件系统中的元数据。
如果不知道最后修改日期和时间，该属性必须按照上述格式返回当前日期和时间。
readyState当 Document 正在加载时返回 "loading"，
一旦结束解析但仍在加载子资源时返回 "interactive"，
一旦加载完成返回 "complete"。
当这个值变化时触发 Document 对象上的
readystatechange 事件。
Support in all current engines.
每个文档有一个 当前文档就绪状态。
创建 Document 对象时，
如果文档有关联的 HTML 解析器，XML 解析器，或 XSLT 处理器，
必须将 当前文档就绪状态
设置为字符串 "loading"；
否则必须将 当前文档就绪状态
设置为字符串 "complete"。
在页面加载过程中很多算法都会影响这个值。当这个值被设置时用户代理必须
在 Document 对象上
触发 一个名为 readystatechange 的事件。
如果文档有关联的 HTML 解析器 或 XML 解析器，
且该解析器还没有 停止 或 中止，
那么就称该 Document 有一个 活动的解析器。
当读取时，readyState IDL 属性必须返回
当前文档就绪状态。
如果文档是一个 html 元素，
它的 html 元素 就是它的 文档元素，
否则就是 null。
headSupport in all current engines.
返回 head 元素。
文档的 head 元素 是它的
html 元素 的第一个 head 子元素。
如果没有这样的元素，则为 null。
当读取 head 属性时，必须返回
文档的 head 元素（一个 head 元素或 null）。
title [ = value ]返回文档的标题，对于 HTML 由 title 元素 给出，
对于 SVG 由 SVG title 元素给出。
可以被设置来更新文档的标题。如果没有合适的元素来更新，新的值将被忽略。
文档的 title 元素
是文档中的第一个 title 元素（按 树序），
如果没有则为 null。
Support in all current engines.
当读取时，title
属性必须执行以下算法：
如果 文档元素 是一个 SVG svg 元素，
那么令 value 为 文档元素 的第一个子 SVG title 元素的
子文本内容。
否则，令 value 为 title 元素
的 子文本内容，
如果 title 元素 是 null 则令 value 为空字符串。
返回 value。
当设置时，必须执行匹配下列第一个条件的对应步骤：
svg 元素什么都不做。
body [ = value ]Support in all current engines.
返回 body 元素。
被设置时替换 body 元素。
如果新值不是一个 body 或 frameset 元素，这将会抛出一个
"HierarchyRequestError" DOMException。
文档的 body 元素 是 html 元素
的第一个是 body 元素或 frameset 元素的子节点，没有的话就是 null。
读取 body 属性时，必须返回
文档的 body 元素（是一个 body 或 frameset 元素，或者是 null）。
当设置时，必须执行以下算法：
body 或 frameset 元素则抛出一个
"HierarchyRequestError" DOMException 并中止这些步骤。HierarchyRequestError" DOMException 并中止这些步骤.imagesSupport in all current engines.
返回一个 Document 中的 img 元素的 HTMLCollection。
embedsSupport in all current engines.
pluginsSupport in all current engines.
返回一个 Document 中的 embed 元素的 HTMLCollection。
linksSupport in all current engines.
返回一个 Document 中的有 href 属性的
a 和 area 元素的 HTMLCollection。
formsSupport in all current engines.
返回一个 Document 中的 form 元素的 HTMLCollection。
scriptsSupport in all current engines.
返回一个 Document 中的 script 元素的 HTMLCollection。
images 属性必须返回一个
根为 Document 节点的 HTMLCollection ，其过滤器只匹配
img 元素。
embeds 属性必须返回一个
根为 Document 节点的 HTMLCollection ，其过滤器只匹配
embed 元素。
plugins 属性必须返回
与 embeds 属性返回的相同的对象。
links 属性必须返回一个
根为 Document 节点的 HTMLCollection ，其过滤器只匹配
有 href 属性的 a 元素，
以及有 href 属性的 area 元素。
forms 属性必须返回一个
根为 Document 节点的 HTMLCollection ，其过滤器只匹配
form 元素。
scripts 属性必须返回一个
根为 Document 节点的 HTMLCollection ，其过滤器只匹配
script 元素。
getElementsByName(name)Support in all current engines.
getElementsByName(name)
方法接受一个字符串 name，必须返回一个 活的 NodeList，
包含该文档中
name 属性的值（大小写敏感地地）等于 name 参数
的所有 HTML 元素，按照 树序 排列。
在 Document 对象上再次使用同样的参数调用该方法时，用户代理可以返回之前调用时返回的对象。
其他情况必须返回一个新的 NodeList 对象。
currentScriptSupport in all current engines.
返回正在执行的 script 元素或 SVG script 元素，
只要该元素表示一个 经典的脚本。
对于再次进入的脚本，返回还没结束执行的脚本中最近开始执行的那个。
如果 Document 当前没有在执行 script 或 SVG script
（例如，因为运行的脚本是一个事件处理器或定时器），如果当前执行的
script 或 SVG script 元素表示一个 module script 则返回 null。
读取 currentScript 属性时，必须返回最近设置的值。
创建 Document 时，currentScript 必须初始化为 null。
实现和标准社区已经不再青睐这一 API 因为它会全局暴露
script 或 SVG script 元素。
因此它在新的环境中不再可用，比如执行 module scripts 时，
或者在 shadow tree 执行中执行脚本时。我们正在寻求一种解决方案，
在这些环境中识别正在执行的脚本，同时又不让它全局可用。参见 Issue：#1013。
Document 接口 支持命名属性。
任何时候 Document 对象 document 支持的属性名 包括
（根据贡献的元素按照树序 排列，忽略后续重复项，
对同一元素从 id 属性来的值排在从 name 属性来的值前面）：
在文档树中 root 为 document，
且 name 内容属性非空的所有 applet, exposed embed, form, iframe,
img, 和 暴露的 object 元素的 name 内容属性的值；
在文档树中 root 为 document，
且 id 内容属性非空的所有 applet 和 暴露的 object 元素
的 id 内容属性的值；
在文档树中 root 为 document，
且 id 和 name 内容属性均非空的所有 img 元素的
id 内容属性的值。
为了 确定 Document
的命名属性 name 的值，用户代理返回必须执行以下算法得到的值：
令 elements 为 在文档树中 root 为 document， 且名为 name 的 命名元素 列表。
按照定义，至少会有一个这样的元素。
如果 elements 只有一个元素且该元素为 iframe 元素，
且该 iframe 元素的 嵌套浏览环境 不是 null，
则返回该元素的 嵌套浏览环境 的 WindowProxy 对象。
否则，如果 elements 只有一个元素，返回该元素。
否则返回一个根为 Document 节点的 HTMLCollection，
其过滤器只匹配名为 name 的 命名元素。
上述算法中名为 name 的 命名元素 包括：
name 内容属性的值是 name 的
applet, exposed embed, form,
iframe, img, 或 暴露的 object 元素；id 内容属性的值为 name 的
applet 或 暴露的 object 元素；id 内容属性的值为 name 且 name 内容属性的值非空的
img 元素。如果 embed 或 object 元素没有 暴露的 object 祖先，
而且对于 object 元素没有显示它的 后备内容 或没有 object
或 embed 后代，就说它是 暴露的。
Document 接口上的 dir 属性
与 dir 内容属性定义在一起。
HTML 中的元素、属性和属性值（由本规范）定义有确定的含义（语义）。
例如，ol 元素表示一个有序列表，
lang 属性表示内容的语言。
这些定义允许诸如 Web 浏览器或搜索引擎的 HTML 处理器可能在作者不曾考虑过的 各种上下文中呈现和使用文档或应用。
作为一个简单的例子，考虑一个由只考虑了桌面电脑 Web 浏览器的作者编写的 Web 页面：
<!DOCTYPE HTML> <html lang="en"> <head> <title>My Page</title> </head> <body> <h1>Welcome to my page</h1> <p>I like cars and lorries and have a big Jeep!</p> <h2>Where I live</h2> <p>I live in a small hut on a mountain!</p> </body> </html>
因为 HTML 表达了 含义，而不是呈现，同样的页面也可以用于移动电话的小浏览器而不需要改动页面。 比如在桌面浏览器中标题使用大号字体，但在移动浏览器中可能整页使用同样大小的字体，但标题使用粗体。
这不仅仅是屏幕尺寸的差异：同一页面可以通过基于语音合成的浏览器同样地被盲人使用， 这时就不是在屏幕上显示页面，而是将页面朗读给用户，比如使用耳机。 标题可能不再是大号的文本，语音浏览器可能会对它使用不同的音量或较慢的声音。
带来的好处不止于此。由于浏览器知道页面的哪些部分是标题，它们可以创建文档大纲来使得用户可以快速浏览文档， 使用“跳转到下一个标题”或“跳转到上一个标题”的快捷键。 这些功能在语音浏览器中尤其常见，不然用户可能很难快速浏览页面。
甚至除浏览器之外的软件也可以利用这些信息。 搜索引擎可以使用标题更有效地对页面进行索引，或者从结果中提供到页面子部分的快速链接。 而工具可以使用标题来创建一个目录（实际上这个规范的目录就是这样生成的）。
这个例子集中在标题上，但是相同的原则也适用于 HTML 中的所有语义。
作者不得将元素，属性或属性值用于适当的语义目的之外的其他意图，因为这样做会使软件无法正确处理页面。
例如，以下代表公司网站标题的代码片段是不合规范的， 因为第二行不是子部分的标题，而是副标题（同一部分的下级标题）。
<body> <h1>ACME Corporation</h1> <h2>The leaders in arbitrary fast delivery since 1920</h2> ...
hgroup 元素适用于这些情况：
<body> <hgroup> <h1>ACME Corporation</h1> <h2>The leaders in arbitrary fast delivery since 1920</h2> </hgroup> ...
下一个例子中的文档类似，也不合规范。尽管语法是正确的，
因为放在单元格中的数据显然不是表格数据，而且误用了 cite 元素：
<!DOCTYPE HTML>
<html lang="en-GB">
<head> <title> Demonstration </title> </head>
<body>
<table>
<tr> <td> My favourite animal is the cat. </td> </tr>
<tr>
<td>
—<a href="https://example.org/~ernest/"><cite>Ernest</cite></a>,
in an essay from 1992
</td>
</tr>
</table>
</body>
</html>
这将使得依赖于这些语义的软件出错： 例如支持盲人浏览文档中的表格的语音浏览器将把上述引用误报为表格，这会使用户困惑； 同样地，从网页中提取作品标题的工具也会将“Ernest”提取为作品的标题，即使它实际上是一个人名而不是标题。
这个文档的更正版本可能是：
<!DOCTYPE HTML>
<html lang="en-GB">
<head> <title> Demonstration </title> </head>
<body>
<blockquote>
<p> My favourite animal is the cat. </p>
</blockquote>
<p>
—<a href="https://example.org/~ernest/">Ernest</a>,
in an essay from 1992
</p>
</body>
</html>
作者不得使用本规范或 其他适用规范 不允许的元素，属性或属性值，因为这样做会使以后的语言扩展变得更加困难。
下一个示例中有一个不符合规范的属性值（“carpet”）和不符合规范的属性（“texture”），这是本规范不允许的：
<label>Carpet: <input type="carpet" name="c" texture="deep pile"></label>
以下是标记这一内容的正确的替代方式：
：<label>Carpet: <input type="text" class="carpet" name="c" data-texture="deep pile"></label>
其 节点文档 不具有 浏览环境 的 DOM 节点 不受 HTML 语法 要求和 XML 语法 要求之外的所有文档一致性要求的限制。
特别地，template 元素的 模板内容
的 节点文档 没有浏览上下文。
例如，内容模型 要求
和属性值微语法要求不适用于
template 元素的 模板内容。
在此示例中，img 元素的属性值是占位符，如果在 template 元素外就是无效的。
<template>
<article>
<img src="{{src}}" alt="{{alt}}">
<h1></h1>
</article>
</template>
但是，如果上述标记省略了 </h1> 结束标记，将会违反
HTML 语法，会被符合性检查器标记为错误。
通过脚本和使用其他机制，在用户代理处理文档时，属性值，文本以及文档的整体结构都可能会动态变化。 某一时刻文档的语义由这一时刻的文档的状态来表示，因此文档的语义会随时间变化。 当这种情况发生时，用户代理 must 更新文档的显示。
HTML 有一个表示进度条的 progress 元素。
如果其 “value” 属性由脚本动态地更新，那么 UA 将更新呈现以显示进度变化。
在 DOM 中表示 HTML 元素 的节点 必须 实现本规范相关章节中的对应接口，并暴露给脚本。这包括 XML 文档 中的 HTML 元素， 即使那些文档在另外的上下文中（比如在一个 XSLT 转换中）。
DOM 中的元素 表示着 事物； 也就是说它们具有内在的 意义，也被称为语义。
例如一个 ol 元素表示一个有序列表。
元素可以被显式地或者隐式地 引用。
一种显式地引用 DOM 中元素的方式是给该元素一个 id 属性，
然后创建一个 超链接，
用那个 id 属性值作为该
超链接 的 href 属性值的
片段。其实引用不需要超链接的存在，任何其他引用元素的方式都是足够的。
<figure id="module-script-graph">
<img src="module-script-graph.svg"
alt="Module A depends on module B, which depends
on modules C and D.">
<figcaption>Figure 27: a simple module graph</figcaption>
</figure>
As we can see in <a href="#module-script-graph">figure 27</a>, ...
"As depicted in the figure of modules A, B, C, and D..."
"In Figure 27..." (without a hyperlink)
"From the contents of the 'simple module graph' figure..."
"In the figure below..." (but this is discouraged)
所有 HTML 元素 的接口都继承自一个基本接口，没有额外要求的元素必须使用这一接口。这就是 HTMLElement 接口。
Support in all current engines.
Support in all current engines.
[Exposed=Window,
HTMLConstructor]
interface HTMLElement : Element {
// 元数据属性
[CEReactions] attribute DOMString title;
[CEReactions] attribute DOMString lang;
[CEReactions] attribute boolean translate;
[CEReactions] attribute DOMString dir;
[SameObject] readonly attribute DOMStringMap dataset;
// 用户交互
[CEReactions] attribute boolean hidden;
void click();
[CEReactions] attribute long tabIndex;
void focus(optional FocusOptions options);
void blur();
[CEReactions] attribute DOMString accessKey;
readonly attribute DOMString accessKeyLabel;
[CEReactions] attribute boolean draggable;
[CEReactions] attribute boolean spellcheck;
[CEReactions] attribute DOMString autocapitalize;
[CEReactions] attribute [TreatNullAs=EmptyString] DOMString innerText;
};
HTMLElement includes GlobalEventHandlers;
HTMLElement includes DocumentAndElementEventHandlers;
HTMLElement includes ElementContentEditable;
// 注意：有意 不是 [HTMLConstructor]
[Exposed=Window]
interface HTMLUnknownElement : HTMLElement { };
HTMLElement 接口保存着很多不同功能的方法和属性，因此该接口的成员在本规范的不同部分中描述。
HTML 命名空间 中名为 name 的元素的 元素接口 这样决定：
如果 name 是 applet，bgsound，blink，isindex，
keygen， multicol， nextid， 或 spacer，
则返回 HTMLUnknownElement。
如果 name 是 acronym， basefont， big，
center， nobr， noembed， noframes，
plaintext， rb， rtc， strike，或
tt，则返回 HTMLElement。
如果 name 是 listing 或 xmp，则返回
HTMLPreElement。
否则，如果本规范定义了一个 local name name 对应的 元素类型 的接口，则返回那个接口。
如果 其他适用规范 为 name 定义了一个合适的接口， 则返回他们定义的接口。
如果 name 是一个 合法的 Custom Element 名字，
则返回HTMLElement。
在 合法的 Custom Element 名 时，
使用 HTMLElement 而不是 HTMLUnknownElement 是为了确保任何将来可能的
升级 只造成元素原型链的线性转变，
从 HTMLElement 到一个子类，而不是后者从 HTMLUnknownElement 到一个不相关的子类。
HTML 和 SVG 元素共同的特性使用 HTMLOrSVGElement 接口 mixin: [SVG]
interface mixin HTMLOrSVGElement {
[SameObject] readonly attribute DOMStringMap dataset;
attribute DOMString nonce;
[CEReactions] attribute long tabIndex;
void focus(optional FocusOptions options);
void blur();
};
HTMLElement includes HTMLOrSVGElement;
SVGElement includes HTMLOrSVGElement;
为支持 Custom Element 特性，
所有 HTML 元素都有特定的构造器行为。该行为 [HTMLConstructor] IDL 扩展属性 表示。
它指示着给定接口的接口对象将在调用时具有特定的行为，如下所述：
[HTMLConstructor] 扩展属性必须接受无参数，
不得出现在非接口的对象上。它在接口上必须只出现一次，且该接口不得以 [Constructor]
或 [NoInterfaceObject] 扩展属性标注。（但是该接口可以用
[NamedConstructor] 标注，这里没有冲突。）它不得作为回调接口使用。
以 [HTMLConstructor] 扩展属性标注的接口的
接口对象 必须作为相应 JavaScript 函数对象的 [[Call]] 和 [[Construct]]
调用的函数体行为运行以下步骤。从 [[Call]] 调用时，NewTarget 值未定义，因此下面的算法将立即抛出异常。
从 [[Construct]] 调用时，[[Construct]] 的 newTarget 参数提供了 NewTarget 值。
令 registry 为 当前全局对象 的
CustomElementRegistry 对象。
如果 NewTarget 等于 活动的函数对象，则抛出一个
TypeError 并中止这些步骤。
这在使用 元素接口 作为构造器定义 Custom Element 时可能发生：
customElements.define("bad-1"，HTMLButtonElement);
new HTMLButtonElement(); // (1)
document.createElement("bad-1"); // (2)
在这种情形下，HTMLButtonElement（可能是显式地，比如 1，也可能是隐式地，比如 2）的执行期间，
活动的函数对象 和 NewTarget 都是 HTMLButtonElement。
如果没有这项检查，就可能会创建一个 local name 是 bad-1 的 HTMLButtonElement 实例。
令definition 为 registry 中 constructor 等于 NewTarget 的条目。
如果没有这样一个定义，则抛出一个 TypeError 并中止这些步骤。
由于 registry 中没有 constructor 是 undefined 的条目， 这一步可以防止 HTML 元素构造器作为函数被调用（在该情况下 NewTarget 将是 undefined）。
如果 definition的 local name 等于 definition的 name （即 definition 是一个 自主的 Custom Element），则：
如果 活动的函数对象 不是 HTMLElement，则抛出一个
TypeError 并中止这些步骤。
当 Custom Element 定义时没有扩展任何 local name 但继承了非 HTMLElement 类时就会发生这种情况：
customElements.define("bad-2"，class Bad2 extends HTMLParagraphElement {});
这种情形下，在（隐含地）在创建 Bad2 实例的 super() 调用期间，
活动的函数对象 是 HTMLParagraphElement 而不是 HTMLElement。
否则（如果 definition 是一个 定制的内建元素）：
令valid local name 为本规范 或 其他适用规范 中定义的 使用 活动的函数对象 作为 元素接口 的元素的 local name 列表。
如果 valid local names 不包含 definition 的 local name，则抛出一个
TypeError 并中止这些步骤。
这在定义扩展了给定 local name 但继承了错误的类的 Custom Element 时会发生：
customElements.define("bad-3"，class Bad3 extends HTMLQuoteElement {}，{ extends： "p" });
这种情形下，在（隐含地）在创建 Bad3 实例的 super() 调用期间，
valid local names 列表包含 q 和 blockquote，但 definition 的 local name 是 p，不在这个列表中。
令prototype 为 Get(NewTarget，"prototype")。 重新抛出任何异常。
如果 Type(prototype) 不是 Object，则：
令 realm 为 GetFunctionRealm(NewTarget)。
活动函数对象 的 realm 可能不是 realm，所以我们用更通用的概念 "Realms 中同样的接口" 而不寻找 相等的 接口对象。设计这个退化行为是为了匹配 JavaScript 内建对象的类似行为， 包括使用 NewTarget 的 Realm 以及在那里查找合适的原型。
如果 definition 的 构造栈 为空，则：
令 element 为实现了
活动函数对象 对应接口的新元素，
没有属性，命名空间设为 HTML 命名空间，local name 设为 definition 的 local name，节点文档
设为 当前全局对象的 关联 Document。
执行 element.[[SetPrototypeOf]](prototype)。重新抛出任何异常。
设置 element的 Custom Element 状态 为 "custom"。
设置 element的 Custom Element 定义 为 definition。
返回 element。
当作者脚本直接构建了一个新的 Custom Element 时（例如通过 new MyCustomElement()），就会发生这种情况。
令 element 为 definition 的 构造栈 中的最后一个条目。
如果 element 是一个 已经构造 标记，
则抛出一个 "InvalidStateError" DOMException 并中止这些步骤。
Custom Element 构造器 中的作者代码
在调用 super() 之前
不合规范地 创建了
正在创建的类的另一个实例时，就会发生这种情况：
let doSillyThing = false;
class DontDoThis extends HTMLElement {
constructor() {
if (doSillyThing) {
doSillyThing = false;
new DontDoThis();
// 现在构造栈将会包含一个 已经构造 标记。
}
// 这将会失败并抛出 "InvalidStateError" DOMException：
super();
}
}
当 Custom Element 构造器 中的作者代码
不合规范地 调用两次 super() 时也会发生。
因为根据 JavaScript 规范，这确实会在抛出异常前执行两次父类构造器（即当前这个算法）：
class DontDoThisEither extends HTMLElement {
constructor() {
super();
// 这将会抛出异常，但这时已经调用进入了 HTMLElement 构造器
super();
}
}
执行 element.[[SetPrototypeOf]](prototype)。重新抛出任何异常。
返回 element。
当 升级 Custom Element 时就会自然地进入这一步，
返回现有元素，以便 Custom Element 构造器 中的
super() 调用将该现有元素赋值给 this。
除了 [HTMLConstructor] 隐含的构造器行为之外，
有些元素还有 命名的构造器（其实是改过 prototype 属性的工厂函数）。
当定义 Custom Element 构造器 时，
HTML 元素的命名构造器也可以用于 extends 子句中：
class AutoEmbiggenedImage extends Image {
constructor(width，height) {
super(width * 10，height * 10);
}
}
customElements.define("auto-embiggened"，AutoEmbiggenedImage，{ extends： "img" });
const image = new AutoEmbiggenedImage(15，20);
console.assert(image.width === 150);
console.assert(image.height === 200);
在本规范中的每个元素都有一个定义，包含以下信息：
该元素可以在哪里使用的 非规范化的 描述。 该信息与允许该元素为子元素的元素内容模型之间存在冗余，只是为了方便而提供。
必须包含哪些内容作为子元素和后代元素的规范性描述。
非规范性的 描述：在 text/html 语法中，
是否可以忽略 开始 和 结束 标签。
该信息与 可选标签 部分给出的规范性要求之间存在冗余，
在元素定义中提供只是为了方便。
可以在该元素上指定的（除了不允许的情况）一个规范性的属性列表， 以及对这些属性的非规范性的描述。（破折号左侧是规范性的，右侧是非规范性的。）
对于使用方：遵循 ARIA in HTML 定义的
ARIA role 和 aria-* 属性的要求。
[ARIA] [ARIAHTML]
对于实现方：用户代理实现可访问性 API 语义的要求定义在 HTML Accessibility API Mappings。[HTMLAAM]
一个关于这样的元素必须实现的 DOM 接口的规范性的定义。
之后是更多的描述：元素所 代表 的内容， 以及可能适用于作者 和实现 的任何其他规范性的一致性标准，有时也包括实例。
一个属性值是一个字符串。 除非另有说明，HTML 元素 上的属性值可以是任何字符串值， 包括空字符串，而且对这些属性值中可以指定的文本没有任何限制。
本标准中定义的每个元素都有内容模型：描述了元素期望的 内容。 HTML 元素 的内容必须匹配元素内容模型的要求。 元素的 内容 是它在 DOM 中的子节点。
元素之间总是允许 ASCII 空白。用户代理将源码标记中元素之间的字符表示为
DOM 中的 Text 节点。 空的 Text 节点和
只包含那些字符序列的 Text 节点都被认为是 元素间空白。
当决定元素内容是否匹配元素内容模型的要求时必须忽略 元素间空白， 注释节点，以及处理指令节点。 在执行定义文档和元素语义的算法时也必须忽略这些节点。
因此，元素 A 与元素 B 相邻 是指
A 和 B 有相同的父节点且它们之间没有元素节点或 Text 节点
（元素间空白 除外）。
类似地，元素的 唯一子节点 是指其父元素只包含 元素箭空白，
注释节点，以及处理指令节点。
作者不得随意使用 HTML 元素，除非明确允许（每个元素都有定义）， 或其他规范有明确要求。对于 XML 复合文档，如果这些元素定义为提供相关上下文，那么这些上下文可以位于其他命名空间的元素之内。
例如 Atom 规范定义了一个 content 元素。当它的
type 属性值为 xhtml 时，Atom
规范要求它包含单个 HTML div 元素。因此该上下文中允许出现一个a
div 元素，虽然本规范没有明确地声明这一点。[ATOM]
此外，HTML 元素 可能是孤立节点（即没有父节点）。
例如创建一个 td 元素并把它存储为脚本中的全局变量是符合规范的，即使
td 元素只期望用于 tr 元素中。
var data = {
name： "Banana"，
cell： document.createElement('td')，
};
当元素的内容模型是 nothing 时，
该元素不得包含 Text 节点（元素间空白 除外）
和元素节点。
方便起见，多数内容模型为 “nothing” 的 HTML 元素同时也是 void 元素 （HTML 语法 中没有 结束标签 的元素）。 但是，这些是完全独立的概念。
HTML 中的每个元素都属于一个或多个 类别 一个类别中的元素有着相似的特点。本规范使用以下几个大类：
有些元素属于其他类别，这些类别定义在本规范的其他部分。
这些类别之间的关系如下：
章节内容，标题内容，短语内容，嵌入内容和交互式内容都是流式内容。元数据有时也是流式内容。 元数据和交互式内容有时是短语内容。嵌入内容也是短语内容，有时也是交互式内容。
有时为了特定意图也会使用其他的类别，比如为了定义通用要求，为表单控件指定了多种类别。 有些元素有独特的要求，不属于任何特定类别。
元数据内容 是设置其余内容的呈现或行为， 设置文档与其他文档的关系，传达其他“带外”信息的内容。
其语义主要与元数据相关的其他命名空间的元素（比如 RDF） 也是 元数据内容。
因此在 XML 序列化中，可以像这样使用 RDF：
<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:r="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xml:lang="en">
<head>
<title>Hedral's Home Page</title>
<r:RDF>
<Person xmlns="http://www.w3.org/2000/10/swap/pim/contact#"
r:about="https://hedral.example.com/#">
<fullName>Cat Hedral</fullName>
<mailbox r:resource="mailto:hedral@damowmow.com"/>
<personalTitle>Sir</personalTitle>
</Person>
</r:RDF>
</head>
<body>
<h1>My home page</h1>
<p>I like playing with string, I guess. Sister says squirrels are fun
too so sometimes I follow her to play with them.</p>
</body>
</html>
然而，这在 HTML 序列化中是不可能的。
多数文档和应用的 body 中的元素都归类为 流式内容。
aabbraddressarea （如果是 map 元素的后代）articleasideaudiobbdibdoblockquotebrbuttoncanvascitecodedatadatalistdeldetailsdfndialogdivdlemembedfieldsetfigurefooterformh1h2h3h4h5h6headerhgrouphriiframeimginputinskbdlabellink （如果 在 body 中允许）mainmapmarkmathmenumeta （如果 itemprop 属性存在）meternavnoscriptobjectoloutputppicturepreprogressqrubyssampscriptsectionselectslotsmallspanstrongsubsupsvgtabletemplatetextareatimeuulvarvideowbr每个 章节内容 元素可能具有标题和 大纲。 有关详细信息请参阅 标题和章节。
有些元素属于 章节根元素。 它们与 章节内容 不同，但也可以有 大纲。
标题内容 定义了章节的标题 （可以显式地用 章节内容 元素标记， 也可以隐式地用标题内容自己来标记）。
短语内容 是文档中的文本， 以及标记该文本的段内级别的元素。 多个 短语内容 形成 段落。
aabbrarea （如果是 map 元素的后代）audiobbdibdobrbuttoncanvascitecodedatadatalistdeldfnemembediiframeimginputinskbdlabellink （如果 在 body 中允许）mapmarkmathmeta （如果 itemprop 属性存在）meternoscriptobjectoutputpictureprogressqrubyssampscriptselectslotsmallspanstrongsubsupsvgtemplatetextareatimeuvarvideowbr多数短语内容元素只能包含短语内容元素，而不是任何流式内容元素。
内容模型中的 文本内容 意味着什么都没有，
或者有 Text 节点。有时 文本内容 自身就是
一种内容模型，但它也是 短语内容，
也可以是 元素间空白
（如果 Text 节点为空或者只包含 ASCII 空白）。
Text 节点和属性值必须由
标量值 组成，不包含 非字符，
以及除 ASCII 空白 之外的 控制字符。
本规范对 Text 节点的值和属性的值，基于其确切的上下文会有额外的约束。
嵌入内容 是指 引入其他资源的内容，或插入到文档的其他词汇表的内容。
来自除 HTML 命名空间 之外的，表达内容而不是元数据的元素， 是 嵌入内容，用于本规范定义的内容模型 （例如 MathML 或 SVG。）
有些嵌入内容元素可以包含 fallback 内容： 在不能使用外部资源时使用的内容（例如，因为它是不受支持的格式）。 如果有 fallback 内容的话，元素定义会加以说明。
交互式内容 是专门用于用户交互的内容。
a （如果 href 属性存在）audio （如果 controls 属性存在）buttondetailsembediframeimg （如果 usemap 属性存在）input （如果 type 属性 不处于 状态）labelobject （如果 usemap 属性存在）selecttextareavideo （如果 controls 属性存在）作为一般规则，其内容模型允许任何 流式内容 或 短语内容 的元素应在其 内容 中至少包含一个 可感知内容 的节点，并且它没有指定 属性。
可感知内容
通过提供一些非空的后代 文本，
可以听到（audio 元素）或看到（video、img 或 canvas 元素），
或可以与之交互的内容（例如可交互的表单控件）来让一个元素变得非空。
但这个要求并不是一个硬性要求，因为有许多情况下空元素是合理的，比如当它被用作一个占位符之后用脚本填充， 或者对于模板元素，大多数页面都会填充，只有某些页面表现为空白，这也无关紧要。
推荐一致性检查工具为作者提供工具，来定位不遵守该要求的元素，帮助作者的代码编写。
下列元素是可感知内容：
aabbraddressarticleasideaudio （如果存在 controls 属性）bbdibdoblockquotebuttoncanvascitecodedatadetailsdfndivdl （如果元素的子节点至少包含一个键值对）emembedfieldsetfigurefooterformh1h2h3h4h5h6headerhgroupiiframeimginput （如果 type 属性 不处于 状态）inskbdlabelmainmapmarkmathmenu （如果元素的子节点至少包含一个 li 元素）meternavobjectol （如果元素的子节点至少包含一个 li 元素）outputppreprogressqrubyssampsectionselectsmallspanstrongsubsupsvgtabletextareatimeuul （如果元素的子节点至少包含一个 li 元素）varvideo脚本支持元素 是指那些本身不 表示 任何东西（即它们不被渲染），但是被用于支持脚本， 为用户提供功能。
下列元素是脚本支持元素：
有些元素被描述为 透明的；它们的内容模型描述中有 "透明" 一词。 透明 元素的内容模型从其父元素的内容模型派生： “透明”的内容模型中某一部分所要求的元素与父元素的内容模型中对应部分所要求的元素相同。
在有些情况下，透明元素互相嵌套时，必须迭代地应用这个过程。
考虑下面的标记片段：
<p><object><param><ins><map><a href="/">Apples</a></map></ins></object></p>
为了检查 a 元素中是否允许 "Apples"，要检查内容模型。
a 元素的内容模型是透明的，map 元素也是，
ins 元素也是，object 元素中 ins 元素所在的部分也是。
object 元素位于 p 元素中，后者的内容模型是 短语内容。
因此 "Apples" 是允许的，因为文本是短语内容。
当透明元素秘佑父元素时，它的内容模型中透明的那一部分必须被当做可以接受任何 流式内容。
本小节定义的术语 段落 的用途不止是
p 元素的定义。这里定义的 段落 概念用于描述如何解释文档。
p 元素仅仅是标记 段落 的一种方式。
一个 段落 通常是由一系列 短语内容 形成的一个文本块， 在排版中可能包含一个或多个句子，讨论一个特定的主题（也可以用于更一般的专题分组）。 例如，一个地址也可以是一个段落，可以是表单的一部分，一行署名，或者一首诗中的一节。
下面的例子中，一个章节中有两段话。还有一个标题，它包含的短语内容不构成段落。 注意注释和 元素间空白 也不构成段落。
<section> <h1>Example of paragraphs</h1> This is the <em>first</em> paragraph in this example. <p>This is the second.</p> <!-- This is not a paragraph. --> </section>
流式内容 中的段落相对于文档的样子来定义，与
a，ins，del，和 map 元素造成的复杂情况无关。
因为那些元素的混合内容模型使它们可以跨越段落边界，就像下面两个例子那样。
通常来讲，最好避免让元素跨越段落边界。这样的标记很难维护。
下面的例子取自前一个例子中的标记，在某些标记处加入了 ins 和 del 元素，
用来展示文本的变更（尽管这种情况下，这一变更确实没啥意义）。注意这个例子中的段落与上一个例子完全相同，
尽管插入了 ins 和 del 元素 — ins 元素跨越了标题和第一个段落，
del 元素跨越了两个段落的边界。
<section> <ins><h1>Example of paragraphs</h1> This is the <em>first</em> paragraph in</ins> this example<del>. <p>This is the second.</p></del> <!-- This is not a paragraph. --> </section>
令 view 为 DOM 的一个视图，将文档中所有的 a,
ins, del, 和 map 元素替换为它们的
内容。然后对 view 中除了接受
短语内容 还接受非 短语内容
的元素中的每一个没有被其他类型内容打断的 短语内容 节点序列，
令 first 为序列中第一个节点，令 last 为序列中最后一个节点。
对每一个上述序列中，包含至少一个非
嵌入内容 或 元素间空白
节点的序列，在原始 DOM 中从 first 前面到 last 后面形成了一个段落。（因此段落可以横跨 a,
ins, del, 和 map 元素。）
一致性检查器可以在段落相互重叠时警告作者（
在 object, video, audio, 和 canvas 元素中有可能发生这种情况，
允许再次嵌入 HTML 的其他命名空间的元素也可能间接地发生这种情况，比如 SVG svg 或 MathML
math）。
p 元素可以用于包裹普通的短语内容来形成相互独立的段落。
在下面的例子中，链接横跨了半个第一个段落，分离两个段落的标题，以及半个第二个段落。它同时横跨了段落和标题。
<header> Welcome! <a href="about.html"> This is home of... <h1>The Falcons!</h1> The Lockheed Martin multirole jet fighter aircraft! </a> This page discusses the F-16 Fighting Falcon's innermost secrets. </header>
这是上述标记的另一种写法，这次显式地声明了段落，将一个链接分割成三部分：
<header> <p>Welcome! <a href="about.html">This is home of...</a></p> <h1><a href="about.html">The Falcons!</a></h1> <p><a href="about.html">The Lockheed Martin multirole jet fighter aircraft!</a> This page discusses the F-16 Fighting Falcon's innermost secrets.</p> </header>
当使用某些元素来定义 fallback 内容时，段落可以重叠。例如下面的章节：
<section> <h1>My Cats</h1> You can play with my cat simulator. <object data="cats.sim"> To see the cat simulator, use one of the following links: <ul> <li><a href="cats.sim">Download simulator file</a> <li><a href="https://sims.example.com/watch?v=LYds5xY4INU">Use online simulator</a> </ul> Alternatively, upgrade to the Mellblom Browser. </object> I'm quite proud of it. </section>
共有5个段落：
object 元素。第一个段落与后面4个重叠。支持 "cats.sim" 资源的用户代理只会显示第一个段落，但显示 fallback 的用户代理就会 将第一个段落的第一句话与第二段显示为一个段落，最后一个段落会显示在第一个段落的第二个句子之前。
为避免困惑，可以显式地使用 p 元素。例如：
<section> <h1>My Cats</h1> <p>You can play with my cat simulator.</p> <object data="cats.sim"> <p>To see the cat simulator, use one of the following links:</p> <ul> <li><a href="cats.sim">Download simulator file</a> <li><a href="https://sims.example.com/watch?v=LYds5xY4INU">Use online simulator</a> </ul> <p>Alternatively, upgrade to the Mellblom Browser.</p> </object> <p>I'm quite proud of it.</p> </section>
下列属性是通用的，所有 HTML 元素 都可以指定。 （即使是本规范中没有定义的那些元素）：
accesskeyautocapitalizeautofocuscontenteditabledirdraggableenterkeyhintisitemiditempropitemrefitemscopeitemtypelangspellcheckstyletabindextitletranslate本规范定义的这些属性只用于 HTML 元素 的属性。 当本规范引用具有这些属性的元素时， 没有定义这些属性的命名空间下的元素不得当作具有这些属性的元素处理。
例如，在下面的 XML 片段中，根据本标准的定义 "bogus" 元素没有
dir 属性，尽管它字面上有一个名为
"dir" 的属性。因此，最里面的 span 元素的
方向 是 'rtl'，
通过 "bogus" 元素间接地继承自 div 元素。
<div xmlns="http://www.w3.org/1999/xhtml" dir="rtl">
<bogus xmlns="https://example.net/ns" dir="ltr">
<span xmlns="http://www.w3.org/1999/xhtml">
</span>
</bogus>
</div>Support in all current engines.
WHATWG DOM 标准为任何命名空间下的任何元素的
class，
id，和
slot 属性定义了用户代理的要求。[DOM]
class，id，和 slot 属性可以在任何 HTML 元素 上指定。
当 HTML 元素 上指定了 class
属性时，该属性的值必须是 一组空格分隔的令牌，
表示该元素所属的各种类。
设置元素的类会影响 CSS 选择符的匹配，DOM 的
getElementsByClassName() 方法，
以及其他这类特性。
作者在 class 属性中可以使用的令牌没有额外的限制，
但鼓励作者使用描述内容本质的取值，而不是描述内容表示的取值。
当 HTML 元素 上指定了 id 属性时，
该属性的值必须在该元素所在的 树 的所有 IDs 中是唯一的，
必须至少包含一个字符，且不得包含任何 ASCII 空白。
对 ID 的形式没有其他限制；ID 甚至可以只包含数字，以数字起始，以下划线起始，或者只包含标点等等。
元素的 唯一标识符 有很多用途， 最值得一提的是通过 URL 片段 链接到文档的特定部分， 在脚本中指向元素的一种方式，以及从 CSS 中指定特定元素的样式。
标识符是不透明的字符串。不得通过 id 属性的值来推断特定的含义。
slot 属性用于为元素
设置一个槽：具有 slot 属性的元素会被
指定到
name 属性值匹配该 slot 属性值的
slot 元素创建的 slot 中。
— 但条件是 slot 元素所在的 shadow tree 的
root 的 host 有相应的 slot 属性值。
为了使辅助技术产品提供相比于 HTML 元素和属性更加细粒度的界面，有一组
用于辅助技术产品的注解 可以指定
（ARIA role 和 aria-* 属性)。[ARIA]
下面的 事件处理器内容属性 可以在任何 HTML 元素 上指定：
onabortonauxclickonblur*oncanceloncanplayoncanplaythroughonchangeonclickoncloseoncontextmenuoncopyoncuechangeoncutondblclickondragondragendondragenterondragleaveondragoverondragstartondropondurationchangeonemptiedonendedonerror*onfocus*onformdataoninputoninvalidonkeydownonkeypressonkeyuponload*onloadeddataonloadedmetadataonloadstartonmousedownonmouseenteronmouseleaveonmousemoveonmouseoutonmouseoveronmouseuponpasteonpauseonplayonplayingonprogressonratechangeonresetonresize*onscroll*onseekedonseekingonselectonslotchangeonstalledonsubmitonsuspendontimeupdateontoggleonvolumechangeonwaitingonwheel用星号标记的属性在用于 body 元素时有不同的含义，
因为该元素暴露了 Window 对象上同名的 事件处理器。
虽然这些属性适用于所有元素，但并不是对所有元素都有用。
例如，只有 媒体元素 才会接收到用户代理触发的
音量变化 事件。
任何 HTML 元素 都可以指定
自定义数据属性 （例如 data-foldername 或 data-msgid），
用于存储页面相关的自定义数据，状态，注解这样的东西。
在 HTML 文档 中，HTML 命名空间
中的元素可以指定一个 xmlns 属性，但它的值只能是
"http://www.w3.org/1999/xhtml" 。这不适用于 XML 文档。
在 HTML 中，xmlns 属性完全没有效果，它只是一个标志。
它的存在只是为了方便 XML 与 HTML 之间的迁移。
被 HTML 解析器 解析时，该属性的结果是没有命名空间，
而不是像 XML 中的命名空间声明那样，解析为 "http://www.w3.org/2000/xmlns/" 命名空间。
在 XML 中，xmlns 属性是命名空间机制的一部分，
没有指定命名空间时，元素不能有 xmlns 属性。
在 XML 文档 中的任何元素上，XML 也遵循
XML 命名空间 中的 xml:space 属性的使用。
该属性在 HTML 元素 上没有作用，因为 HTML 的默认行为就是保留空白。 [XML]
在 text/html 语法中，无法序列化
HTML 元素 的 xml:space 属性。
title 属性title 属性 表示
元素的建议信息，比如适用于工具提示。
在链接上，这可能是目标资源的标题或描述；在图片上，它可以是图片来源或图片的描述；
在段落上，它可能是文本的脚注或评论；在引用上，可以是来源的进一步信息；
在 交互式内容 上，它可能是元素的标签或使用说明；
等等。值为文本。
目前不鼓励依赖于 title 属性，
因为许多用户代理不按照本规范中要求的可访问的方式来暴露该属性
（例如，要求像鼠标这样的定点设备来触发工具提示，这排除了纯键盘用户和触摸设备用户，比如手机或平板电脑的用户）。
如果从元素中省略此属性，则默认关联到离该元素最近的设置了 title 属性的
祖先 HTML 元素。
设置该属性可以覆盖这一行为，显式地声明了任何祖先的建议信息与此元素无关。
将属性设置为空字符串表示该元素没有建议信息。
如果 title 属性值包含 U+000A LINE FEED （LF）字符，
其内容将被分割为多行。每个 U+000A LINE FEED （LF） 字符表示一个换行。
在 title 属性中使用换行要多加小心。
比如下面的代码片段其实定义了一个缩写的解释，其中包含一个换行：
<p>My logs show that there was some interest in <abbr title="Hypertext Transport Protocol">HTTP</abbr> today.</p>
有些元素比如 link， abbr，和 input
为 title 属性定义了额外的语义。
元素的 建议信息 是下面算法的返回值， 只要有值返回算法就中止。当该算法返回空字符串时，没有任何建议信息。
如果该元素是一个 link， style， dfn，或
abbr 元素，那么：如果该元素有 title 属性，
返回该属性的值，否则返回空字符串。
否则，如果该元素有一个 title 属性则返回它的值。
否则，如果该属性有父元素，则返回父元素的 建议信息。
否则，返回空字符串。
当元素有 建议信息 时， 用户代理应该通知用户，不然就无法发现该信息。
Support in all current engines.
lang 和 xml:lang 属性Support in all current engines.
lang 属性（没有命名空间）指定了该元素内容，
以及该元素包含文本的属性的主要语言。它的值必须是一个合法的 BCP 47 语言标签，
或者空字符串。将该属性设置为控制付出表示主要语言是未知的。 [BCP47]
在 XML 命名空间 中的
lang 属性
定义在 XML 中。 [XML]
如果元素省略了这些属性，该元素的语言就是其父元素的语言（如果有的话）。
没有命名空间的 lang 属性可以用于任何 HTML 元素。
在 XML 命名空间 中的
lang 属性 可以用于 XML 文档 中的
HTML 元素，
也可以用户其他命名空间中的元素，只要相关规范允许（特别低，
MathML 和 SVG 允许其元素上指定 XML 命名空间
中的lang 属性 ）。如果同时在同一个元素上指定了
无命名空间的 lang 属性，和
XML 命名空间 中的 lang 属性，
它们必须有 ASCII 大小写不敏感的 同样的值。
作者不得在 HTML 文档 中的
HTML 元素 上使用
XML 命名空间 中的 lang 属性。
为了便于与 XML 之间迁移，作者可以在
HTML 文档 中的 HTML 元素 上
指定一个没有命名空间的，没有前缀的属性，其 localname 为 "xml:lang"，
但指定该属性时必须同时指定一个无命名空间的 lang 属性，
它们必须有 ASCII 大小写不敏感的 同样的值。
没有命名空间没有前缀的 localname 是 "xml:lang" 的属性对语言处理没有任何影响。
确定节点的 language，用户代理必须寻找设置了
XML 命名空间 中的 lang 属性
或无命名空间的 lang 属性的，
最近的祖先元素（包括该元素自身，如果该节点是元素的话）。
该属性指定了节点的语言（不管它的值是什么）。
如果同时在同一个元素上指定了
无命名空间的 lang 属性，和
XML 命名空间 中的 lang 属性，
在确定元素的语言时，用户代理必须使用
XML 命名空间 中的 lang 属性，
必须 忽略 无命名空间的 lang 属性。
如果节点的 包括自身在内的祖先 两个属性都没设置， 但是设置了一个 编译指示的默认语言，那么这就是该节点的语言。 如果没有设置 编译指示的默认语言， 那么必须使用上级协议（比如 HTTP）的语言信息（如果有的话）作为最终的 fallback 语言。 如果没有该语言信息，或者上级协议报告了多种语言，该节点的语言就是未知的，对应的语言标签是空字符串。
如果结果值不是可识别的的语言标签，那么它必须被视为具有给定语言标签的未知语言，区别于所有其他语言。 为了与预期该语言标签的其他服务进行往返或通信，用户代理应通过未修改的方式传递未知语言标签， 并将其标记为 BCP 47 语言标签，以防后续服务将数据解释为另一种语言描述。 [BCP47]
因此，例如具有 lang="xyzzy"
的元素将被选择器 :lang(xyzzy) 匹配（例如在 CSS 中），
但是它不会被 :lang(abcde) 匹配，虽然两者都是无效的。
类似地，如果要 Web 浏览器和屏幕阅读器一致地传达关于元素的语言，
则浏览器将告诉屏幕阅读器该语言是 “xyzzy”，即使它知道这个语言是无效的，
以防屏幕阅读器实际上支持该标签的语言。即使屏幕阅读器支持 BCP 47 和用于编码语言名称的另一语法，
比如在这一语法中字符串 “xyzzy” 表示白俄罗斯语言，那么屏幕阅读器将会 错误地 把文本当做白俄罗斯语言处理，
因为 “xyzzy” 不是 BCP 47 规范中描述的白俄罗斯语言（BCP 47 使用 “be” 表示白俄罗斯语言）。
如果结果值是空字符串，那么必须把它解释为该节点的语言显式地声明为未知。
用户代理可以使用元素的语言来决定适当的处理和渲染（例如选择合适的字体和标点， 选择合适的字典，或者用于表单控件的 UI 比如日期选择器）。
Support in all current engines.
lang IDL 属性必须 反映
无命名空间的 lang 内容属性。
translate 属性translate 属性是一个
枚举属性，用于指定元素在本地化时应该翻译其属性值和
Text 子节点的值，还是让它们保持不变。
该属性的关键字是空字符串，yes，和 no。
空字符串和 yes 关键字映射为 yes 状态。
no 关键字映射为 no 状态。
此外还有第三种状态，inherit 状态，这是 缺失值默认
（以及 非法值默认）。
每个元素（甚至非 HTML 元素）都有 翻译模式，处于
启用翻译 状态或者 不翻译 状态。
如果 HTML 元素 的 translate
属性处于 yes 状态，那么该元素的 翻译模式 处于
启用翻译 状态；否则该元素的 translate
属性处于 no 状态，那么该元素的 翻译模式 处于
不翻译 状态。 否则，元素的 translate
属性处于 inherit 状态，或者该元素不是 HTML 元素
因此没有 translate 属性；任何一种情况下该元素的
翻译模式 都将与其父元素处于同样的状态（如果有父元素的话），
否则该元素是一个 文档元素，则处于 启用翻译 状态。
当元素处于 启用翻译 状态时，该元素的
可翻译的属性 及其 Text 子节点的值将会在页面本地化时进行翻译。
当元素处于 不翻译 状态时，该元素的属性值及其 Text
子节点的值将会在页面本地化时保留原样。例如因为该元素包含人名或者计算机程序名。
下面的属性是 可翻译属性：
th 元素上的 abbrarea，img，
和 input 元素上的 alt
meta 元素上的 content，
如果 name 属性指定的元数据名对应的值已知可翻译的话a 和 area 元素上的 download optgroup，option，
和 track 元素上的 label
lang；
必须经过 "翻译" 来匹配翻译中使用的语言input 和 textarea 元素上的
placeholder
iframe 元素上的 srcdoc；必须进行解析和递归处理style；必须进行解析和递归处理
（例如 'content' 属性的值）titletype 属性处于 按钮 状态
或 重置按钮 状态的 input 元素
上的 value其他规范可能会定义其他 可翻译的属性。例如 ARIA 可能会把 aria-label 属性定义为可翻译的。
在读取时，translate IDL 属性必须返回 true
如果元素的 翻译模式 是 启用翻译；
否则返回 false。当设置时，它必须设置该内容属性的值为 "yes" 如果新值为 true；
否则设置该内容属性的值为 "no"。
在这个例子中，在本地化时文档中的所有东西都会被翻译，除了示例键盘输入和示例程序输出：
<!DOCTYPE HTML> <html lang=en> <!-- default on the document element is translate=yes --> <head> <title>The Bee Game</title> <!-- implied translate=yes inherited from ancestors --> </head> <body> <p>The Bee Game is a text adventure game in English.</p> <p>When the game launches，the first thing you should do is type <kbd translate=no>eat honey</kbd>。 The game will respond with：</p> <pre><samp translate=no>Yum yum! That was some good honey!</samp></pre> </body> </html>
dir 属性dir 属性指定了元素的文本方向。
该属性是一个 枚举属性 关键字和状态如下：
ltr 关键字，映射到 ltr 状态表示该元素的内容是明确的独立方向隔离的从左到右的文本。
rtl 关键字，映射到 rtl 状态表示该元素的内容是明确的独立方向隔离的从右到左的文本。
auto 关键字，映射到 auto 状态表示该元素的内容是明确的方向隔离的文本，但其方向需要通过元素的内容以编程方式确定（描述如下）。
这种状态使用的启发式是非常粗糙的（类似于双向算法中的段落级别确定，查看第一个具有较强方向性的字符）。 强烈建议作者只有在文本的方向是未知，并且服务器端没有更好的启发式算法的情况下才使用该值作为最后的手段。 [BIDI]
元素（任意元素，不仅是 HTML 元素）的 方向 或者是 'ltr'，或者是'rtl'， 由下列列表中第一个适用的步骤确定：
dir 属性处于 ltr 状态dir
属性不处于已定义状态（即未出现或具有非法值）input 元素
type 属性处于 电话 状态，
且 dir
属性不处于已定义状态（即未出现或具有非法值）dir 属性处于 rtl 状态input 元素，且它的 type 属性处于 文本，
搜索，
电话，
URL，或
E-mail 状态，
且 dir 属性处于 auto 状态textarea 元素，且它的 dir
属性处于 auto 状态如果该元素的 value 包含一个双向字符类型 AL 或 R 的字符， 且它之前不包含双向字符类型 L 的字符，则该元素的 方向 为 'rtl'。 [BIDI]
dir 属性处于 auto 状态bdi 元素且 dir
属性不处于已定义状态（即未出现或具有非法值）找到按照 树序 的第一个匹配下列条件的字符：
该字符属于双向字符类型 L，AL，或 R。 [BIDI]
如果找到了这样的字符且它属于双向字符类型 AL 或 R，该元素的 方向 是 rtl'。
如果找到了这样的字符且它属于双向字符类型 L，该元素的 方向 是 ltr'。
dir 属性处于未定义状态
（即未出现或具有非法值）因为只有 HTML 元素 定义有 dir 属性，
它不得出现在其他命名空间的元素上。所以其他命名空间的元素只从它们的父元素继承
方向，如果没有父元素则默认为 'ltr'。
该属性 有涉及双向算法的渲染要求。
当该属性的文本用于渲染过程时，HTML 元素 属性的方向 由第一个合适的步骤决定：
dir 属性处于 auto 状态找到该属性值中第一个属于双向字符类型 L，AL，或 R （按照逻辑顺序）的字符。 [BIDI]
下列属性属于 有方向的属性：
th 元素上的 abbrarea，
img，和
input 元素上的 altmeta 元素上的 content，
如果 name 属性的元数据名对应的值应该是人类可读的optgroup， option，和
track 元素上的 label input 和 textarea
元素上的 placeholdertitledir [ = value ]可以设置为 "ltr"，"rtl"，或 "auto"
来替换掉 html 元素 的 dir 属性值。
如果不存在 html 元素，则返回空字符串，忽略新的值。
Support in all current engines.
元素的 dir IDL 属性必须
反映 其 dir 内容属性，
仅限于已知值。
Support in all current engines.
Document 对象上的 dir IDL 属性必须
反映 html 元素 的
dir 内容属性，如果有的话，
仅限于已知值。如果没有这样的元素，
该属性必须返回空字符串，且在设置时什么都不做。
强烈建议作者使用 dir
属性而不是 CSS 来指明文本方向。因为这样的话即使没有 CSS 文档也会正确地渲染。
（例如被搜索引擎解释时）。
这个标记片段是关于 IM 对话的。
<p dir=auto class="u1"><b><bdi>Student</bdi>：</b> How do you write "What的 your name?" in Arabic?</p> <p dir=auto class="u2"><b><bdi>Teacher</bdi>：</b> ما اسمك؟</p> <p dir=auto class="u1"><b><bdi>Student</bdi>：</b> Thanks.</p> <p dir=auto class="u2"><b><bdi>Teacher</bdi>：</b> That的 written "شكرًا".</p> <p dir=auto class="u2"><b><bdi>Teacher</bdi>：</b> Do you know how to write "Please"?</p> <p dir=auto class="u1"><b><bdi>Student</bdi>：</b> "من فضلك"，right?</p>
给定 p 元素合适的样式表和默认对齐样式，也就是说把文本对齐到段落 起始端，渲染结果可能是这样的：
就像之前提到的，auto 值不是万能药。
这个例子中最后一个段落被错误地解释为从右向左的文本，
因为它以一个阿拉伯字符开始，这会导致 "right?" 出现在阿拉伯文字的左边。
style 属性所有 HTML 元素 都可以设置 style 内容属性。
这个 style 属性 定义在
CSS Style Attributes 规范中。 [CSSATTR]
在支持 CSS 的用户代理中，当添加或改变这个属性时必须 ， 根据 style 属性 的规则解析该属性的值。 [CSSATTR]
但是如果 是否应该根据内容安全策略禁止元素的内联行为 算法对
属性的 元素，"style attribute"，和属性值返回了 "Blocked" ，
那么该属性值中定义的样式规则不得应用于该 元素。 [CSP]
在任何元素上使用了 style 属性的文档必须
在那些属性移除之后仍然可以理解和使用。
特别是使用 style 属性来隐藏和显示内容，
或者传达文档中本没有的含义，这些都是不合规范的。
（隐藏和显示内容，可以使用 属性）
style返回该元素的 style 属性的 CSSStyleDeclaration 对象。
style IDL 属性定义在 CSS Object Model (CSSOM) 规范中。 [CSSOM]
下面的例子中，使用
span 元素和 style 属性
来标记指代颜色的词，来让它们在视觉媒体中显示相关的颜色。
<p>My sweat suit is <span style="color： green; background： transparent">green</span> and my eyes are <span style="color： blue; background： transparent">blue</span>.</p>
data-* 属性 嵌入自定义不可见数据Support in all current engines.
自定义数据属性 是一个无命名空间的属性，其名字以
字符串 "data-" 起始，连字符后有至少一个字符，
兼容于 XML，且不得包含非
ASCII 大写字母。
HTML 文档 中 HTML 元素 上所有属性名都会被自动 ASCII 小写化，所以 ASCII 大写字母的限制不会影响这种文档。
自定义数据属性 用于存储页面或应用私有的，自定义的数据、状态、注解和类似东西，只有在找不到更合适的属性或元素时才能使用。
这些属性不适用于所属站点管理员不知道的软件。对于很多独立工具使用的通用扩展， 或者需要扩展本规范来显式地提供该特性，或者应该使用类似 microdata 的技术（使用标准化的词汇）。
例如一个音乐相关的站点可能会在表示专辑音轨的列表项中， 使用自定义数据属性来表示每个音轨的长度。 用户可能会用该信息对列表进行排序，或者按照某个长度来过滤列表。
<ol> <li data-length="2m11s">Beyond The Sea</li> ..。 </ol>
但是用户使用与这个音乐站点不相关的通用软件来根据这个数据搜索某个长度的音轨是不合适的。
这是因为这些属性只适用于该站点自己的脚本，并不是一种公众可用的元数据的通用扩展机制。
类似地，页面作者可能会编写一些标记来为他们打算使用的翻译工具提供一些信息：
<p>The third <span data-mytrans-de="Anspruch">claim</span> covers the case of <span translate="no">HTML</span> markup.</p>
这个例子中，"data-mytrans-de" 属性给出的文字
提供给 MyTrans 产品使用，把 "claim" 翻译到德语。但是标准的
translate 属性用于指示，
在所有语言环境中 "HTML" 要保持不变。当有标准属性可用时，就没必要使用
自定义数据属性 了。
在这个例子中，自定义数据属性用于存储
PaymentRequest 特性探测的结果，可能会用于 CSS 中，对付款页面做特殊的样式。
<script>
if ('PaymentRequest' in window) {
document.documentElement.dataset.hasPaymentRequest = '';
}
</script>
这里 data-has-payment-request 属性被当做
布尔属性 来使用；检查该属性是否出现就足够了。
但是如果作者希望，它后续也可以填充一些值，比如用来标识该特性的功能限制。
每个 HTML 元素 都可以指定任意个 自定义数据属性，指定为任意值。
作者应该仔细地设计这样的扩展，当属性被忽略或者任何相关 CSS 被丢弃时，页面仍然可用。
用户代理不得从这些属性和值腿短任何实现行为。用户代理的相关规范不得为这些属性定义任何有意义的值。
JavaScript 库可以使用 自定义数据属性，因为它们也是页面的一部分。 鼓励有很多人用的库的作者在属性名中包含名字，来减小冲突的风险。 如果可行的话，鼓励库的作者支持自定义实际在用的名字，这样不小心选了重复名字的库也可以用于同一个页面， 同一个库的多个版本也可以用于同一个页面，即使它们互相不兼容。
例如一个叫做 "DoQuery" 的库可能会使用叫做 data-doquery-range 的属性名，
另一个叫做 "jJo" 的库可能会使用叫做 data-jjo-range 的属性名。
jJo 库也可以提供一个 API 来设置使用哪个前缀（例如 J.setDataPrefix('j2')，
使属性的名字变成 data-j2-range）。
datasetSupport in all current engines.
Support in all current engines.
返回该元素的 data-* 属性的 DOMStringMap 对象。
连字符连接的名字会变成驼峰的。例如 data-foo-bar=""
变成了 element.dataset.fooBar。
dataset IDL 属性
为元素上的所有 data-* 属性提供了便利访问器。
当读取时，dataset IDL 属性必须返回一个与该元素关联的
DOMStringMap。
DOMStringMap 接口用于 dataset 属性。每个 DOMStringMap 有一个 关联的元素。
[OverrideBuiltins]
interface DOMStringMap {
getter DOMString (DOMString name);
[CEReactions] setter void (DOMString name,DOMString value);
[CEReactions] deleter void (DOMString name);
};
获取 DOMStringMap 的键值对，需呀执行以下算法：
令 list 为一个键值对的空列表。
对于 DOMStringMap 的 关联元素
上的每个前五个字符是 "data-" 且剩余字符（如果有的话）不包含任何
ASCII 大写字母 的内容属性，
按照这些属性在 属性列表 中出现的顺序，
在 list 中新增一个键值对，属性名移除前五个字符后作为键，属性值作为值。
对 list 中的每个键中的每个后面跟随着一个 ASCII 小写字母 的 U+002D HYPHEN-MINUS 字符 （-） ，移除这个 U+002D HYPHEN-MINUS 字符 （-）并且将后面的这个字符替换为 同一个字母 转换为 ASCII 大写 的结果。
返回 list。
任一时刻，DOMStringMap 对象上 支持的属性名 都是那一时刻
获取 DOMStringMap 的键值对
返回的每个键值对的键构成的列表，并按照返回的顺序排序。
对一个 DOMStringMap
确定命名属性 name 的值，
获取 DOMStringMap 的键值对
返回的列表中键是 name 的键值对的值。
给定属性名 name 和一个新值 value，
为一个 DOMStringMap
设置一个新命名属性的值 或
设置已有属性的值 ，需要执行以下步骤：
如果 name 包含一个 U+002D HYPHEN-MINUS 字符（-） 紧跟着 ASCII 小写字母，
则抛出一个 "SyntaxError" DOMException 并中止这些步骤。
对每一个 name 中的 ASCII 大写字母 ， 在该字符前插入一个 U+002D HYPHEN-MINUS 字符（-），并把这个字母替换为同一字母 转换为 ASCII 小写 的结果。
在 name 之前插入字符串 data-。
如果 name 不匹配 XML Name 生成式，
抛出一个 "InvalidCharacterError" DOMException 并中止这些步骤。
使用 name 和 value 为 DOMStringMap 的 关联元素
设置属性值。
为一个 DOMStringMap
删除一个既有命名属性
name ，需要执行以下步骤：
对每一个 name 中的 ASCII 大写字母 ， 在该字符前插入一个 U+002D HYPHEN-MINUS 字符（-），并把这个字母替换为同一字母 转换为 ASCII 小写 的结果。
在 name 之前插入字符串 data-。
给定 name 和 DOMStringMap 的 关联元素
通过属性名移除属性。
Web IDL 规范只会使用前述
获取 DOMStringMap 的键值对
算法给出的名字调用该算法。 [WEBIDL]
如果一个 Web 页面想要一个元素来表示宇宙飞船，比如作为游戏的一部分，他可能必须同时使用
class 属性与 data-* 属性：
<div class="spaceship" data-ship-id="92432"
data-weapons="laser 2" data-shields="50%"
data-x="30" data-y="10" data-z="90">
<button class="fire"
onclick="spaceships[this.parentNode.dataset.shipId].fire()">
Fire
</button>
</div>
注意连字符连接的属性名在 API 中变成了驼峰命名的。
给定下面的相似构造的片段和元素：
<img class="tower" id="tower5" data-x="12" data-y="5"
data-ai="robotarget" data-hp="46" data-ability="flames"
src="towers/rocket.png" alt="Rocket Tower">
...可能会想到用一个 splashDamage() 函数，它接受的第一个参数是被处理的元素：
function splashDamage(node，x，y，damage) {
if (node.classList.contains('tower') && // checking the 'class' attribute
node.dataset.x == x && // reading the 'data-x' attribute
node.dataset.y == y) { // reading the 'data-y' attribute
var hp = parseInt(node.dataset.hp); // reading the 'data-hp' attribute
hp = hp - damage;
if (hp < 0) {
hp = 0;
node.dataset.ai = 'dead'; // setting the 'data-ai' attribute
delete node.dataset.ability; // removing the 'data-ability' attribute
}
node.dataset.hp = hp; // setting the 'data-hp' attribute
}
}
innerText getter and
setterSupport in all current engines.
innerText [ = value ]Returns the element's text content "as rendered".
Can be set, to replace the element's children with the given value, but with line breaks
converted to br elements.
The innerText getter steps are:
If this is not being rendered or if the user agent is a non-CSS user agent, then return this's descendant text content.
This step can produce surprising results, as when the innerText getter is invoked on an element not being
rendered, its text contents are returned, but when accessed on an element that is
being rendered, all of its children that are not being rendered have
their text contents ignored.
Let results be a new empty list.
For each child node node of this:
Let current be the list resulting in running the inner text collection steps with node. Each item in results will either be a string or a positive integer (a required line break count).
Intuitively, a required line break count item means that a certain number of line breaks appear at that point, but they can be collapsed with the line breaks induced by adjacent required line break count items, reminiscent to CSS margin-collapsing.
For each item item in current, append item to results.
Remove any items from results that are the empty string.
Remove any runs of consecutive required line break count items at the start or end of results.
Replace each remaining run of consecutive required line break count items with a string consisting of as many U+000A LINE FEED (LF) characters as the maximum of the values in the required line break count items.
Return the concatenation of the string items in results.
The inner text collection steps, given a node node, are as follows:
Let items be the result of running the inner text collection steps with each child node of node in tree order, and then concatenating the results to a single list.
If node's computed value of 'visibility' is not 'visible', then return items.
If node is not being rendered, then return items. For the purpose of this step, the following elements must act as described if the computed value of the 'display' property is not 'none':
select elements have an associated non-replaced inline CSS box
whose child boxes include only those of optgroup and option element
child nodes;optgroup elements have an associated non-replaced block-level CSS
box whose child boxes include only those of option element child nodes;
andoption element have an associated non-replaced block-level CSS
box whose child boxes are as normal for non-replaced block-level CSS boxes.items can be non-empty due to 'display:contents'.
If node is a Text node, then for each CSS text box produced by
node, in content order, compute the text of the box after application of the CSS
'white-space' processing rules and 'text-transform' rules, set
items to the list of the resulting strings, and return items.
The CSS 'white-space' processing rules are slightly modified: collapsible spaces at
the end of lines are always collapsed, but they are only removed if the line is the last line of
the block, or it ends with a br element. Soft hyphens should be preserved. [CSSTEXT]
If node is a br element, then append a string containing a single U+000A LINE FEED (LF) character to
items.
If node's computed value of 'display' is 'table-cell', and node's CSS box is not the last 'table-cell' box of its enclosing 'table-row' box, then append a string containing a single U+0009 CHARACTER TABULATION (tab) character to items.
If node's computed value of 'display' is 'table-row', and node's CSS box is not the last 'table-row' box of the nearest ancestor 'table' box, then append a string containing a single U+000A LINE FEED (LF) character to items.
If node is a p element, then append 2 (a required line break count) at the beginning and end of
items.
If node's used value of 'display' is block-level or 'table-caption', then append 1 (a required line break count) at the beginning and end of items. [CSSDISPLAY]
Floats and absolutely-positioned elements fall into this category.
Return items.
Note that descendant nodes of most replaced elements (e.g., textarea,
input, and video — but not button) are not rendered
by CSS, strictly speaking, and therefore have no CSS boxes for the
purposes of this algorithm.
This algorithm is amenable to being generalized to work on ranges. Then we can use it as the basis for Selection's
stringifier and maybe expose it directly on ranges. See Bugzilla bug 10583.
The innerText setter steps are:
Let document be this's node document.
Let fragment be a new DocumentFragment object whose node
document is document.
Let input be the given value.
Let position be a pointer into input, initially pointing at the start of the string.
Let text be the empty string.
While position is not past the end of input:
Collect a sequence of code points that are not U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters from input given position. Set text to the collected characters.
If text is not the empty string, then append a new Text node whose data is text and node document is
document to fragment.
While position is not past the end of input, and the character at position is either a U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) character:
If the character at position is a U+000D CARRIAGE RETURN (CR) character and the next character is a U+000A LINE FEED (LF) character, then advance position to the next character in input.
Advance position to the next character in input.
Append the result of creating an element given document,
br, and the HTML namespace to fragment.
Replace all with fragment within this.
内容 中包含 Text 节点的
HTML 元素 中的 文本内容，
以及允许自由文本的 HTML 元素 属性中的文本，
可以包含 U+202A 到 U+202E，和 U+2066 到 U+2069 （双向算法的格式化字符） 范围内的字符。 [BIDI]
鼓励作者使用 dir 属性，
bdo 元素，以及 bdi 元素，
而不是手动地维护双向算法字符。双向算法格式化字符与 CSS 的交互性很差。
用户代理必须实现 Unicode 双向算法，以便在渲染文档或文档部分时确定合适的字符顺序。[BIDI]
HTML 到 Unicode 双向算法的映射必须以这三种方式中的任何一种来完成。 一，用户代理必须实现 CSS，特别是 CSS 'unicode-bidi'， 'direction'，和 'content' 属性， 并且在其用户代理样式表中必须包含在本规范的 渲染 章节中给出的用到了这些属性的规则。 二，用户代理必须表现得就像实现了前述属性一样，并且用户代理样式表中包含所有前述规则，但没有让文档中指定的样式表覆盖它们。 三，用户代理必须实现另一种具有等价语义的样式语言。 [CSSGC]
以下元素和属性具有由 渲染 部分定义的要求， 由于本节中的要求，那些要求对所有用户代理都有效（不仅仅是那些 支持建议默认渲染的）：
在 HTML 元素 上实现可访问性 API 语义的用户代理要求定义在 HTML Accessibility API Mappings 中。 [HTMLAAM]
对一致性检查器检查 HTML 元素 上
ARIA role 和 aria-* 属性的使用
的相关要求定义在 ARIA in HTML。 [ARIAHTML]
html 元素Support in all current engines.
Support in all current engines.
head 元素紧接着 body 元素。html element's start tag can be omitted
if the first thing inside the html element is not a comment.html element's end tag can be omitted if
the html element is not immediately followed by a comment.manifest — Application cache manifest[HTMLConstructor]
interface HTMLHtmlElement : HTMLElement {};
鼓励作者在根 html 元素上指定 lang 属性，
给出文档的语言。这可以帮助语音合成工具确定要使用的发音，帮助翻译工具确定要使用的规则，
等等。
manifest 属性给出文档的
应用缓存
manifest 的地址，如果有的话。
如果出现了该属性，其值必须是一个
可能被空格包裹的合法的非空URL。
manifest 属性是是遗留的
"离线 Web 应用" 特性的一部分，
该特性正在从 Web 平台移除。（这是一个很长的过程，可能持续很多年。）
现在强烈不推荐使用 manifest 属性。
使用 service workers 来替代。 [SW]
manifest 属性只在文档加载的早期阶段
起作用。
因此动态改变该属性没有效果（因此没有为该属性提供任何 DOM API）。
应用缓存选择 中，后续的 base
元素影响不了 manifest 属性的
URL 解析，因为在还没看到这些元素时就进行了这些属性的处理。
window.applicationCache IDL
属性提供了对离线 应用缓存 机制的脚本访问。
下面例子中的 html 元素声明了该文档的语言是英语。
<!DOCTYPE html> <html lang="en"> <head> <title>Swapping Songs</title> </head> <body> <h1>Swapping Songs</h1> <p>Tonight I swapped some of the songs I wrote with some friends,who gave me some of the songs they wrote. I love sharing my music.</p> </body> </html>
head 元素Support in all current engines.
Support in all current engines.
html 元素的第一个元素。iframe srcdoc 文档，
或者如果在更高级协议上有标题信息可用：大于等于0个 元数据内容 元素，其中不多于一个 title 元素，
不多于一个 base 元素。title 元素，
且不多于一个 base 元素。head element's start tag can be omitted if
the element is empty, or if the first thing inside the head element is an
element.head element's end tag can be omitted if
the head element is not immediately followed by ASCII whitespace or a
comment.[HTMLConstructor]
interface HTMLHeadElement : HTMLElement {};
head 元素中的元数据集合可大可小。这里有一个非常小的例子：
<!doctype html> <html lang=en> <head> <title>A document with a short head</title> </head> <body> ...
这是一个更长的例子：
<!DOCTYPE HTML> <HTML LANG="EN"> <HEAD> <META CHARSET="UTF-8"> <BASE HREF="https://www.example.com/"> <TITLE>An application with a long head</TITLE> <LINK REL="STYLESHEET" HREF="default.css"> <LINK REL="STYLESHEET ALTERNATE" HREF="big.css" TITLE="Big Text"> <SCRIPT SRC="support.js"></SCRIPT> <META NAME="APPLICATION-NAME" CONTENT="Long headed application"> </HEAD> <BODY> ...
title 元素在多数场景下都是必须的子元素，但是当更高级协议提供了标题信息时，
例如在 e-mail 中使用 HTML 作为编写格式，有标题行时就可以忽略 title 元素。
title 元素Support in all current engines.
Support in all current engines.
title 的 head 元素中。[HTMLConstructor]
interface HTMLTitleElement : HTMLElement {
[CEReactions] attribute DOMString text;
};
title 元素 表示 文档的标题或名称。
作者应该使用标识其文档的标题，这个标题在其他上下文中也会被使用，例如在用户的历史、书签，或搜索结果中。
文档的标题常常与第一个标题元素不同，因为第一个标题不需要在上下文之外有意义。
每个文档不得有多个 title 元素。
如果 Document 没有标题是合理的，则可能不要求有
title 元素。在 head 元素的内容模型中描述了何时该元素是必须的。
text [ = value ]返回该元素的 子文本内容。
设置时用给定的值替换该元素的子内容。
text IDL 属性必须返回
title 元素的 子文本内容。
当设置时，必须与 textContent IDL 属性的表现一致。
这是一些合适的标题的例子，以及同一页面中可能有的顶层标题。
<title>Introduction to The Mating Rituals of Bees</title>
...
<h1>Introduction</h1>
<p>This companion guide to the highly successful
<cite>Introduction to Medieval Bee-Keeping</cite> book is...
The next page might be a part of the same site。 Note how the title describes the subject matter unambiguously，while the first heading assumes the reader knows what the context is and therefore won't wonder if the dances are Salsa or Waltz:
<title>Dances used during bee mating rituals</title>
...
<h1>The Dances</h1>
document.title IDL 属性用作文档的标题。
当用户代理在其用户界面中引用文档时，应该使用该文档的标题。
当这样使用 title 元素的内容时，
应该使用该 title 元素的 方向
设置用户界面中该文档标题的方向。
base 元素Support in all current engines.
base 元素的 head 元素中。href — Document base URLtarget — Default browsing context for hyperlink navigation and form submission[Exposed=Window,
HTMLConstructor]
interface HTMLBaseElement : HTMLElement {
[CEReactions] attribute USVString href;
[CEReactions] attribute DOMString target;
};
base 元素允许作者指定用于 解析 URL 的
文档基 URL，
以及用于 跟随超链接 的默认
浏览上下文 的名称。
该元素不 表示 超出该信息的任何内容。
每个文档中不得包含多个 base 元素。
base 元素有一个 href 属性，
或者 target 属性，或者两者都有。
如果指定了 href 内容属性，
必须包含一个 可能被空白包裹的合法的 URL。
如果 base 元素有 href 属性，
则必须出现在树中的任何使用 URL 的其他元素之前，
html 元素除外（其 manifest 属性不受
base 元素的影响)。
如果有多个有 href 属性的 base 元素，
则忽略除第一个之外的。
如果指定了 target 属性，则必须包含一个
合法的浏览上下文名称或关键字，
它指定了当 Document 中的 超链接 和
表单 产生 导航 时默认使用哪个
浏览上下文。
base 元素如果有 target 属性，
必须出现在树中表示 超链接 的其他元素之前。
如果有多个有 target 属性的
base 元素，则忽略除第一个之外的。
给定 a， area， 或 form 元素 element，
要 获得元素的 target，执行这些步骤：
如果 element 有 target 属性，则返回该属性的值。
如果 element 的 节点文档 包含一个
有 target 属性的 base 元素，
则返回第一个这样的 base 元素的这一
target 属性的值。
返回空字符串。
在文档树中 第一个有
href 内容属性的 base 元素元素有一个
冻结的 base URL。当下列情况发生时，
必须 立即 设置
元素的 冻结的 base URL：
base 元素成为其 Document 中按照
树序 的第一个有
href 内容属性的 base 元素。base 元素是 Document 中按照
树序 的第一个有
href 内容属性的 base 元素，
它的 href 内容属性发生了变化。为元素 element 设置冻结的 base URL 步骤如下：
令document 为 element的 节点文档。
令urlRecord 为用 document 的 fallback base URL，
以及 document的 字符编码
解析
element的 href 内容属性的值的结果。
（因此 base 元素不影响它自己。）
如果 urlRecord 是失败或者
在 结果 URL 记录 和 document 上执行
Document 是否允许 base？ 返回了 "Blocked"，
则设置 element 的 冻结的 base URL 为
document 的 fallback base URL；否则设置为 urlRecord。
在读取时，href IDL 属性必须返回运行以下算法的结果：
令document 为 element的 节点文档。
令url 该元素的 href 属性的值，
如果没有则为空字符串。
令urlRecord 为用 document 的 fallback base URL，
以及 document的 字符编码
解析
element的 href 内容属性的值的结果。
（因此 base 元素不影响它自己。）
如果 urlRecord 是失败，则返回 url。
返回 urlRecord 的 序列化。
在设置 href IDL 属性时，必须设置 href 内容属性为给定的新值。
target IDL 属性必须
反映 同名的内容属性。
在这个例子中，用一个 base 元素来设置 文档的 base URL：
<!DOCTYPE html>
<html lang="en">
<head>
<title>This is an example for the <base> element</title>
<base href="https://www.example.com/news/index.html">
</head>
<body>
<p>Visit the <a href="archives.html">archives</a>.</p>
</body>
</html>
上面例子中的链接地址是 "https://www.example.com/news/archives.html"。
link 元素Support in all current engines.
head 中的 noscript 元素中。href — Address of the hyperlinkcrossorigin — How the element handles crossorigin requestsrel — Relationship between the document containing the hyperlink and the destination resourcemedia — Applicable mediaintegrity — Integrity metadata used in Subresource Integrity checks [SRI]hreflang — Language of the linked resourcetype — Hint for the type of the referenced resourcereferrerpolicy — Referrer policy for fetches initiated by the elementsizes — Sizes of the icons (for rel="icon")imagesrcset — Images to use in different situations, e.g., high-resolution displays, small monitors, etc. (for rel="preload")imagesizes — Image sizes for different page layouts (for rel="preload")as — Potential destination for a preload request (for rel="preload" and rel="modulepreload")color — Color to use when customizing a site's icon (for rel="mask-icon")disabled — Whether the link is disabledtitle 属性 有特殊的语义。 — Title of the link — CSS style sheet set name[Exposed=Window]
interface HTMLLinkElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute USVString href;
[CEReactions] attribute DOMString? crossOrigin;
[CEReactions] attribute DOMString rel;
[CEReactions] attribute DOMString as; // (default "")
[SameObject，PutForwards=value] readonly attribute DOMTokenList relList;
[CEReactions] attribute DOMString media;
[CEReactions] attribute DOMString integrity;
[CEReactions] attribute DOMString hreflang;
[CEReactions] attribute DOMString type;
[SameObject，PutForwards=value] readonly attribute DOMTokenList sizes;
[CEReactions] attribute USVString imageSrcset;
[CEReactions] attribute DOMString imageSizes;
[CEReactions] attribute DOMString referrerPolicy;
[CEReactions] attribute boolean disabled;
// also has obsolete members
};
HTMLLinkElement includes LinkStyle;link 元素允许作者把他们的文档链接到其他资源。
链接的地址由 href 属性给出。
如果存在 href 属性，那么它的值必须是一个
可能由空格包裹的合法的非空 URL。
必须存在 href 或 imagesrcset 属性。
如果 href 和 imagesrcset 属性都不存在，那么该元素就不包含链接的定义。
crossorigin
属性是一个 CORS 设置属性，用于 外部资源链接。
链接指示的类型（关系）由 rel
属性的值给出。该属性如果出现则它的值必须是一个
由空格分隔的唯一令牌的无序集合。
允许的关键字及其含义 定义在后续章节中。
如果 rel 属性缺失，
不包含关键字，或者没有出现（根据本规范中的定义）允许的关键字，
则该元素不创建任何链接。
rel
支持的令牌 是
HTML 链接类型 中定义的，
link 元素上允许的，影响处理模型的，用户代理支持的关键字。
可能的 支持的令牌 包括
alternate，
dns-prefetch，
icon，
modulepreload,
next，
pingback，
preconnect，
prefetch，
preload，
prerender，
search，和
stylesheet。
rel 的
支持的令牌 必须只包含这个列表中用户代理实现了相应处理模型的令牌。
理论上用户代理可能会支持
canonical 关键字的处理模型，
比如用户代理是一个执行 JavaScript 的搜索引擎的话。虽然实践中基本不会发生。
所以大多数场景下，canonical 不应属于
rel
支持的令牌。
link 元素必须有一个 rel
属性或者 itemprop 属性，但不能同时有这两个。
如果 link 元素有一个 itemprop 属性，
或者有一个只包含 body-ok 关键字的 rel 属性，
则称该元素 允许出现在 body 中。
这意味着该元素可以用于期望 短语内容 的地方。
如果用了 rel 属性，
该元素只能用于页面中的 body 部分。如果用了 itemprop 属性，该元素既可以用于页面中的
head 元素也可以用于 body 元素，取决于元数据模型的约束。
使用 link 元素可以创建两类链接： 外部资源链接 和 超链接。
链接类型一章 定义了
特定的链接类型是外部资源还是超链接。
一个 link 元素可以创建多个链接（其中有些可能是
外部资源链接，有些可能是
超链接）；具体创建了哪种链接，创建了多少个取决于
rel 属性中给出的关键字。
用户代理必须逐链接而不是逐元素进行处理。
为 link 元素创建的每个链接都会被单独处理。
例如，如果有两个有 rel="stylesheet" 的 link 元素，
它们每个都被当做独立的外部资源，每个都会独立地受自己的属性影响。
类似地，如果单个 link 元素有一个
值为 next stylesheet
rel 属性，
它（为 next 关键字）创建了一个
超链接，同时也
（为 stylesheet 关键字）创建了一个
外部资源链接，
它们分别受其他属性（例如 media 或
title）的影响。
例如下面的 link 元素创建了两个 超链接
（指向同一页面）：
<link rel="author license" href="/about">该元素创建的这两个链接，一个的语义是目标页面具有关于当前页面作者的信息， 另一个的语义是目标页面具有提供当前页面的授权信息。
link 元素和它的 rel 属性创建的
超链接 适用于整个文档。
这区别于 a 和 area 元素的
rel 属性，它们指定了链接的类型，
链接的上下文由链接在文档中的位置给出。
外部资源链接 的确切行为取决于由相关的 链接类型 定义的具体关系。
media 属性指定了该资源适用哪种媒体。
它的值必须是 合法的媒体查询列表。
nonce 属性表示一个加密随机数
（"一次性数字"）用于 Content Security Policy 来确定
该链接指定的外部资源
是否会被加载和应用于该文档。它的值是文本。 [CSP]
integrity
属性表示该元素负责的请求的 integrity 元信息。
值为文本。该属性只能用于 rel 属性包含
stylesheet，
preload 或
modulepreload 关键字的 link 元素。[SRI]
link 元素上的 hreflang
属性与 a 元素上的 hreflang
属性 具有相同的语义。
type 属性给出了被链接资源的
MIME 类型。它纯属建议性的。它的值必须是
合法的 MIME 类型。
对于 外部资源链接，
type 属性用于提示用户代理避免获取它们不支持的资源。
referrerpolicy 属性是一个
引荐策略属性。
它用于 外部资源链接，
帮助设置 获取和处理 该外部资源时的
引荐策略。 [REFERRERPOLICY]。
title 属性给出了链接的标题。
有一个特例中，它是纯建议性的。它的值是文本。
特例是 在文档树 中的样式表链接，
对它而言 title 属性定义了
CSS 样式表集合。
link 元素上的 title 属性
与多数其他元素的全局 title 属性不同，
没有标题的链接不继承父元素的标题：它只是没有标题。
imagesrcset
属性可以是一个 srcset 属性。
imagesrcset 和 href 属性（如果没有使用 宽度描述符）
一起给 source set 提供 图片源。
如果 imagesrcset 属性存在且有任一个
图片候选字符串 使用了 宽度描述符，
imagesizes 属性就必须存在，且是一个
尺寸属性。imagesizes 属性给
source set 提供 源尺寸。
imagesrcset 和 imagesizes 属性只能用于
rel 属性指定了 preload 关键字，且
as 属性在处于 "image" 状态的
link 元素上。
这些属性运行预加载适当的资源，
后续 img 被使用
srcset 和 sizes 属性：
<link rel="preload" as="image"
imagesrcset="wolf_400px.jpg 400w, wolf_800px.jpg 800w, wolf_1600px.jpg 1600w"
imagesizes="50vw">
<!-- ... later, or perhaps inserted dynamically ... -->
<img src="wolf.jpg" alt="A rad wolf"
srcset="wolf_400px.jpg 400w, wolf_800px.jpg 800w, wolf_1600px.jpg 1600w"
sizes="50vw">注意我们忽略了 href 属性，因为它会在不支持
imagesrcset 的浏览器中起作用，可能会预加载错误的图片。
imagesrcset 属性可以和
media 属性一起使用来预加载适当的资源
picture 元素选择的资源，来实现 美术设计：
<link rel="preload" as="image"
imagesrcset="dog-cropped-1x.jpg, dog-cropped-2x.jpg 2x"
media="(max-width: 800px)">
<link rel="preload" as="image"
imagesrcset="dog-wide-1x.jpg, dog-wide-2x.jpg 2x"
media="(min-width: 801px)">
<!-- ... later, or perhaps inserted dynamically ... -->
<picture>
<source srcset="dog-cropped-1x.jpg, dog-cropped-2x.jpg 2x"
media="(max-width: 800px)">
<img src="dog-wide-1x.jpg" srcset="dog-wide-2x.jpg 2x"
alt="An awesome dog">
</picture>sizes 属性给出了
视觉媒体的图标大小。它的值（如果存在的话）只是建议性的。
用户代理可以根据它的值来在多个可用的图标中选择使用哪个。
如果指定了，这个属性的值必须是 ASCII 大小写不敏感 的
空格分隔的无序不重复令牌集合。
每个值必须 ASCII 大小写不敏感 地匹配字符串
"any"，或包含两个
合法的非负整数 且不得有前导 U+0030 DIGIT ZERO (0) 字符，
并以单个 U+0078 LATIN SMALL LETTER X 或 U+0058 LATIN CAPITAL LETTER X 字符分隔。
该属性只能用于 rel 属性指定了
icon 关键字或 apple-touch-icon 关键字的 link 元素。
apple-touch-icon 关键字是一个已注册的 链接类型的预定义集合扩展，但不要求用户代理支持它。
as 属性为
href 指定的资源的预加载请求指定了
可能的地址。
这是一个 枚举属性。
每个 可能的地址 是一个关键字，
映射为一个同名的状态。该属性必须指定在
rel 属性包含 preload 关键字的
link 元素上。也可以指定在
rel 属性包含 modulepreload 关键字的
link 元素上；这种情况下它的值必须是
script-like 地址。
其他 link 元素不能使用这个属性。
如何使用 as 属性的处理模型在
每个 link 类型的 获取和处理链接资源 算法中给出。
该属性没有 缺失值默认 或 非法值默认，意味着非法值或缺失值不映射到任何状态。 这在处理模型中有所解释。
color 属性用于 mask-icon 链接类型。该属性不得指定在
rel 属性不包含 mask-icon 关键字的
link 元素上。它的值必须是一个匹配 CSS <color> 生成式的字符串，
用于定义一个用户代理在用户固定一个网站时用来自定义图标展示的建议颜色。
本规范在 color 属性上对用户代理没有任何要求。
mask-icon 关键字是一个已注册的 链接类型的预定义集合扩展，但不要求用户代理支持。
link 元素有一个关联的 explicitly enabled 布尔。它默认为假。
disabled
属性是一个 布尔属性，用在 stylesheet 链接类型上。该属性只能用在
rel 属性包含 stylesheet
关键字的 link 元素上。
只要 disabled 属性被移除，就设置
link 元素的 explicitly enabled 属性为真。
动态地移除 disabled 属性，例如用
document.querySelector("link").removeAttribute("disabled")，会去获取并应用样式：
<link disabled rel="alternate stylesheet" href="css/pooh">Support in all current engines.
IDL 属性
href，
hreflang，
integrity，
media，
nonce，
rel，
scope，
sizes，
type，和
disabled
都必须 反映 对应的同名内容属性。
对 color 属性没有相应的 IDL 属性，但今后可能会增加。
Support in all current engines.
as IDL 属性必须 反映
as 内容属性，
并限制为已知的值。
crossOrigin IDL 属性必须
反映 crossorigin 内容属性。
HTMLLinkElement/referrerPolicy
Support in all current engines.
referrerPolicy IDL 属性必须
反映 referrerpolicy
内容属性，并 限制到已知的值。
The imageSrcset IDL 属性必须
反映 imagesrcset 内容属性。
The imageSizes IDL 属性必须
反映 the imagesizes 内容属性。
Support in all current engines.
media 属性如果该链接是 超链接 那么
media 属性就是纯建议性的，
描述了该文档是为哪种媒体设计的。
但如果该链接是 外部资源链接，则 media 属性则是指定的。
当 media 属性值
匹配当前环境 以及其他相关条件时，
用户代理必须应用这一外部资源。其他情况不得应用该资源。
如果忽略 media 属性，
默认值是 "all"，表示默认情况下链接适用于所有媒体。
外部资源可能会在此限制范围内限定其适用性。
例如，CSS样式表可能有一些 @media 块。
本规范不会覆盖这些进一步的限制或要求。
type 属性如果存在 type 属性，那么用户代理必须假定
该资源属于给定的类型（即使它不是 合法的 MIME 类型，
例如空字符串）。 如果忽略了该属性，但
外部资源链接 类型定义有默认值，则用户代理必须假定该资源属于那种类型。
如果 UA 对指定的链接关系不支持给定的 MIME 类型，
那么 UA 不应 获取和处理 该资源；如果 UA
对指定的链接关系支持给定的 MIME 类型，
那么 UA 应该按照 外部资源链接 的特定类型所指定的
适当时机 获取和处理 该资源。
如果忽略了该属性，且 外部资源链接
类型没有定义默认值，但如果已知类型且支持的话用户代理会
获取和处理 该资源，
那么用户代理就应该假设支持该资源并且 获取和处理 它。
用户代理不得将 type 属性当做权威的
— 也就是说获取该资源时用户代理不得使用 type
属性来决定实际的类型。只能用实际的类型（定义在下一段）而不是前述的假设的类型
来决定是否应用该资源。
如果 外部资源链接 类型定义了处理该资源的 Content-Type 元数据 的规则，那么应用那些规则。 否则，如果该资源期望是一张图片，用户代理可以应用 图片嗅探规则， 其中 official type 为从该资源的 Content-Type 元数据 确定的类型， 然后把 计算得到的资源类型 当做实际的类型使用。 否则，如果上述条件都不符合或者用户代理选择不应用图片嗅探规则，那么用户代理必须使用该资源的 Content-Type 元数据 来确定资源类型。 如果没有类型元数据，但 外部资源链接 类型定义有默认值， 那么该用户代理必须假设该资源是那种类型。
stylesheet 链接类型定义了处理资源的
Content-Type 元数据 的规则。
一旦用户代理建立了该资源的类型，而且这是一个支持的类型并满足其他相关条件，就必须应用该资源。 否则就必须忽略该资源。
如果一个文档包含如下标记的样式表链接：
<link rel="stylesheet" href="A" type="text/plain">
<link rel="stylesheet" href="B" type="text/css">
<link rel="stylesheet" href="C">那么一个只支持 CSS 样式表的满足规范的 UA 会获取 B 和 C 文件，
并跳过 A （因为 text/plain 不是 CSS 样式表的 MIME type）。
对于文件 B 和 C，它会检查服务器返回的实际类型。对于作为 text/css 发送的，
它将会应用样式，但对于标记为 text/plain 或其他类型则不会。
如果这两个文件中的一个返回时没有 Content-Type 元数据，
或者有一个语法错误的类型比如 Content-Type： "null"，
那么就使用 stylesheet 链接的默认类型。
因为这个默认类型是 text/css，所以这个样式表 将会 被应用。
link 元素获取和处理资源所有 外部资源链接
都有一个 获取和处理链接资源 算法，
该算法接受一个 link 元素 el 作为参数。
还有一个 链接资源获取配置步骤，
它接受 link 元素 el 和 请求 request。
每个链接类型可能提供它们自己的 获取和处理链接资源 算法，
但除非特别说明，它们都使用 默认的获取和处理链接资源 算法。
类似地，每个链接类型也可能提供它自己的 链接资源获取配置步骤，
但除非特别说明，这些步骤只返回 true。
给定 link 元素 el，
默认的获取和处理链接资源 如下：
如果 href 属性的值是空字符串，则中止这些步骤。
给定 href 属性，相对于该元素的 节点文档
解析 URL。如果失败则返回。否则，令 url 为
结果 URL 记录。
令 corsAttributeState 为该元素的 crossorigin 内容属性的当前状态。
令request 为给定 url，空字符串，以及 corsAttributeState 创建一个可能跨域的请求 的结果。
设置 request 的 异步标志。
设置request 的
nonce 元数据 为
link 元素的 [[CryptographicNonce]] 内部槽的当前值。
设置request的
referrer policy 为
link 元素的 referrerpolicy
属性的为当前状态。
执行 链接资源获取配置步骤 并传入 el 和 request。如果结果为 false 就返回。
并行地 执行以下步骤：
令 response 为 获取 request 的结果。
令 success 为 true。
如果 response 是 网络错误 或它的 状态 不是 OK， 这是 success 为 false。
注意内容相关错误比如 CSS 解析错误或 PNG 解码错误不影响 success。
如果 success 为 true，等待 链接资源 的 关键子资源 结束加载。
定义链接类型的 关键子资源 的标准（比如 CSS） 应该描述这些如何获取和处理这些子资源。本标准描述描述等待 链接资源 的 关键子资源 获取和处理时，假设它得到了正确地处理。
用户代理在需要这些资源时才会去 获取和处理， 而不是预先获取所有未应用的 外部资源。
和 获取和处理链接资源 算法类似，所有
外部资源链接 都有对应的
处理链接资源 算法，
接受一个 link 元素 el，布尔值 success 和 响应 response。
除非个别链接类型提供了它自己的 处理链接资源 算法，
就是用给定 link 元素 el 和布尔值 success （忽略 response）的
默认的处理链接资源 算法：
除非给定的 rel 关键字另有说明，
在获取该资源及其 关键子资源 的所有尝试完成之前，
必须 推迟 该元素的
节点文档 的 load 事件。
（用户代理还没尝试获取的资源不会 推迟 load 事件，比如因为它在等待需要的资源。）
Link` 协议头如果支持 HTTP `Link` 协议头，
则必须假定它们在文档的 link 之前，按照在 HTTP 消息中的顺序到达。
这些头将会按照相关规范给出的规则进行处理。 [HTTP] [WEBLINK]
HTTP `Link` 中的关系类型注册
与 HTML link 类型 不同，
因此它们与同名 HTML 类型的语义也可能不同。
link 元素创建的超链接的方式交互式的用户代理可能会在用户界面里为用户提供一种 跟随 使用 link 元素创建的超链接的方式。
本规范没有定义具体的界面，但是它可以对文档中每个由
link 元素创建的 hyperlink，
包含某种形式的（可能会被简化）以下信息（从元素的属性获取，下文有相应定义）：
rel 属性给出）之间的关系。title 属性给出）。href 属性给出）。hreflang 属性给出）。media 属性给出）。用户代理也可以包含其他信息，比如资源类型（由 type 属性给出）。
创建 超链接 的 link 元素的
激活行为 就是如下步骤：
meta 元素Support in all current engines.
Support in all current engines.
itemprop 属性： 流式内容。itemprop 属性： 短语内容。charset 属性，或者元素的 http-equiv 属性处于 编码声明状态：在 head 元素中。http-equiv 属性但不在 编码声明状态：在 head 元素中。http-equiv 属性但不在 编码声明状态：在 head 元素的 noscript 子元素中。name 属性：期望 元数据内容 的地方。itemprop 属性： 期望元数据内容 的地方。itemprop 属性： 期望短语内容 的地方。name — Metadata namehttp-equiv — Pragma directivecontent — Value of the elementcharset — Character encoding declaration[HTMLConstructor]
interface HTMLMetaElement : HTMLElement {
[CEReactions] attribute DOMString name;
[CEReactions] attribute DOMString httpEquiv;
[CEReactions] attribute DOMString content;
};
meta 元素 表示 很多不能用
title，base，link，style，
和 script 元素表达的元数据。
meta 元素可以通过 name 属性表示文档级别的元数据，
通过 http-equiv 属性表示编译指示，
通过 charset 属性表示 HTML 文档序列化为字符串时（例如用于网络传输或者磁盘存储时）
文件的 字符编码声明。
必须指定这些属性中的一个：name，http-equiv，charset，
和 itemprop。
如果指定了 name，http-equiv，
或 itemprop，那么必须也指定 content 属性。
否则必须忽略它。
charset 属性指定了文档使用的字符编码。
这是一个 字符编码声明。如果在
XML 文档 上指定了该属性，其值必须
ASCII 大小写不敏感地 匹配 "utf-8"
（且该文档必须使用 UTF-8 作为它的编码）。
meta 元素上 charset 属性
对 XML 文档不起作用，允许它只是为了方便与 XML 之间进行迁移。
每个文档必须存在不少于一个有 charset 属性的 meta 元素。
content 属性给出了文档元数据或编译指令的值。
允许的值取决于确切的上下文，见本规范的后续章节的描述。
如果 meta 元素有一个 name 属性，
它设置了文档元数据。文档元数据以 name-value 对表达，meta 元素的
name 属性给出了名字，同一元素的 content 属性
给出了值。名字指定了要设置哪方面的元数据；合法的名字连同它的值的含义在后续章节描述。
如果 meta 元素没有 content 属性，那么 name-value 对的值的部分是空字符串。
name 和 content IDL 属性必须 反映
相应的同名内容属性。IDL 属性 httpEquiv 必须 反映
http-equiv 内容属性。
名字是大小写不敏感的，必须以 ASCII 大小写不敏感 的方式进行比较。
application-name值必须是一个简短的格式自由的字符串，给出本页面表示的 Web 应用的名字。
如果该页面不是一个 Web 应用，不得使用 application-name 元数据名。
必须使用 lang 属性来指定该 Web 应用的每个名字的语言。
给定一个 language，一个文档中不得有多个
name 属性设为 application-name
的 meta 元素。
用户代理可能会在 UI 中使用应用名来取代 title，因为标题可能包含某一特定时刻页面的状态信息。
为了从给定语言的应用名有序列表（比如 British English, American English, 和 English）中选取要用的应用名， 用户代理必须执行以下步骤：
令 languages 为语言列表。
如果有一个 default language, 并且如果它与 languages 中的语言都不同, 把它追加到 languages 上。
令 winning language 为 languages 中的第一个满足这个条件的语言：
在 Document 中存在一个
name 属性设为 application-name
且 language 是那个语言的 meta 元素。
如果所有语言都没有对应的 meta 元素，那么中止这些步骤；没有给定的应用名。
返回 content 属性的值。
Document 中 按照树序 的第一个
name 属性为 application-name
且 language 是 winning language 的 meta 元素。
浏览器需要页面名字时可能会使用本算法，比如设置书签的标签时。 提供给本算法的语言列表可能来自用户的偏好设置。
author值必须是格式自由的字符串，给出了本页面的作者的名字。
description值必须是描述本页面的格式自由的字符串，必须适合用于页面字典，例如搜索引擎。
每个文档中不得有多个 meta 元素的 name
属性设为 description。
generator值必须是一个表示用于产生该文档的软件包的格式自由的字符串。 这个值不得用于不是由软件产生的页面上，例如用户在文本编辑器上编写的页面。
这是一个名为 "Frontweaver" 的工具在页面 head 元素中的输出，来标识该页面是由它这个工具生成的：
<meta name=generator content="Frontweaver 8.2">
keywords值必须是一个 逗号分隔的令牌集合, 其中每一项都是页面的关键字。
关于英国高速公路的字体的页面使用 meta 元素来指定一些关键字（用户可能用这些关键字来查找该页面）：
<!DOCTYPE HTML> <html lang="en-GB"> <head> <title>Typefaces on UK motorways</title> <meta name="keywords" content="british,type face,font,fonts,highway,highways"> </head> <body> ...
很多搜索引擎不会考虑这些关键字，因为该特性是不可靠的甚至会导致垃圾结果，对用户并没有帮助。
为了获得作者指定的关键字列表，用户代理必须执行以下步骤：
令 keywords 为一个空列表。
对每一个 name 属性的值是 keywords，
且有一个 content 属性的 meta 元素：
用逗号 分割该元素的 content 属性的值。
把得到的结果添加到 keywords，如果有的话。
从 keywords 移除所有重复。
返回 keywords。这就是作者指定的页面关键字列表。
对值的可靠性没有足够的信心时，用户代理不应使用这一信息。
例如，内容管理系统可能会使用页面关键字来填充站内搜索引擎的索引， 但使用这一信息进行大规模的内容聚合时可能会发现某些用户会尝试利用不合适的关键字来对排序机制作弊。
referrer值必须是 引荐策略, 它为 Document 定义了默认的
引荐策略。 [REFERRERPOLICY]
如果有 meta 元素 插入文档 或 从文档中移除，或者现有的 meta 元素的
name 或 content 属性变化，
用户代理必须执行以下算法：
对 candidate elements 中的每一个 element：
令 value 为 element 的 content 属性
转化为 ASCII 小写 后的值。
如果 value 的值属于下表中第一列，则设置 value 为第二列的值：
| 遗留的值 | 引荐策略 |
|---|---|
never
| no-referrer
|
default
| no-referrer-when-downgrade
|
always
| unsafe-url
|
origin-when-crossorigin
| origin-when-cross-origin
|
对每个元素应用这些步骤使得 旧的用户代理仍然可以使用这些值。 [REFERRERPOLICY]
theme-color值必须是匹配 CSS <color> 生成式的字符串, 为用户代理定义了一个建议的颜色， 用来自定义该页面或周边 UI 的显示。例如，浏览器可能会用指定的颜色渲染页面的标题栏， 或者把它作为任标签栏或务栏的高亮颜色。
每个文档至少要有一个 meta 元素的 name 属性设为
theme-color。
This standard itself uses "WHATWG green" as its theme color:
<!DOCTYPE HTML> <title>HTML Standard</title> <meta name="theme-color" content="#3c790a"> ...
用户代理必须执行以下步骤来获取页面的主题颜色：
令 candidate elements 为所有符合以下条件的 meta 元素，
按照 树序 排列：
name 属性，其值为 theme-colorcontent 属性对 candidate elements 中每一个 element：
令 value 为把
element 的 content 属性的值
移除前后 ASCII 空白
后的结果。
如果 value 可以解析为 CSS 颜色值, 返回解析后的值。
如果到达了这一步，该页面没有主题颜色。
如果有任何 meta 元素 插入到文档中 或者 从文档中移除, 或者现有 meta 元素的
name 或 content
属性发生了改变，用户代理必须重新执行以上算法，并应用所有影响 UI 的结果。
在 UI 中使用主题颜色时，用户代理可能会调整具体的实现细节。例如，如果用户代理要用主题色作背景在上面显示白色的文字， 则可能会在 UI 的这一部分使用主题颜色的较暗变体，以确保充分的对比度。
任何人都可以 对预定义元数据名称集的扩展， 创建和使用自己的元数据。而且无需注册此类扩展。
但是在以下任何情况下，都不应创建新的元数据名：
如果名称或它的 content 属性值是 URL；在着这些情况下，建议把它注册为一个 超链接预定义类型的扩展（而不是创建一个新的元数据名称）。
如果该名称需要用户代理进行处理，那么它应该被标准化。
另外，在创建和使用新的元数据名称之前，建议查阅 WHATWG Wiki MetaExtensions 页面 — 来避免选择已经在使用的元数据名，以及与既有元数据名的用途冗余， 以及避免新标准化的名称与你选择的冲突。 [WHATWGWIKI]
任何人在任何时候都可以编辑 WHATWG Wiki MetaExtensions 页面，添加想要的元数据名称。 新的元数据名称可以用下列信息来说明：
要定义的实际名称。名称不应与任何其他定义的名称混淆相似（比如，仅区分大小写）。
对这个元数据名称的含义的一个简短的非规范的描述，包括它的取值所需的格式。
具有完全相同的处理要求的其他名称的列表。 作者不应使用定义为同义词的名称（它们只是用于允许用户代理支持遗留内容）。 任何人都可以移除实践中不再使用的同义词；只有需要当做同义词做兼容处理的名称才能以这种方式注册。
以下之一：
如果一个元数据名称与其他现存的值冗余，那么应该把它移除并列为那个现存值的同义词。
如果一个元数据名称添加到 “已提议” 状态经过一个多月的时间并未被使用或标准化， 那么应该把它从 WHATWG Wiki MetaExtensions 页面移除。
如果一个元数据名称添加到 “已提议” 状态，并且发现与现存的值冗余， 就应该把它移除并列为该现存值的同义词。 如果一个名称被添加到 “已提议” 状态但被发现有害，则应该把它改为 “已终止” 状态。
任何人都可以随时更新状态，但应该根据上述定义进行更新。
When the http-equiv attribute is specified on a
meta element, the element is a pragma directive.
The http-equiv attribute is an enumerated
attribute. The following table lists the keywords defined for this attribute. The states
given in the first cell of the rows with keywords give the states to which those keywords map.
Some of the keywords are non-conforming, as noted in the last
column.
| State | Keyword | Notes |
|---|---|---|
| Content Language | content-language
| Non-conforming |
| Encoding declaration | content-type
| |
| Default style | default-style
| |
| Refresh | refresh
| |
| Set-Cookie | set-cookie
| Non-conforming |
| X-UA-Compatible | x-ua-compatible
| |
| Content security policy | content-security-policy
|
When a meta element is inserted
into the document, if its http-equiv attribute is
present and represents one of the above states, then the user agent must run the algorithm
appropriate for that state, as described in the following list:
http-equiv="content-language")
This feature is non-conforming. Authors are encouraged to use the lang attribute instead.
This pragma sets the pragma-set default language. Until such a pragma is successfully processed, there is no pragma-set default language.
If the element's content attribute contains a
U+002C COMMA character (,) then return.
Let input be the value of the element's content attribute.
Let position point at the first character of input.
Skip ASCII whitespace within input given position.
Collect a sequence of code points that are not ASCII whitespace from input given position.
Let candidate be the string that resulted from the previous step.
If candidate is the empty string, return.
Set the pragma-set default language to candidate.
If the value consists of multiple space-separated tokens, tokens after the first are ignored.
This pragma is almost, but not quite, entirely unlike the HTTP `Content-Language` header of the same name. [HTTP]
http-equiv="content-type")
The Encoding declaration state is
just an alternative form of setting the charset
attribute: it is a character encoding declaration. This state's user
agent requirements are all handled by the parsing section of the specification.
For meta elements with an http-equiv
attribute in the Encoding declaration
state, the content attribute must have a value
that is an ASCII case-insensitive match for a string that consists of: the literal
string "text/html;", optionally followed by any number of ASCII
whitespace, followed by the literal string "charset=utf-8".
A document must not contain both a meta element with an http-equiv attribute in the Encoding declaration state and a
meta element with the charset attribute
present.
The Encoding declaration state may be
used in HTML documents, but elements with an http-equiv attribute in that state must not be used in
XML documents.
http-equiv="default-style")
This pragma sets the name of the default CSS style sheet set.
If the meta element has no content
attribute, or if that attribute's value is the empty string, then return.
Change the preferred CSS style sheet set name with the name being the value
of the element's content attribute. [CSSOM]
http-equiv="refresh")
This pragma acts as timed redirect.
A Document object has an associated will declaratively
refresh (a boolean). It is initially false.
If the meta element has no content
attribute, or if that attribute's value is the empty string, then return.
Let input be the value of the element's content attribute.
Run the shared declarative refresh steps with the meta
element's node document, input, and the meta
element.
The shared declarative refresh steps, given a Document object
document, string input, and optionally a meta element
meta, are as follows:
If document's will declaratively refresh is true, then return.
Let position point at the first code point of input.
Skip ASCII whitespace within input given position.
Let time be 0.
Collect a sequence of code points that are ASCII digits from input given position, and let the result be timeString.
If timeString is the empty string, then:
If the code point in input pointed to by position is not U+002E (.), then return.
Otherwise, set time to the result of parsing timeString using the rules for parsing non-negative integers.
Collect a sequence of code points that are ASCII digits and U+002E FULL STOP characters (.) from input given position. Ignore any collected characters.
Let urlRecord be document's URL.
If position is not past the end of input, then:
If the code point in input pointed to by position is not U+003B (;), U+002C (,), or ASCII whitespace, then return.
Skip ASCII whitespace within input given position.
If the code point in input pointed to by position is U+003B (;) or U+002C (,), then advance position to the next code point.
Skip ASCII whitespace within input given position.
If position is not past the end of input, then:
Let urlString be the substring of input from the code point at position to the end of the string.
If the code point in input pointed to by position is U+0055 (U) or U+0075 (u), then advance position to the next code point. Otherwise, jump to the step labeled skip quotes.
If the code point in input pointed to by position is U+0052 (R) or U+0072 (r), then advance position to the next code point. Otherwise, jump to the step labeled parse.
If the code point in input pointed to by position is U+004C (L) or U+006C (l), then advance position to the next code point. Otherwise, jump to the step labeled parse.
Skip ASCII whitespace within input given position.
If the code point in input pointed to by position is U+003D (=), then advance position to the next code point. Otherwise, jump to the step labeled parse.
Skip ASCII whitespace within input given position.
Skip quotes: If the code point in input pointed to by position is U+0027 (') or U+0022 ("), then let quote be that code point, and advance position to the next code point. Otherwise, let quote be the empty string.
Set urlString to the substring of input from the code point at position to the end of the string.
If quote is not the empty string, and there is a code point in urlString equal to quote, then truncate urlString at that code point, so that it and all subsequent code points are removed.
Parse: Parse urlString relative to document. If that fails, return. Otherwise, set urlRecord to the resulting URL record.
Set document's will declaratively refresh to true.
Perform one or more of the following steps:
After the refresh has come due (as defined below), if the user has not canceled the
redirect and, if meta is given, document's active sandboxing
flag set does not have the sandboxed automatic features browsing context
flag set, then navigate
document's browsing context to
urlRecord, with historyHandling set to "replace" and the source browsing context set to
document's browsing context.
For the purposes of the previous paragraph, a refresh is said to have come due as soon as the later of the following two conditions occurs:
It is important to use document here, and not meta's
node document, as that might have changed between the initial set of steps and
the refresh coming due and meta is not always given (in case of the HTTP
`Refresh` header).
Provide the user with an interface that, when selected, navigates a browsing context to urlRecord, with document's browsing context as the source browsing context.
Do nothing.
In addition, the user agent may, as with anything, inform the user of any and all aspects of its operation, including the state of any timers, the destinations of any timed redirects, and so forth.
For meta elements with an http-equiv
attribute in the Refresh state, the content attribute must have a value consisting either of:
URL",
followed by a U+003D EQUALS SIGN character (=), followed by a valid URL string
that does not start with a literal U+0027 APOSTROPHE (') or U+0022 QUOTATION MARK (")
character.In the former case, the integer represents a number of seconds before the page is to be reloaded; in the latter case the integer represents a number of seconds before the page is to be replaced by the page at the given URL.
A news organization's front page could include the following markup in the page's
head element, to ensure that the page automatically reloads from the server every
five minutes:
<meta http-equiv="Refresh" content="300">A sequence of pages could be used as an automated slide show by making each page refresh to the next page in the sequence, using markup such as the following:
<meta http-equiv="Refresh" content="20; URL=page4.html">http-equiv="set-cookie")
This pragma is non-conforming and has no effect.
User agents are required to ignore this pragma.
http-equiv="x-ua-compatible")
In practice, this pragma encourages Internet Explorer to more closely follow the specifications.
For meta elements with an http-equiv
attribute in the X-UA-Compatible state, the
content attribute must have a value that is an
ASCII case-insensitive match for the string "IE=edge".
User agents are required to ignore this pragma.
http-equiv="content-security-policy")
This pragma enforces a Content Security
Policy on a Document. [CSP]
If the meta element is not a child of a head element,
return.
If the meta element has no content
attribute, or if that attribute's value is the empty string, then return.
Let policy be the result of executing Content Security Policy's parse
a serialized Content Security Policy algorithm on the meta element's
content attribute's value, with a source of "meta",
and a disposition of "enforce".
Remove all occurrences of the report-uri, frame-ancestors, and sandbox directives from policy.
Enforce the policy policy.
For meta elements with an http-equiv
attribute in the Content security
policy state, the content attribute must have a
value consisting of a valid Content Security
Policy, but must not contain any report-uri,
frame-ancestors, or sandbox directives.
The Content Security Policy given in the content attribute will be enforced upon the current document. [CSP]
A page might choose to mitigate the risk of cross-site scripting attacks by preventing the execution of inline JavaScript, as well as blocking all plugin content, using a policy such as the following:
<meta http-equiv="Content-Security-Policy" content="script-src 'self'; object-src 'none'">There must not be more than one meta element with any particular state in the
document at a time.
A character encoding declaration is a mechanism by which the character encoding used to store or transmit a document is specified.
The Encoding standard requires use of the UTF-8 character
encoding and requires use of the "utf-8" encoding label
to identify it. Those requirements necessitate that the document's character encoding
declaration, if it exists, specifies an encoding label using an ASCII
case-insensitive match for "utf-8". Regardless of whether a
character encoding declaration is present or not, the actual character encoding used to encode the document must be
UTF-8. [ENCODING]
To enforce the above rules, authoring tools must default to using UTF-8 for newly-created documents.
The following restrictions also apply:
In addition, due to a number of restrictions on meta elements, there can only be
one meta-based character encoding declaration per document.
If an HTML document does not start with a BOM, and its
encoding is not explicitly given by Content-Type
metadata, and the document is not an iframe srcdoc document, then the encoding must be specified
using a meta element with a charset attribute
or a meta element with an http-equiv
attribute in the Encoding declaration
state.
A character encoding declaration is required (either in the Content-Type metadata or explicitly in the file) even when all characters are in the ASCII range, because a character encoding is needed to process non-ASCII characters entered by the user in forms, in URLs generated by scripts, and so forth.
Using non-UTF-8 encodings can have unexpected results on form submission and URL encodings, which use the document's character encoding by default.
If the document is an iframe srcdoc
document, the document must not have a character encoding declaration. (In
this case, the source is already decoded, since it is part of the document that contained the
iframe.)
In XML, the XML declaration should be used for inline character encoding information, if necessary.
In HTML, to declare that the character encoding is UTF-8, the author could
include the following markup near the top of the document (in the head element):
<meta charset="utf-8">In XML, the XML declaration would be used instead, at the very top of the markup:
<?xml version="1.0" encoding="utf-8"?>style elementSupport in all current engines.
Support in all current engines.
noscript element that is a child of a head element.media — Applicable mediatitle attribute has special semantics on this element: CSS style sheet set name.[Exposed=Window]
interface HTMLStyleElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString media;
// also has obsolete members
};
HTMLStyleElement includes LinkStyle;The style element allows authors to embed CSS style sheets in their documents.
The style element is one of several inputs to the styling processing
model. The element does not represent content for the
user.
The media attribute
says which media the styles apply to. The value must be a valid media query list.
The user agent must apply the styles when the media attribute's value matches the environment and
the other relevant conditions apply, and must not apply them otherwise.
The styles might be further limited in scope, e.g. in CSS with the use of @media blocks. This specification does not override such further restrictions or
requirements.
The default, if the media
attribute is omitted, is "all", meaning that by default styles apply to all
media.
The title attribute on style elements defines
CSS style sheet sets. If the style element
has no title attribute, then it has no title; the title attribute of ancestors does not apply to the style
element. If the style element is not in a document tree, then the title attribute is ignored. [CSSOM]
The title attribute on style
elements, like the title attribute on link
elements, differs from the global title attribute in that a
style block without a title does not inherit the title of the parent element: it
merely has no title.
The child text content of a style element must be that of a
conformant style sheet.
The user agent must run the update a style block algorithm whenever
one of the following conditions occur:
The element is popped off the stack of open elements of an HTML parser or XML parser.
The element is not on the stack of open elements of an HTML parser or XML parser, and it becomes connected or disconnected.
The element's children changed steps run.
The update a style block algorithm is as follows:
Let element be the style element.
If element has an associated CSS style sheet, remove the CSS style sheet in question.
If element's root is neither a shadow root nor a document, then return.
If element's type attribute is present and
its value is neither the empty string nor an ASCII case-insensitive match for
"text/css", then return.
In particular, a type value with
parameters, such as "text/css; charset=utf-8", will cause this algorithm
to return early.
If the Should element's inline behavior be blocked by Content Security
Policy? algorithm returns "Blocked" when executed upon the
style element, "style", and the style
element's child text content, then return. [CSP]
Create a CSS style sheet with the following properties:
element
The media attribute of element.
This is a reference to the (possibly absent at this time) attribute, rather than a copy of the attribute's current value. CSSOM defines what happens when the attribute is dynamically set, changed, or removed.
The title attribute of element, if
element is in a document tree, or the empty string otherwise.
Again, this is a reference to the attribute.
Unset.
Set.
null
Left at its default value.
Left uninitialized.
This doesn't seem right. Presumably we should be using the element's child text content? Tracked as issue #2997.
Once the attempts to obtain the style sheet's critical subresources, if any, are complete, or, if the style sheet has no critical subresources, once the style sheet has been parsed and processed, the user agent must run these steps:
Let element be the style element associated with the style sheet
in question.
Let success be true.
If the attempts to obtain any of the style sheet's critical subresources failed for any reason (e.g., DNS error, HTTP 404 response, a connection being prematurely closed, unsupported Content-Type), set success to false.
Note that content-specific errors, e.g., CSS parse errors or PNG decoding errors, do not affect success.
Queue an element task on the networking task source given element and the following steps:
If success is true, fire an event
named load at element.
Otherwise, fire an event named error at element.
If element contributes a script-blocking style sheet:
Assert: element's node document's script-blocking style sheet counter is greater than 0.
Decrement element's node document's script-blocking style sheet counter by 1.
The element must delay the load event of the element's node document until all the attempts to obtain the style sheet's critical subresources, if any, are complete.
This specification does not specify a style system, but CSS is expected to be supported by most web browsers. [CSS]
Support in all current engines.
The media IDL
attribute must reflect the content attribute of the same name.
The LinkStyle interface is also implemented by this element. [CSSOM]
The following document has its stress emphasis styled as bright red text rather than italics text, while leaving titles of works and Latin words in their default italics. It shows how using appropriate elements enables easier restyling of documents.
<!DOCTYPE html>
<html lang="en-US">
<head>
<title>My favorite book</title>
<style>
body { color: black; background: white; }
em { font-style: normal; color: red; }
</style>
</head>
<body>
<p>My <em>favorite</em> book of all time has <em>got</em> to be
<cite>A Cat's Life</cite>. It is a book by P. Rahmel that talks
about the <i lang="la">Felis Catus</i> in modern human society.</p>
</body>
</html>If the style sheet referenced no other resources (e.g., it was an internal style sheet given by
a style element with no @import rules), then the style rules
must be immediately made available to script; otherwise, the style rules must only be
made available to script once the event loop reaches its update the
rendering step.
An element el in the context of a
Document of an HTML parser or XML parser contributes a
script-blocking style sheet if all of the following conditions are true:
el was created by that Document's parser.
el is either a style element or a link element that
was an external resource link that contributes to the styling
processing model when the el was created by the parser.
If the el is a link element, it's media attribute's value matches the
environment.
el's style sheet was enabled when the element was created by the parser.
The last time the event loop reached step 1,
el's root was that Document.
The user agent hasn't given up on loading that particular style sheet yet. A user agent may give up on loading a style sheet at any time.
Giving up on a style sheet before the style sheet loads, if the style sheet eventually does still load, means that the script might end up operating with incorrect information. For example, if a style sheet sets the color of an element to green, but a script that inspects the resulting style is executed before the sheet is loaded, the script will find that the element is black (or whatever the default color is), and might thus make poor choices (e.g., deciding to use black as the color elsewhere on the page, instead of green). Implementers have to balance the likelihood of a script using incorrect information with the performance impact of doing nothing while waiting for a slow network request to finish.
It is expected that counterparts to the above rules also apply to
<?xml-stylesheet?> PIs and HTTP `Link` headers.
However, this has not yet been thoroughly investigated.
A Document has a script-blocking style sheet counter, which is a
number, initially 0.
A Document has a style sheet that is blocking scripts if its
script-blocking style sheet counter is greater than 0, or if that
Document has a non-null browsing context
whose container document is non-null and has a
script-blocking style sheet counter greater than 0.
A Document has no style sheet that is blocking scripts if it does not
have a style sheet that is blocking
scripts as defined in the previous paragraph.
body elementSupport in all current engines.
Support in all current engines.
html element.body element's start tag can be omitted
if the element is empty, or if the first thing inside the body element is not
ASCII whitespace or a comment, except if the
first thing inside the body element is a meta, link,
script, style, or template element. body element's end tag can be omitted if the
body element is not immediately followed by a comment.onafterprintonbeforeprintonbeforeunloadonhashchangeonlanguagechangeonmessageonmessageerroronofflineononlineonpagehideonpageshowonpopstateonrejectionhandledonstorageonunhandledrejectiononunload[Exposed=Window]
interface HTMLBodyElement : HTMLElement {
[HTMLConstructor] constructor();
// also has obsolete members
};
HTMLBodyElement includes WindowEventHandlers;The body element represents the contents of the document.
In conforming documents, there is only one body element. The document.body IDL attribute provides scripts with easy access to
a document's body element.
Some DOM operations (for example, parts of the drag and drop model)
are defined in terms of "the body element". This refers to a particular element in
the DOM, as per the definition of the term, and not any arbitrary body element.
The body element exposes as event handler content attributes a number
of the event handlers of the Window object. It also mirrors their
event handler IDL attributes.
The event handlers of the Window object named by the
Window-reflecting body element event handler set, exposed on the
body element, replace the generic event handlers with the same names
normally supported by HTML elements.
Thus, for example, a bubbling error event
dispatched on a child of the body element of a Document would first
trigger the onerror event handler content
attributes of that element, then that of the root html element, and only
then would it trigger the onerror event handler content attribute on the
body element. This is because the event would bubble from the target, to the
body, to the html, to the Document, to the
Window, and the event handler on the
body is watching the Window not the body. A regular event
listener attached to the body using addEventListener(),
however, would be run when the event bubbled through the body and not when it reaches
the Window object.
This page updates an indicator to show whether or not the user is online:
<!DOCTYPE HTML>
<html lang="en">
<head>
<title>Online or offline?</title>
<script>
function update(online) {
document.getElementById('status').textContent =
online ? 'Online' : 'Offline';
}
</script>
</head>
<body ononline="update(true)"
onoffline="update(false)"
onload="update(navigator.onLine)">
<p>You are: <span id="status">(Unknown)</span></p>
</body>
</html>article elementSupport in all current engines.
HTMLElement.The article element represents a complete, or self-contained,
composition in a document, page, application, or site and that is, in principle, independently
distributable or reusable, e.g. in syndication. This could be a forum post, a magazine or
newspaper article, a blog entry, a user-submitted comment, an interactive widget or gadget, or any
other independent item of content.
When article elements are nested, the inner article elements
represent articles that are in principle related to the contents of the outer article. For
instance, a blog entry on a site that accepts user-submitted comments could represent the comments
as article elements nested within the article element for the blog
entry.
Author information associated with an article element (q.v. the
address element) does not apply to nested article elements.
When used specifically with content to be redistributed in syndication, the
article element is similar in purpose to the entry element in
Atom. [ATOM]
The schema.org microdata vocabulary can be used to provide the publication date
for an article element, using one of the CreativeWork subtypes.
When the main content of the page (i.e. excluding footers, headers, navigation blocks, and
sidebars) is all one single self-contained composition, that content may be marked with an
article, but it is technically redundant in that case (since it's self-evident that
the page is a single composition, as it is a single document).
This example shows a blog post using the article element, with some schema.org
annotations:
<article itemscope itemtype="http://schema.org/BlogPosting">
<header>
<h1 itemprop="headline">The Very First Rule of Life</h1>
<p><time itemprop="datePublished" datetime="2009-10-09">3 days ago</time></p>
<link itemprop="url" href="?comments=0">
</header>
<p>If there's a microphone anywhere near you, assume it's hot and
sending whatever you're saying to the world. Seriously.</p>
<p>...</p>
<footer>
<a itemprop="discussionUrl" href="?comments=1">Show comments...</a>
</footer>
</article>Here is that same blog post, but showing some of the comments:
<article itemscope itemtype="http://schema.org/BlogPosting">
<header>
<h1 itemprop="headline">The Very First Rule of Life</h1>
<p><time itemprop="datePublished" datetime="2009-10-09">3 days ago</time></p>
<link itemprop="url" href="?comments=0">
</header>
<p>If there's a microphone anywhere near you, assume it's hot and
sending whatever you're saying to the world. Seriously.</p>
<p>...</p>
<section>
<h1>Comments</h1>
<article itemprop="comment" itemscope itemtype="http://schema.org/UserComments" id="c1">
<link itemprop="url" href="#c1">
<footer>
<p>Posted by: <span itemprop="creator" itemscope itemtype="http://schema.org/Person">
<span itemprop="name">George Washington</span>
</span></p>
<p><time itemprop="commentTime" datetime="2009-10-10">15 minutes ago</time></p>
</footer>
<p>Yeah! Especially when talking about your lobbyist friends!</p>
</article>
<article itemprop="comment" itemscope itemtype="http://schema.org/UserComments" id="c2">
<link itemprop="url" href="#c2">
<footer>
<p>Posted by: <span itemprop="creator" itemscope itemtype="http://schema.org/Person">
<span itemprop="name">George Hammond</span>
</span></p>
<p><time itemprop="commentTime" datetime="2009-10-10">5 minutes ago</time></p>
</footer>
<p>Hey, you have the same first name as me.</p>
</article>
</section>
</article>Notice the use of footer to give the information for each comment (such as who
wrote it and when): the footer element can appear at the start of its
section when appropriate, such as in this case. (Using header in this case wouldn't
be wrong either; it's mostly a matter of authoring preference.)
In this example, article elements are used to host widgets on a portal page. The
widgets are implemented as customized built-in
elements in order to get specific styling and scripted behavior.
<!DOCTYPE HTML>
<html lang=en>
<title>eHome Portal</title>
<script src="/scripts/widgets.js"></script>
<link rel=stylesheet href="/styles/main.css">
<article is="stock-widget">
<h1>Stocks</h1>
<table>
<thead> <tr> <th> Stock <th> Value <th> Delta
<tbody> <template> <tr> <td> <td> <td> </template>
</table>
<p> <input type=button value="Refresh" onclick="this.parentElement.refresh()">
</article>
<article is="news-widget">
<h1>News</h1>
<ul>
<template>
<li>
<p><img> <strong></strong>
<p>
</template>
</ul>
<p> <input type=button value="Refresh" onclick="this.parentElement.refresh()">
</article>section elementSupport in all current engines.
HTMLElement.The section element represents a generic section of a document or
application. A section, in this context, is a thematic grouping of content, typically with a
heading.
Examples of sections would be chapters, the various tabbed pages in a tabbed dialog box, or the numbered sections of a thesis. A web site's home page could be split into sections for an introduction, news items, and contact information.
Authors are encouraged to use the article element instead of the
section element when it would make sense to syndicate the contents of the
element.
The section element is not a generic
container element. When an element is needed only for styling purposes or as a convenience for
scripting, authors are encouraged to use the div element instead. A general rule is
that the section element is appropriate only if the element's contents would be
listed explicitly in the document's outline.
In the following example, we see an article (part of a larger web page) about apples, containing two short sections.
<article>
<hgroup>
<h1>Apples</h1>
<h2>Tasty, delicious fruit!</h2>
</hgroup>
<p>The apple is the pomaceous fruit of the apple tree.</p>
<section>
<h1>Red Delicious</h1>
<p>These bright red apples are the most common found in many
supermarkets.</p>
</section>
<section>
<h1>Granny Smith</h1>
<p>These juicy, green apples make a great filling for
apple pies.</p>
</section>
</article>Notice how the use of section means that the author can use h1
elements throughout, without having to worry about whether a particular section is at the top
level, the second level, the third level, and so on.
Here is a graduation programme with two sections, one for the list of people graduating, and one for the description of the ceremony. (The markup in this example features an uncommon style sometimes used to minimize the amount of inter-element whitespace.)
<!DOCTYPE Html>
<Html Lang=En
><Head
><Title
>Graduation Ceremony Summer 2022</Title
></Head
><Body
><H1
>Graduation</H1
><Section
><H1
>Ceremony</H1
><P
>Opening Procession</P
><P
>Speech by Validactorian</P
><P
>Speech by Class President</P
><P
>Presentation of Diplomas</P
><P
>Closing Speech by Headmaster</P
></Section
><Section
><H1
>Graduates</H1
><Ul
><Li
>Molly Carpenter</Li
><Li
>Anastasia Luccio</Li
><Li
>Ebenezar McCoy</Li
><Li
>Karrin Murphy</Li
><Li
>Thomas Raith</Li
><Li
>Susan Rodriguez</Li
></Ul
></Section
></Body
></Html>In this example, a book author has marked up some sections as chapters and some as appendices, and uses CSS to style the headers in these two classes of section differently.
<style>
section { border: double medium; margin: 2em; }
section.chapter h1 { font: 2em Roboto, Helvetica Neue, sans-serif; }
section.appendix h1 { font: small-caps 2em Roboto, Helvetica Neue, sans-serif; }
</style>
<header>
<hgroup>
<h1>My Book</h1>
<h2>A sample with not much content</h2>
</hgroup>
<p><small>Published by Dummy Publicorp Ltd.</small></p>
</header>
<section class="chapter">
<h1>My First Chapter</h1>
<p>This is the first of my chapters. It doesn't say much.</p>
<p>But it has two paragraphs!</p>
</section>
<section class="chapter">
<h1>It Continues: The Second Chapter</h1>
<p>Bla dee bla, dee bla dee bla. Boom.</p>
</section>
<section class="chapter">
<h1>Chapter Three: A Further Example</h1>
<p>It's not like a battle between brightness and earthtones would go
unnoticed.</p>
<p>But it might ruin my story.</p>
</section>
<section class="appendix">
<h1>Appendix A: Overview of Examples</h1>
<p>These are demonstrations.</p>
</section>
<section class="appendix">
<h1>Appendix B: Some Closing Remarks</h1>
<p>Hopefully this long example shows that you <em>can</em> style
sections, so long as they are used to indicate actual sections.</p>
</section>nav elementSupport in all current engines.
HTMLElement.The nav element represents a section of a page that links to other
pages or to parts within the page: a section with navigation links.
Not all groups of links on a page need to be in a nav element —
the element is primarily intended for sections that consist of major navigation blocks. In
particular, it is common for footers to have a short list of links to various pages of a site,
such as the terms of service, the home page, and a copyright page. The footer element
alone is sufficient for such cases; while a nav element can be used in such cases, it
is usually unnecessary.
User agents (such as screen readers) that are targeted at users who can benefit from navigation information being omitted in the initial rendering, or who can benefit from navigation information being immediately available, can use this element as a way to determine what content on the page to initially skip or provide on request (or both).
In the following example, there are two nav elements, one for primary navigation
around the site, and one for secondary navigation around the page itself.
<body>
<h1>The Wiki Center Of Exampland</h1>
<nav>
<ul>
<li><a href="/">Home</a></li>
<li><a href="/events">Current Events</a></li>
...more...
</ul>
</nav>
<article>
<header>
<h1>Demos in Exampland</h1>
<p>Written by A. N. Other.</p>
</header>
<nav>
<ul>
<li><a href="#public">Public demonstrations</a></li>
<li><a href="#destroy">Demolitions</a></li>
...more...
</ul>
</nav>
<div>
<section id="public">
<h1>Public demonstrations</h1>
<p>...more...</p>
</section>
<section id="destroy">
<h1>Demolitions</h1>
<p>...more...</p>
</section>
...more...
</div>
<footer>
<p><a href="?edit">Edit</a> | <a href="?delete">Delete</a> | <a href="?Rename">Rename</a></p>
</footer>
</article>
<footer>
<p><small>© copyright 1998 Exampland Emperor</small></p>
</footer>
</body>In the following example, the page has several places where links are present, but only one of those places is considered a navigation section.
<body itemscope itemtype="http://schema.org/Blog">
<header>
<h1>Wake up sheeple!</h1>
<p><a href="news.html">News</a> -
<a href="blog.html">Blog</a> -
<a href="forums.html">Forums</a></p>
<p>Last Modified: <span itemprop="dateModified">2009-04-01</span></p>
<nav>
<h1>Navigation</h1>
<ul>
<li><a href="articles.html">Index of all articles</a></li>
<li><a href="today.html">Things sheeple need to wake up for today</a></li>
<li><a href="successes.html">Sheeple we have managed to wake</a></li>
</ul>
</nav>
</header>
<main>
<article itemprop="blogPosts" itemscope itemtype="http://schema.org/BlogPosting">
<header>
<h1 itemprop="headline">My Day at the Beach</h1>
</header>
<div itemprop="articleBody">
<p>Today I went to the beach and had a lot of fun.</p>
...more content...
</div>
<footer>
<p>Posted <time itemprop="datePublished" datetime="2009-10-10">Thursday</time>.</p>
</footer>
</article>
...more blog posts...
</main>
<footer>
<p>Copyright ©
<span itemprop="copyrightYear">2010</span>
<span itemprop="copyrightHolder">The Example Company</span>
</p>
<p><a href="about.html">About</a> -
<a href="policy.html">Privacy Policy</a> -
<a href="contact.html">Contact Us</a></p>
</footer>
</body>You can also see microdata annotations in the above example that use the schema.org vocabulary to provide the publication date and other metadata about the blog post.
A nav element doesn't have to contain a list, it can contain other kinds of
content as well. In this navigation block, links are provided in prose:
<nav>
<h1>Navigation</h1>
<p>You are on my home page. To the north lies <a href="/blog">my
blog</a>, from whence the sounds of battle can be heard. To the east
you can see a large mountain, upon which many <a
href="/school">school papers</a> are littered. Far up thus mountain
you can spy a little figure who appears to be me, desperately
scribbling a <a href="/school/thesis">thesis</a>.</p>
<p>To the west are several exits. One fun-looking exit is labeled <a
href="https://games.example.com/">"games"</a>. Another more
boring-looking exit is labeled <a
href="https://isp.example.net/">ISP™</a>.</p>
<p>To the south lies a dark and dank <a href="/about">contacts
page</a>. Cobwebs cover its disused entrance, and at one point you
see a rat run quickly out of the page.</p>
</nav>In this example, nav is used in an email application, to let the user switch
folders:
<p><input type=button value="Compose" onclick="compose()"></p>
<nav>
<h1>Folders</h1>
<ul>
<li> <a href="/inbox" onclick="return openFolder(this.href)">Inbox</a> <span class=count></span>
<li> <a href="/sent" onclick="return openFolder(this.href)">Sent</a>
<li> <a href="/drafts" onclick="return openFolder(this.href)">Drafts</a>
<li> <a href="/trash" onclick="return openFolder(this.href)">Trash</a>
<li> <a href="/customers" onclick="return openFolder(this.href)">Customers</a>
</ul>
</nav>aside elementSupport in all current engines.
HTMLElement.The aside element represents a section of a page that consists of
content that is tangentially related to the content around the aside element, and
which could be considered separate from that content. Such sections are often represented as
sidebars in printed typography.
The element can be used for typographical effects like pull quotes or sidebars, for
advertising, for groups of nav elements, and for other content that is considered
separate from the main content of the page.
It's not appropriate to use the aside element just for
parentheticals, since those are part of the main flow of the document.
The following example shows how an aside is used to mark up background material on Switzerland in a much longer news story on Europe.
<aside>
<h1>Switzerland</h1>
<p>Switzerland, a land-locked country in the middle of geographic
Europe, has not joined the geopolitical European Union, though it is
a signatory to a number of European treaties.</p>
</aside>The following example shows how an aside is used to mark up a pull quote in a longer article.
...
<p>He later joined a large company, continuing on the same work.
<q>I love my job. People ask me what I do for fun when I'm not at
work. But I'm paid to do my hobby, so I never know what to
answer. Some people wonder what they would do if they didn't have to
work... but I know what I would do, because I was unemployed for a
year, and I filled that time doing exactly what I do now.</q></p>
<aside>
<q> People ask me what I do for fun when I'm not at work. But I'm
paid to do my hobby, so I never know what to answer. </q>
</aside>
<p>Of course his work — or should that be hobby? —
isn't his only passion. He also enjoys other pleasures.</p>
...The following extract shows how aside can be used for blogrolls and other side
content on a blog:
<body>
<header>
<h1>My wonderful blog</h1>
<p>My tagline</p>
</header>
<aside>
<!-- this aside contains two sections that are tangentially related
to the page, namely, links to other blogs, and links to blog posts
from this blog -->
<nav>
<h1>My blogroll</h1>
<ul>
<li><a href="https://blog.example.com/">Example Blog</a>
</ul>
</nav>
<nav>
<h1>Archives</h1>
<ol reversed>
<li><a href="/last-post">My last post</a>
<li><a href="/first-post">My first post</a>
</ol>
</nav>
</aside>
<aside>
<!-- this aside is tangentially related to the page also, it
contains twitter messages from the blog author -->
<h1>Twitter Feed</h1>
<blockquote cite="https://twitter.example.net/t31351234">
I'm on vacation, writing my blog.
</blockquote>
<blockquote cite="https://twitter.example.net/t31219752">
I'm going to go on vacation soon.
</blockquote>
</aside>
<article>
<!-- this is a blog post -->
<h1>My last post</h1>
<p>This is my last post.</p>
<footer>
<p><a href="/last-post" rel=bookmark>Permalink</a>
</footer>
</article>
<article>
<!-- this is also a blog post -->
<h1>My first post</h1>
<p>This is my first post.</p>
<aside>
<!-- this aside is about the blog post, since it's inside the
<article> element; it would be wrong, for instance, to put the
blogroll here, since the blogroll isn't really related to this post
specifically, only to the page as a whole -->
<h1>Posting</h1>
<p>While I'm thinking about it, I wanted to say something about
posting. Posting is fun!</p>
</aside>
<footer>
<p><a href="/first-post" rel=bookmark>Permalink</a>
</footer>
</article>
<footer>
<p><a href="/archives">Archives</a> -
<a href="/about">About me</a> -
<a href="/copyright">Copyright</a></p>
</footer>
</body>h1, h2, h3, h4, h5, and
h6 elementsSupport in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
hgroup element.[Exposed=Window]
interface HTMLHeadingElement : HTMLElement {
[HTMLConstructor] constructor();
// also has obsolete members
};These elements represent headings for their sections.
The semantics and meaning of these elements are defined in the section on headings and sections.
These elements have a rank given by the number in their name. The h1
element is said to have the highest rank, the h6 element has the lowest rank, and two
elements with the same name have equal rank.
As far as their respective document outlines (their heading and section structures) are concerned, these two snippets are semantically equivalent:
<body>
<h1>Let's call it a draw(ing surface)</h1>
<h2>Diving in</h2>
<h2>Simple shapes</h2>
<h2>Canvas coordinates</h2>
<h3>Canvas coordinates diagram</h3>
<h2>Paths</h2>
</body><body>
<h1>Let's call it a draw(ing surface)</h1>
<section>
<h1>Diving in</h1>
</section>
<section>
<h1>Simple shapes</h1>
</section>
<section>
<h1>Canvas coordinates</h1>
<section>
<h1>Canvas coordinates diagram</h1>
</section>
</section>
<section>
<h1>Paths</h1>
</section>
</body>Authors might prefer the former style for its terseness, or the latter style for its convenience in the face of heavy editing; which is best is purely an issue of preferred authoring style.
The two styles can be combined, for compatibility with legacy tools while still future-proofing for when that compatibility is no longer needed. This third snippet again has the same outline as the previous two:
<body>
<h1>Let's call it a draw(ing surface)</h1>
<section>
<h2>Diving in</h2>
</section>
<section>
<h2>Simple shapes</h2>
</section>
<section>
<h2>Canvas coordinates</h2>
<section>
<h3>Canvas coordinates diagram</h3>
</section>
</section>
<section>
<h2>Paths</h2>
</section>
</body>hgroup elementSupport in all current engines.
h1, h2, h3, h4,
h5, h6 elements, optionally intermixed with script-supporting
elements.HTMLElement.The hgroup element represents the heading of a section, which
consists of all the h1–h6 element children of the
hgroup element. The element is used to group a set of
h1–h6 elements when the heading has multiple levels, such as
subheadings, alternative titles, or taglines.
The rank of an hgroup element is the rank of the highest-ranked
h1–h6 element descendant of the hgroup element, if
there are any such elements, or otherwise the same as for an h1 element (the highest
rank). Other h1–h6 elements of heading content in the
hgroup element indicate subheadings or subtitles or (secondary) alternative
titles.
The section on headings and sections defines how hgroup elements are
assigned to individual sections.
Here are some examples of valid headings.
<hgroup>
<h1>The reality dysfunction</h1>
<h2>Space is not the only void</h2>
</hgroup><hgroup>
<h1>Dr. Strangelove</h1>
<h2>Or: How I Learned to Stop Worrying and Love the Bomb</h2>
</hgroup>The point of using hgroup in these examples is to prevent the h2
element (which acts as a secondary title) from creating a separate section of its own in any
outline and to instead cause the contents of the h2 to be shown in
rendered output from the outline algorithm in some way to indicate that it is not
the title of a separate section but instead just a secondary title in a group of titles.
How a user agent exposes such multi-level headings in user interfaces (e.g. in tables of contents or search results) is left open to implementers, as it is a user interface issue. The first example above could be rendered as:
The reality dysfunction: Space is not the only void
Alternatively, it could look like this:
The reality dysfunction (Space is not the only void)
In interfaces where a title can be rendered on multiple lines, it could be rendered as follows, maybe with the first line in a bigger font size:
The reality dysfunction Space is not the only void
The following two examples show ways in which two h1 headings could be used
within an hgroup element to group the US and UK names for the same movie.
<hgroup>
<h1>The Avengers</h1>
<h1>Avengers Assemble</h1>
</hgroup>
<hgroup>
<h1>Avengers Assemble</h1>
<h1>The Avengers</h1>
</hgroup>The first example above shows how the movie names might be grouped in a publication in the US, with the US name The Avengers as the (primary) title, and the UK name Avengers Assemble as the (secondary) alternative title. The second example above shows how the movie names might be grouped in a publication in the UK, with the UK name as the (primary) title, and the US name as the (secondary) alternative title.
In both cases it is important to note the use of the hgroup element to group the
two titles indicates that the titles are not equivalent; instead the first h1 gives
the (primary) title while the second gives the (secondary) alternative title. Even though both
the title and alternative title are marked up with h1 elements, in a rendered view
of output from the outline algorithm, the second h1 in the
hgroup will be shown in some way that clearly indicates it is secondary; for
example:
In a US publication:
The Avengers (Avengers Assemble)
In a UK publication:
Avengers Assemble (The Avengers)
In the following example, an hgroup element is used to mark up a two-level
heading in a wizard-style dialog box:
<dialog onclose="walletSetup.continue(this.returnValue)">
<hgroup>
<h1>Wallet Setup</h1>
<h2>Configure your Wallet funding source</h2>
</hgroup>
<p>Your Wallet can be used to buy wands at the merchant in town, to buy potions from travelling
salesmen you may find in the dungeons, and to pay for mercenaries.</p>
<p>We support two payment sources:</p>
<form method=dialog>
<fieldset oninput="this.getElementsByTagName('input')[0].checked = true">
<legend> <label> <input type=radio name=payment-type value=cc> Credit Card </label> </legend>
<p><label>Name on card: <input name=cc1 autocomplete="section-cc cc-name" placeholder="Y. Name"></label>
<p><label>Card number: <input name=cc2 inputmode=numeric autocomplete="section-cc cc-number" placeholder="6331 1019 9999 0016"></label>
<p><label>Expiry Date: <input name=cc3 type=month autocomplete="section-cc cc-exp" placeholder="2020-02"></label>
<p><label>Security Code: <input name=cc4 inputmode=numeric autocomplete="section-cc cc-csc" placeholder="246"></label>
</fieldset>
<fieldset oninput="this.getElementsByTagName('input')[0].checked = true">
<legend> <label> <input type=radio name=payment-type value=bank> Checking Account </label> </legend>
<p><label>Name on account: <input name=bank1 autocomplete="section-bank cc-name"></label>
<p><label>Routing number: <input name=bank2 inputmode=numeric></label>
<p><label>Account number: <input name=bank3 inputmode=numeric></label>
</fieldset>
<button type=submit value="back"> ← Back </button>
<button type=submit value="next"> Next → </button>
</form>
</dialog>header elementSupport in all current engines.
header or footer element
descendants.HTMLElement.The header element represents a group of introductory or navigational
aids.
A header element is intended to usually contain the section's heading
(an h1–h6 element or an hgroup element), but this is
not required. The header element can also be used to wrap a section's table of
contents, a search form, or any relevant logos.
Here are some sample headers. This first one is for a game:
<header>
<p>Welcome to...</p>
<h1>Voidwars!</h1>
</header>The following snippet shows how the element can be used to mark up a specification's header:
<header>
<hgroup>
<h1>Fullscreen API</h1>
<h2>Living Standard — Last Updated 19 October 2015</h2>
</hgroup>
<dl>
<dt>Participate:</dt>
<dd><a href="https://github.com/whatwg/fullscreen">GitHub whatwg/fullscreen</a></dd>
<dt>Commits:</dt>
<dd><a href="https://github.com/whatwg/fullscreen/commits">GitHub whatwg/fullscreen/commits</a></dd>
</dl>
</header>The header element is not sectioning content; it doesn't
introduce a new section.
In this example, the page has a page heading given by the h1 element, and two
subsections whose headings are given by h2 elements. The content after the
header element is still part of the last subsection started in the
header element, because the header element doesn't take part in the
outline algorithm.
<body>
<header>
<h1>Little Green Guys With Guns</h1>
<nav>
<ul>
<li><a href="/games">Games</a>
<li><a href="/forum">Forum</a>
<li><a href="/download">Download</a>
</ul>
</nav>
<h2>Important News</h2> <!-- this starts a second subsection -->
<!-- this is part of the subsection entitled "Important News" -->
<p>To play today's games you will need to update your client.</p>
<h2>Games</h2> <!-- this starts a third subsection -->
</header>
<p>You have three active games:</p>
<!-- this is still part of the subsection entitled "Games" -->
...footer elementSupport in all current engines.
header or footer element
descendants.HTMLElement.The footer element represents a footer for its nearest ancestor
sectioning content or sectioning root element. A footer typically
contains information about its section such as who wrote it, links to related documents, copyright
data, and the like.
When the footer element contains entire sections, they represent appendices, indexes, long colophons, verbose license
agreements, and other such content.
Contact information for the author or editor of a section belongs in an
address element, possibly itself inside a footer. Bylines and other
information that could be suitable for both a header or a footer can be
placed in either (or neither). The primary purpose of these elements is merely to help the author
write self-explanatory markup that is easy to maintain and style; they are not intended to impose
specific structures on authors.
Footers don't necessarily have to appear at the end of a section, though they usually do.
When the nearest ancestor sectioning content or sectioning root element is the body element, then it applies to the whole page.
The footer element is not sectioning content; it doesn't
introduce a new section.
Here is a page with two footers, one at the top and one at the bottom, with the same content:
<body>
<footer><a href="../">Back to index...</a></footer>
<hgroup>
<h1>Lorem ipsum</h1>
<h2>The ipsum of all lorems</h2>
</hgroup>
<p>A dolor sit amet, consectetur adipisicing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim
veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex
ea commodo consequat. Duis aute irure dolor in reprehenderit in
voluptate velit esse cillum dolore eu fugiat nulla
pariatur. Excepteur sint occaecat cupidatat non proident, sunt in
culpa qui officia deserunt mollit anim id est laborum.</p>
<footer><a href="../">Back to index...</a></footer>
</body>Here is an example which shows the footer element being used both for a site-wide
footer and for a section footer.
<!DOCTYPE HTML>
<HTML LANG="en"><HEAD>
<TITLE>The Ramblings of a Scientist</TITLE>
<BODY>
<H1>The Ramblings of a Scientist</H1>
<ARTICLE>
<H1>Episode 15</H1>
<VIDEO SRC="/fm/015.ogv" CONTROLS PRELOAD>
<P><A HREF="/fm/015.ogv">Download video</A>.</P>
</VIDEO>
<FOOTER> <!-- footer for article -->
<P>Published <TIME DATETIME="2009-10-21T18:26-07:00">on 2009/10/21 at 6:26pm</TIME></P>
</FOOTER>
</ARTICLE>
<ARTICLE>
<H1>My Favorite Trains</H1>
<P>I love my trains. My favorite train of all time is a Köf.</P>
<P>It is fun to see them pull some coal cars because they look so
dwarfed in comparison.</P>
<FOOTER> <!-- footer for article -->
<P>Published <TIME DATETIME="2009-09-15T14:54-07:00">on 2009/09/15 at 2:54pm</TIME></P>
</FOOTER>
</ARTICLE>
<FOOTER> <!-- site wide footer -->
<NAV>
<P><A HREF="/credits.html">Credits</A> —
<A HREF="/tos.html">Terms of Service</A> —
<A HREF="/index.html">Blog Index</A></P>
</NAV>
<P>Copyright © 2009 Gordon Freeman</P>
</FOOTER>
</BODY>
</HTML>Some site designs have what is sometimes referred to as "fat footers" — footers that contain a lot of material, including images, links to other articles, links to pages for sending feedback, special offers... in some ways, a whole "front page" in the footer.
This fragment shows the bottom of a page on a site with a "fat footer":
...
<footer>
<nav>
<section>
<h1>Articles</h1>
<p><img src="images/somersaults.jpeg" alt=""> Go to the gym with
our somersaults class! Our teacher Jim takes you through the paces
in this two-part article. <a href="articles/somersaults/1">Part
1</a> · <a href="articles/somersaults/2">Part 2</a></p>
<p><img src="images/kindplus.jpeg"> Tired of walking on the edge of
a clif<!-- sic -->? Our guest writer Lara shows you how to bumble
your way through the bars. <a href="articles/kindplus/1">Read
more...</a></p>
<p><img src="images/crisps.jpeg"> The chips are down, now all
that's left is a potato. What can you do with it? <a
href="articles/crisps/1">Read more...</a></p>
</section>
<ul>
<li> <a href="/about">About us...</a>
<li> <a href="/feedback">Send feedback!</a>
<li> <a href="/sitemap">Sitemap</a>
</ul>
</nav>
<p><small>Copyright © 2015 The Snacker —
<a href="/tos">Terms of Service</a></small></p>
</footer>
</body>address elementSupport in all current engines.
header, footer, or
address element descendants.HTMLElement.The address element represents the contact information for its
nearest article or body element ancestor. If that is the body
element, then the contact information applies to the document as a whole.
For example, a page at the W3C web site related to HTML might include the following contact information:
<ADDRESS>
<A href="../People/Raggett/">Dave Raggett</A>,
<A href="../People/Arnaud/">Arnaud Le Hors</A>,
contact persons for the <A href="Activity">W3C HTML Activity</A>
</ADDRESS>The address element must not be used to represent arbitrary addresses (e.g. postal
addresses), unless those addresses are in fact the relevant contact information. (The
p element is the appropriate element for marking up postal addresses in general.)
The address element must not contain information other than contact
information.
For example, the following is non-conforming use of the
address element:
<ADDRESS>Last Modified: 1999/12/24 23:37:50</ADDRESS>Typically, the address element would be included along with other information in a
footer element.
The contact information for a node node is a collection of
address elements defined by the first applicable entry from the following list:
article elementbody elementThe contact information consists of all the address elements that have node as an ancestor and do not have another body or
article element ancestor that is a descendant of node.
article elementbody elementThe contact information of node is the same as the contact information of
the nearest article or body element ancestor, whichever is
nearest.
The contact information of node is the same as the contact information of
the body element of the Document.
There is no contact information for node.
User agents may expose the contact information of a node to the user, or use it for other purposes, such as indexing sections based on the sections' contact information.
In this example the footer contains contact information and a copyright notice.
<footer>
<address>
For more details, contact
<a href="mailto:js@example.com">John Smith</a>.
</address>
<p><small>© copyright 2038 Example Corp.</small></p>
</footer>章节内容 元素中的第一个 标题内容 元素 表示着 那个章节的标题。 后续同样 级别 或更高级别的标题（默认）开启新的章节， 更低 级别 的标题默认开启前述章节的子章节。 两种场景中标题元素都 表示 那个默认章节的标题。
某些元素称为 章节根，
包括 blockquote 和 td 元素。
这些元素可以有自己的目录结构，但这些元素内的章节和标题不会形成它们祖先节点的目录结构。
章节内容 元素总是被视为 最近的祖先 章节根元素 的章节， 或者最近的祖先 章节内容 元素的章节。 这里只要最近的，不论是否属于其他标题默认创建的章节。
对下面的片段：
<body> <h1>Foo</h1> <h2>Bar</h2> <blockquote> <h3>Bla</h3> </blockquote> <p>Baz</p> <h2>Quux</h2> <section> <h3>Thud</h3> </section> <p>Grunt</p> </body>
结构可能是：
body 章节的标题，包含 "Grunt" 一段)
section 章节的标题)
注意上述 section 结束了前一个隐式创建的章节，
这样后续的段落（"Grunt"）重新回到了顶级。
章节可以包含任何 级别 的标题，
但强烈建议作者或者只用 h1 元素，
或者使用与章节的嵌套级别相应 级别 的标题元素。
也强烈建议作者在 章节内容 的元素上显式地包装一个章节元素， 而不是在同一个 章节内容 元素内写多个标题并依赖隐式的章节生成。
例如，下面的片段是正确的：
<body> <h4>Apples</h4> <p>Apples are fruit.</p> <section> <h2>Taste</h2> <p>They taste lovely.</p> <h6>Sweet</h6> <p>Red apples are sweeter than green ones.</p> <h1>Color</h1> <p>Apples come in various colors.</p> </section> </body>
然而同一个文档可以更清晰地表达为：
<body> <h1>Apples</h1> <p>Apples are fruit.</p> <section> <h2>Taste</h2> <p>They taste lovely.</p> <section> <h3>Sweet</h3> <p>Red apples are sweeter than green ones.</p> </section> </section> <section> <h2>Color</h2> <p>Apples come in various colors.</p> </section> </body>
上述两个文档语义上等价，而且在兼容的用户代理上会产生同样的目录结构。
下面第三个例子语义上也是等价的，可能更容易维护 （例如，如果在编辑中经常移动章节的话）：
<body> <h1>Apples</h1> <p>Apples are fruit.</p> <section> <h1>Taste</h1> <p>They taste lovely.</p> <section> <h1>Sweet</h1> <p>Red apples are sweeter than green ones.</p> </section> </section> <section> <h1>Color</h1> <p>Apples come in various colors.</p> </section> </body>
这最后一个例子可能需要加一些显式的样式规则才能在旧浏览器中很好地呈现。 没有 CSS 支持的旧浏览器会把所有标题渲染成顶级标题。
This section defines an algorithm for creating an outline for a sectioning content element or a sectioning root element. It is defined in terms of a walk over the nodes of a DOM tree, in tree order, with each node being visited when it is entered and when it is exited during the walk.
The outline for a sectioning content element or a sectioning root element consists of a list of one or more potentially nested sections. The element for which an outline is created is said to be the outline's owner.
A section is a container that corresponds to some nodes in
the original DOM tree. Each section can have one heading associated with it, and can contain any
number of further nested sections. The algorithm for the outline also
associates each node in the DOM tree with a particular section and potentially a heading.
(The sections in the outline aren't section elements, though some may correspond to
such elements — they are merely conceptual sections.)
The following markup fragment:
<body>
<hgroup id="document-title">
<h1>HTML</h1>
<h2>Living Standard — Last Updated 12 August 2016</h2>
</hgroup>
<p>Some intro to the document.</p>
<h2>Table of contents</h2>
<ol id=toc>...</ol>
<h2>First section</h2>
<p>Some intro to the first section.</p>
</body>...results in the following outline being created for the body node (and thus the
entire document):
Section created for body node.
Associated with heading <hgroup
id="document-title">...</hgroup> consisting of primary heading <h1>HTML</h1> and secondary heading <h2>Living
Standard — Last Updated 12 August 2016</h2>.
Also associated with the paragraph <p>Some intro to the
document.</p> (though it likely would not be shown in a rendered view of the
outline).
Nested sections:
Section implied for first h2 element.
Associated with heading <h2>Table of contents</h2>.
Also associated with the ordered list <ol id=toc>...</ol>
(though it likely would not be shown in a rendered view of the outline).
No nested sections.
Section implied for second h2 element.
Associated with heading <h2>First section</h2>.
Also associated with the paragraph <p>Some intro to the first
section.</p> (though it likely would not be shown in a rendered view of the
outline).
No nested sections.
The following image shows what a rendered view of the outline might look like.
The algorithm that must be followed during a walk of a DOM subtree rooted at a sectioning content element or a sectioning root element to determine that element's outline is as follows:
Let current outline target be null. (It holds the element whose outline is being created.)
Let current section be null. (It holds a pointer to a section, so that elements in the DOM can all be associated with a section.)
Create a stack to hold elements, which is used to handle nesting. Initialize this stack to empty.
Walk over the DOM in tree order, starting with the sectioning content element or sectioning root element at the root of the subtree for which an outline is to be created, and trigger the first relevant step below for each element as the walk enters and exits it.
The element being exited is a heading content element or an element with a attribute.
Pop that element from the stack.
Do nothing.
Push the element being entered onto the stack. (This causes the algorithm to skip that element and any descendants of the element.)
Run these steps:
If current outline target is not null, then:
If the current section has no heading, create an implied heading and let that be the heading for the current section.
Push current outline target onto the stack.
Let current outline target be the element that is being entered.
Let current section be a newly created section for the current outline target element.
Associate current outline target with current section.
Let there be a new outline for the new current outline target, initialized with just the new current section as the only section in the outline.
Run these steps:
If the current section has no heading, create an implied heading and let that be the heading for the current section.
Pop the top element from the stack, and let the current outline target be that element.
Let current section be the last section in the outline of the current outline target element.
Append the outline of the sectioning content element being exited to the current section. (This does not change which section is the last section in the outline.)
Run these steps:
If current outline target is not null, push current outline target onto the stack.
Let current outline target be the element that is being entered.
Let current outline target's parent section be current section.
Let current section be a newly created section for the current outline target element.
Let there be a new outline for the new current outline target, initialized with just the new current section as the only section in the outline.
Run these steps:
If the current section has no heading, create an implied heading and let that be the heading for the current section.
Let current section be current outline target's parent section.
Pop the top element from the stack, and let the current outline target be that element.
The current outline target is the element being exited, and it is the sectioning content element or a sectioning root element at the root of the subtree for which an outline is being generated.
If the current section has no heading, create an implied heading and let that be the heading for the current section.
Skip to the next step in the overall set of steps. (The walk is over.)
If the current section has no heading, let the element being entered be the heading for the current section.
If the element being entered is an hgroup element, that
hgroup as a whole is a multi-level heading for the current
section, with the highest-ranked
h1–h6 descendant of the hgroup providing the
primary heading for the current section, and with other
h1–h6 descendants of the hgroup providing
secondary headings for the current section.
Otherwise, if the element being entered has a rank equal to or higher than the heading of the last section of the outline of the current outline target, or if the heading of the last section of the outline of the current outline target is an implied heading, then create a new section and append it to the outline of the current outline target element, so that this new section is the new last section of that outline. Let current section be that new section. Let the element being entered be the new heading for the current section.
Otherwise, run these substeps:
Let candidate section be current section.
Heading loop: If the element being entered has a rank lower than the rank of the heading of the candidate section, then create a new section, and append it to candidate section. (This does not change which section is the last section in the outline.) Let current section be this new section. Let the element being entered be the new heading for the current section. Abort these substeps.
Let new candidate section be the section that contains candidate section in the outline of current outline target.
Let candidate section be new candidate section.
Return to the step labeled heading loop.
Push the element being entered onto the stack. (This causes the algorithm to skip any descendants of the element.)
Recall that h1 has the highest rank, and h6
has the lowest rank.
Do nothing.
In addition, whenever the walk exits a node, after doing the steps above, if the node is not associated with a section yet, associate the node with the section current section.
Associate all non-element nodes that are in the subtree for which an outline is being created with the section with which their parent element is associated.
Associate all nodes in the subtree with the heading of the section with which they are associated, if any.
The tree of sections created by the algorithm above, or a proper subset thereof, must be used when generating document outlines, for example when generating tables of contents.
The outline created for the body element of a Document is the
outline of the entire document.
When creating an interactive table of contents, entries should jump the user to the relevant sectioning content element, if the section was created for a real element in the original document, or to the relevant heading content element, if the section in the tree was generated for a heading in the above process.
Selecting the first section of the document
therefore always takes the user to the top of the document, regardless of where the first heading
in the body is to be found.
The outline depth of a heading content element associated with a section section is the number of sections that are ancestors of section in the
outermost outline that section finds itself in when the outlines of its Document's elements are created, plus 1. The
outline depth of a heading content element not associated with a section is 1.
User agents should provide default headings for sections that do not have explicit section headings.
Consider the following snippet:
<body>
<nav>
<p><a href="/">Home</a></p>
</nav>
<p>Hello world.</p>
<aside>
<p>My cat is cute.</p>
</aside>
</body>Although it contains no headings, this snippet has three sections: a document (the
body) with two subsections (a nav and an aside). A user
agent could present the outline as follows:
These default headings ("Untitled document", "Navigation", "Sidebar") are not specified by this specification, and might vary with the user's language, the page's language, the user's preferences, the user agent implementer's preferences, etc.
The following JavaScript function shows how the tree walk could be implemented. The root argument is the root of the tree to walk (either a sectioning content element or a sectioning root element), and the enter and exit arguments are callbacks that are called with the nodes as they are entered and exited. [JAVASCRIPT]
function (root, enter, exit) {
var node = root;
start: while (node) {
enter(node);
if (node.firstChild) {
node = node.firstChild;
continue start;
}
while (node) {
exit(node);
if (node == root) {
node = null;
} else if (node.nextSibling) {
node = node.nextSibling;
continue start;
} else {
node = node.parentNode;
}
}
}
}This section is non-normative.
The following document shows a straight-forward application of the outline algorithm. First, here is the document, which is a book with very short chapters and subsections:
<!DOCTYPE HTML>
<html lang=en>
<title>The Tax Book (all in one page)</title>
<h1>The Tax Book</h1>
<h2>Earning money</h2>
<p>Earning money is good.</p>
<h3>Getting a job</h3>
<p>To earn money you typically need a job.</p>
<h2>Spending money</h2>
<p>Spending is what money is mainly used for.</p>
<h3>Cheap things</h3>
<p>Buying cheap things often not cost-effective.</p>
<h3>Expensive things</h3>
<p>The most expensive thing is often not the most cost-effective either.</p>
<h2>Investing money</h2>
<p>You can lend your money to other people.</p>
<h2>Losing money</h2>
<p>If you spend money or invest money, sooner or later you will lose money.
<h3>Poor judgement</h3>
<p>Usually if you lose money it's because you made a mistake.</p>This book would form the following outline:
Notice that the title element does not participate in the outline.
Here is a similar document, but this time using section elements to get the same
effect:
<!DOCTYPE HTML>
<html lang=en>
<title>The Tax Book (all in one page)</title>
<h1>The Tax Book</h1>
<section>
<h1>Earning money</h1>
<p>Earning money is good.</p>
<section>
<h1>Getting a job</h1>
<p>To earn money you typically need a job.</p>
</section>
</section>
<section>
<h1>Spending money</h1>
<p>Spending is what money is mainly used for.</p>
<section>
<h1>Cheap things</h1>
<p>Buying cheap things often not cost-effective.</p>
</section>
<section>
<h1>Expensive things</h1>
<p>The most expensive thing is often not the most cost-effective either.</p>
</section>
</section>
<section>
<h1>Investing money</h1>
<p>You can lend your money to other people.</p>
</section>
<section>
<h1>Losing money</h1>
<p>If you spend money or invest money, sooner or later you will lose money.
<section>
<h1>Poor judgement</h1>
<p>Usually if you lose money it's because you made a mistake.</p>
</section>
</section>This book would form the same outline:
A document can contain multiple top-level headings:
<!DOCTYPE HTML>
<html lang=en>
<title>Alphabetic Fruit</title>
<h1>Apples</h1>
<p>Pomaceous.</p>
<h1>Bananas</h1>
<p>Edible.</p>
<h1>Carambola</h1>
<p>Star.</p>This would form the following simple outline consisting of three top-level sections:
Effectively, the body element is split into three.
Mixing both the h1–h6 model and the
section/h1 model can lead to some unintuitive results.
Consider for example the following, which is just the previous example but with the contents
of the (implied) body wrapped in a section:
<!DOCTYPE HTML>
<html lang=en>
<title>Alphabetic Fruit</title>
<section>
<h1>Apples</h1>
<p>Pomaceous.</p>
<h1>Bananas</h1>
<p>Edible.</p>
<h1>Carambola</h1>
<p>Star.</p>
</section>The resulting outline would be:
This result is described as unintuitive because it results in three subsections even
though there's only one section element. Effectively, the section is
split into three, just like the implied body element in the previous example.
(In this example, "(untitled page)" is the implied heading for the body
element, since it has no explicit heading.)
Headings never rise above other sections. Thus, in the following example, the first
h1 does not actually describe the page header; it describes the header for the
second half of the page:
<!DOCTYPE HTML>
<html lang=en>
<title>Feathers on The Site of Encyclopedic Knowledge</title>
<section>
<h1>A plea from our caretakers</h1>
<p>Please, we beg of you, send help! We're stuck in the server room!</p>
</section>
<h1>Feathers</h1>
<p>Epidermal growths.</p>The resulting outline would be:
Thus, when an article element starts with a nav block and only later
has its heading, the result is that the nav block is not part of the same section as
the rest of the article in the outline. For instance, take this document:
<!DOCTYPE HTML>
<html lang="en">
<title>We're adopting a child! — Ray's blog</title>
<h1>Ray's blog</h1>
<article>
<header>
<nav>
<a href="?t=-1d">Yesterday</a>;
<a href="?t=-7d">Last week</a>;
<a href="?t=-1m">Last month</a>
</nav>
<h1>We're adopting a child!</h1>
</header>
<p>As of today, Janine and I have signed the papers to become
the proud parents of baby Diane! We've been looking forward to
this day for weeks.</p>
</article>
</html>The resulting outline would be:
Also worthy of note in this example is that the header element has no effect
whatsoever on the document outline.
The hgroup element can be used for subheadings. For example:
<!DOCTYPE HTML>
<html lang="en">
<title>Chronotype: CS Student</title>
<hgroup>
<h1> The morning </h1>
<h2> 06:00 to 12:00 </h2>
</hgroup>
<p>We sleep.</p>
<hgroup>
<h1> The afternoon </h1>
<h2> 12:00 to 18:00 </h2>
</hgroup>
<p>We study.</p>
<hgroup>
<h2>Additional Commentary</h2>
<h3>Because not all this is necessarily true</h3>
<h6>Ok it's almost certainly not true</h6>
</hgroup>
<p>Yeah we probably play, rather than study.</p>
<hgroup>
<h1> The evening </h1>
<h2> 18:00 to 00:00 </h2>
</hgroup>
<p>We play.</p>
<hgroup>
<h1> The night </h1>
<h2> 00:00 to 06:00 </h2>
</hgroup>
<p>We play some more.</p>
</html>The resulting outline would be:
Exactly how this is represented by user agents, as most interface issues, is left as a matter
of implementation preference, but the key part is that the hgroup's descendant
h1–h6 elements are what form the element's heading. Thus, the
following would be equally valid:
But so would the following:
The following would also be valid, though maybe less practical in most contexts:
The morning
06:00 to 12:00
The afternoon
12:00 to 18:00
Additional Commentary
Because not all this is necessarily true
Ok it's almost certainly not true
The evening
18:00 to 00:00
The night
00:00 to 06:00
鼓励用户代理暴露页面目录结构给用户以帮助导航，这在非视觉媒体中尤其重要。比如屏幕阅读器。
但是为了减轻作者误用 章节内容 造成的麻烦，也鼓励用户代理提供一种只用 标题内容 来导航的模式。
例如，用户代理可以给箭头键做这些映射：
此外，用户代理还可以把 j 和 k 键映射为导航到上一个/下一个 标题内容 的元素， 无论章节的目录深度并忽略没有标题的章节。
This section is non-normative.
| Element | Purpose |
|---|---|
| Example | |
body
| The contents of the document. |
| |
article
| A complete, or self-contained, composition in a document, page, application, or site and that is, in principle, independently distributable or reusable, e.g. in syndication. This could be a forum post, a magazine or newspaper article, a blog entry, a user-submitted comment, an interactive widget or gadget, or any other independent item of content. |
| |
section
| A generic section of a document or application. A section, in this context, is a thematic grouping of content, typically with a heading. |
| |
nav
| A section of a page that links to other pages or to parts within the page: a section with navigation links. |
| |
aside
| A section of a page that consists of content that is tangentially related to the content around the aside element, and which could be considered separate from that content. Such sections are often represented as sidebars in printed typography.
|
| |
h1–h6
| A section heading |
| |
hgroup
| The heading of a section, which consists of all the h1–h6 element children of the hgroup element. The element is used to group a set of h1–h6 elements when the heading has multiple levels, such as subheadings, alternative titles, or taglines.
|
| |
header
| A group of introductory or navigational aids. |
| |
footer
| A footer for its nearest ancestor sectioning content or sectioning root element. A footer typically contains information about its section such as who wrote it, links to related documents, copyright data, and the like. |
|
This section is non-normative.
一个 section 是其他内容的一部分。一个 article 就是它自己。
但是怎么知道哪个是哪个？大多数情况下真正的答案“取决于作者的意图”。
例如，人们可以想象一本书有一章叫做“Granny Smith”，它只写了：
“These juicy, green apples make a great filling for apple pies.”；
这就是一个 section，因为可能会有很多其他章节来介绍其他类型的苹果。
另一方面，人们也可以想象一条推文，Reddit 评论，Tumblr 文章，或报纸分类广告，
只是说“Granny Smith. These juicy, green apples make a great filling for apple pies.”；
那么它就可以成为一个 article，因为这就是整个事情。
article 的评论不是文章的一部分，因此它是它自己的 article。
p elementSupport in all current engines.
Support in all current engines.
p element's end tag can be omitted if the
p element is immediately followed by an address, article,
aside, blockquote, details, div, dl,
fieldset, figcaption, figure, footer, form, h1, h2,
h3, h4, h5, h6, header,
hgroup, hr, main, menu, nav,
ol, p, pre, section, table, or
ul element, or if there is no more content in the parent element and the parent
element is an HTML element that is not an a,
audio, del, ins, map, noscript,
or video element, or an autonomous custom element.[Exposed=Window]
interface HTMLParagraphElement : HTMLElement {
[HTMLConstructor] constructor();
// also has obsolete members
};The p element represents a paragraph.
While paragraphs are usually represented in visual media by blocks of text that are physically separated from adjacent blocks through blank lines, a style sheet or user agent would be equally justified in presenting paragraph breaks in a different manner, for instance using inline pilcrows (¶).
The following examples are conforming HTML fragments:
<p>The little kitten gently seated herself on a piece of
carpet. Later in her life, this would be referred to as the time the
cat sat on the mat.</p><fieldset>
<legend>Personal information</legend>
<p>
<label>Name: <input name="n"></label>
<label><input name="anon" type="checkbox"> Hide from other users</label>
</p>
<p><label>Address: <textarea name="a"></textarea></label></p>
</fieldset><p>There was once an example from Femley,<br>
Whose markup was of dubious quality.<br>
The validator complained,<br>
So the author was pained,<br>
To move the error from the markup to the rhyming.</p>The p element should not be used when a more specific element is more
appropriate.
The following example is technically correct:
<section>
<!-- ... -->
<p>Last modified: 2001-04-23</p>
<p>Author: fred@example.com</p>
</section>However, it would be better marked-up as:
<section>
<!-- ... -->
<footer>Last modified: 2001-04-23</footer>
<address>Author: fred@example.com</address>
</section>Or:
<section>
<!-- ... -->
<footer>
<p>Last modified: 2001-04-23</p>
<address>Author: fred@example.com</address>
</footer>
</section>List elements (in particular, ol and ul elements) cannot be children
of p elements. When a sentence contains a bulleted list, therefore, one might wonder
how it should be marked up.
For instance, this fantastic sentence has bullets relating to
and is further discussed below.
The solution is to realize that a paragraph, in HTML terms, is not a logical concept, but a structural one. In the fantastic example above, there are actually five paragraphs as defined by this specification: one before the list, one for each bullet, and one after the list.
The markup for the above example could therefore be:
<p>For instance, this fantastic sentence has bullets relating to</p>
<ul>
<li>wizards,
<li>faster-than-light travel, and
<li>telepathy,
</ul>
<p>and is further discussed below.</p>Authors wishing to conveniently style such "logical" paragraphs consisting of multiple
"structural" paragraphs can use the div element instead of the p
element.
Thus for instance the above example could become the following:
<div>For instance, this fantastic sentence has bullets relating to
<ul>
<li>wizards,
<li>faster-than-light travel, and
<li>telepathy,
</ul>
and is further discussed below.</div>This example still has five structural paragraphs, but now the author can style just the
div instead of having to consider each part of the example separately.
hr elementSupport in all current engines.
Support in all current engines.
[Exposed=Window]
interface HTMLHRElement : HTMLElement {
[HTMLConstructor] constructor();
// also has obsolete members
};The hr element represents a paragraph-level thematic
break, e.g. a scene change in a story, or a transition to another topic within a section of a
reference book.
The following fictional extract from a project manual shows two sections that use the
hr element to separate topics within the section.
<section>
<h1>Communication</h1>
<p>There are various methods of communication. This section
covers a few of the important ones used by the project.</p>
<hr>
<p>Communication stones seem to come in pairs and have mysterious
properties:</p>
<ul>
<li>They can transfer thoughts in two directions once activated
if used alone.</li>
<li>If used with another device, they can transfer one's
consciousness to another body.</li>
<li>If both stones are used with another device, the
consciousnesses switch bodies.</li>
</ul>
<hr>
<p>Radios use the electromagnetic spectrum in the meter range and
longer.</p>
<hr>
<p>Signal flares use the electromagnetic spectrum in the
nanometer range.</p>
</section>
<section>
<h1>Food</h1>
<p>All food at the project is rationed:</p>
<dl>
<dt>Potatoes</dt>
<dd>Two per day</dd>
<dt>Soup</dt>
<dd>One bowl per day</dd>
</dl>
<hr>
<p>Cooking is done by the chefs on a set rotation.</p>
</section>There is no need for an hr element between the sections themselves, since the
section elements and the h1 elements imply thematic changes
themselves.
The following extract from Pandora's Star by Peter F. Hamilton shows two
paragraphs that precede a scene change and the paragraph that follows it. The scene change,
represented in the printed book by a gap containing a solitary centered star between the second
and third paragraphs, is here represented using the hr element.
<p>Dudley was ninety-two, in his second life, and fast approaching
time for another rejuvenation. Despite his body having the physical
age of a standard fifty-year-old, the prospect of a long degrading
campaign within academia was one he regarded with dread. For a
supposedly advanced civilization, the Intersolar Commonwealth could be
appallingly backward at times, not to mention cruel.</p>
<p><i>Maybe it won't be that bad</i>, he told himself. The lie was
comforting enough to get him through the rest of the night's
shift.</p>
<hr>
<p>The Carlton AllLander drove Dudley home just after dawn. Like the
astronomer, the vehicle was old and worn, but perfectly capable of
doing its job. It had a cheap diesel engine, common enough on a
semi-frontier world like Gralmond, although its drive array was a
thoroughly modern photoneural processor. With its high suspension and
deep-tread tyres it could plough along the dirt track to the
observatory in all weather and seasons, including the metre-deep snow
of Gralmond's winters.</p>The hr element does not affect the document's
outline.
pre elementSupport in all current engines.
Support in all current engines.
[HTMLConstructor]
interface HTMLPreElement : HTMLElement {};
The pre element represents a block of preformatted text, in which
structure is represented by typographic conventions rather than by elements.
In the HTML syntax, a leading newline character immediately following
the pre element start tag is stripped.
Some examples of cases where the pre element could be used:
Authors are encouraged to consider how preformatted text will be experienced when the formatting is lost, as will be the case for users of speech synthesizers, braille displays, and the like. For cases like ASCII art, it is likely that an alternative presentation, such as a textual description, would be more universally accessible to the readers of the document.
To represent a block of computer code, the pre element can be used with a
code element; to represent a block of computer output the pre element
can be used with a samp element. Similarly, the kbd element can be used
within a pre element to indicate text that the user is to enter.
This element has rendering requirements involving the bidirectional algorithm.
In the following snippet, a sample of computer code is presented.
<p>This is the <code>Panel</code> constructor:</p>
<pre><code>function Panel(element, canClose, closeHandler) {
this.element = element;
this.canClose = canClose;
this.closeHandler = function () { if (closeHandler) closeHandler() };
}</code></pre>
In the following snippet, samp and kbd elements are mixed in the
contents of a pre element to show a session of Zork I.
<pre><samp>You are in an open field west of a big white house with a boarded front door. There is a small mailbox here. ></samp> <kbd>open mailbox</kbd> <samp>Opening the mailbox reveals: A leaflet. ></samp></pre>
The following shows a contemporary poem that uses the pre element to preserve its
unusual formatting, which forms an intrinsic part of the poem itself.
<pre> maxling
it is with a heart
heavy
that i admit loss of a feline
so loved
a friend lost to the
unknown
(night)
~cdr 11dec07</pre>
blockquote elementSupport in all current engines.
Support in all current engines.
cite — Link to the source of the quotation or more information about the edit[Exposed=Window]
interface HTMLQuoteElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute USVString cite;
};The HTMLQuoteElement interface is
also used by the q element.
The blockquote element represents a section that is quoted from
another source.
Content inside a blockquote must be quoted from another source, whose address, if
it has one, may be cited in the cite attribute.
If the cite attribute is present, it must be a
valid URL potentially surrounded by spaces. To obtain the
corresponding citation link, the value of the attribute must be parsed relative to the element's node document. User agents may
allow users to follow such citation links, but they are primarily intended for private use (e.g.,
by server-side scripts collecting statistics about a site's use of quotations), not for
readers.
The content of a blockquote may be abbreviated or may have context added in the
conventional manner for the text's language.
For example, in English this is traditionally done using square brackets. Consider a page with the sentence "Jane ate the cracker. She then said she liked apples and fish."; it could be quoted as follows:
<blockquote>
<p>[Jane] then said she liked [...] fish.</p>
</blockquote>Attribution for the quotation, if any, must be placed outside the blockquote
element.
For example, here the attribution is given in a paragraph after the quote:
<blockquote>
<p>I contend that we are both atheists. I just believe in one fewer
god than you do. When you understand why you dismiss all the other
possible gods, you will understand why I dismiss yours.</p>
</blockquote>
<p>— Stephen Roberts</p>The other examples below show other ways of showing attribution.
The cite IDL
attribute must reflect the element's cite content
attribute.
Here a blockquote element is used in conjunction with a figure
element and its figcaption to clearly relate a quote to its attribution (which is
not part of the quote and therefore doesn't belong inside the blockquote
itself):
<figure>
<blockquote>
<p>The truth may be puzzling. It may take some work to grapple with.
It may be counterintuitive. It may contradict deeply held
prejudices. It may not be consonant with what we desperately want to
be true. But our preferences do not determine what's true. We have a
method, and that method helps us to reach not absolute truth, only
asymptotic approaches to the truth — never there, just closer
and closer, always finding vast new oceans of undiscovered
possibilities. Cleverly designed experiments are the key.</p>
</blockquote>
<figcaption>Carl Sagan, in "<cite>Wonder and Skepticism</cite>", from
the <cite>Skeptical Inquirer</cite> Volume 19, Issue 1 (January-February
1995)</figcaption>
</figure>This next example shows the use of cite alongside blockquote:
<p>His next piece was the aptly named <cite>Sonnet 130</cite>:</p>
<blockquote cite="https://quotes.example.org/s/sonnet130.html">
<p>My mistress' eyes are nothing like the sun,<br>
Coral is far more red, than her lips red,<br>
...This example shows how a forum post could use blockquote to show what post a user
is replying to. The article element is used for each post, to mark up the
threading.
<article>
<h1><a href="https://bacon.example.com/?blog=109431">Bacon on a crowbar</a></h1>
<article>
<header><strong>t3yw</strong> 12 points 1 hour ago</header>
<p>I bet a narwhal would love that.</p>
<footer><a href="?pid=29578">permalink</a></footer>
<article>
<header><strong>greg</strong> 8 points 1 hour ago</header>
<blockquote><p>I bet a narwhal would love that.</p></blockquote>
<p>Dude narwhals don't eat bacon.</p>
<footer><a href="?pid=29579">permalink</a></footer>
<article>
<header><strong>t3yw</strong> 15 points 1 hour ago</header>
<blockquote>
<blockquote><p>I bet a narwhal would love that.</p></blockquote>
<p>Dude narwhals don't eat bacon.</p>
</blockquote>
<p>Next thing you'll be saying they don't get capes and wizard
hats either!</p>
<footer><a href="?pid=29580">permalink</a></footer>
<article>
<article>
<header><strong>boing</strong> -5 points 1 hour ago</header>
<p>narwhals are worse than ceiling cat</p>
<footer><a href="?pid=29581">permalink</a></footer>
</article>
</article>
</article>
</article>
<article>
<header><strong>fred</strong> 1 points 23 minutes ago</header>
<blockquote><p>I bet a narwhal would love that.</p></blockquote>
<p>I bet they'd love to peel a banana too.</p>
<footer><a href="?pid=29582">permalink</a></footer>
</article>
</article>
</article>This example shows the use of a blockquote for short snippets, demonstrating that
one does not have to use p elements inside blockquote elements:
<p>He began his list of "lessons" with the following:</p>
<blockquote>One should never assume that his side of
the issue will be recognized, let alone that it will
be conceded to have merits.</blockquote>
<p>He continued with a number of similar points, ending with:</p>
<blockquote>Finally, one should be prepared for the threat
of breakdown in negotiations at any given moment and not
be cowed by the possibility.</blockquote>
<p>We shall now discuss these points...Examples of how to represent a conversation are shown
in a later section; it is not appropriate to use the cite and blockquote
elements for this purpose.
ol elementSupport in all current engines.
Support in all current engines.
li element: Palpable content.li and script-supporting elements.reversed — Number the list backwardsstart — Starting value of the listtype — Kind of list marker[Exposed=Window]
interface HTMLOListElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute boolean reversed;
[CEReactions] attribute long start;
[CEReactions] attribute DOMString type;
// also has obsolete members
};The ol element represents a list of items, where the items have been
intentionally ordered, such that changing the order would change the meaning of the document.
The items of the list are the li element child nodes of the ol
element, in tree order.
Support in all current engines.
The reversed attribute
is a boolean attribute. If present, it indicates that the list is a descending list
(..., 3, 2, 1). If the attribute is omitted, the list is an ascending list (1, 2, 3, ...).
The start attribute, if
present, must be a valid integer. It is used to determine the starting value of the list.
An ol element has a starting value, which is
an integer determined as follows:
If the ol element has a start attribute,
then:
Let parsed be the result of parsing the value of the attribute as an integer.
If parsed is not an error, then return parsed.
If the ol element has a reversed
attribute, then return the number of owned li
elements.
Return 1.
The type attribute can be
used to specify the kind of marker to use in the list, in the cases where that matters (e.g.
because items are to be referenced by their number/letter). The attribute, if
specified, must have a value that is identical to one of the characters given in the
first cell of one of the rows of the following table. The type attribute represents the state given in the cell in the second
column of the row whose first cell matches the attribute's value; if none of the cells match, or
if the attribute is omitted, then the attribute represents the decimal state.
| Keyword | State | Description | Examples for values 1-3 and 3999-4001 | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 (U+0031)
| decimal | Decimal numbers | 1. | 2. | 3. | ... | 3999. | 4000. | 4001. | ... |
a (U+0061)
| lower-alpha | Lowercase latin alphabet | a. | b. | c. | ... | ewu. | ewv. | eww. | ... |
A (U+0041)
| upper-alpha | Uppercase latin alphabet | A. | B. | C. | ... | EWU. | EWV. | EWW. | ... |
i (U+0069)
| lower-roman | Lowercase roman numerals | i. | ii. | iii. | ... | mmmcmxcix. | i̅v̅. | i̅v̅i. | ... |
I (U+0049)
| upper-roman | Uppercase roman numerals | I. | II. | III. | ... | MMMCMXCIX. | I̅V̅. | I̅V̅I. | ... |
User agents should render the items of the list in a manner consistent with the state of the
type attribute of the ol element. Numbers less than
or equal to zero should always use the decimal system regardless of the type attribute.
For CSS user agents, a mapping for this attribute to the 'list-style-type' CSS property is given in the rendering section (the mapping is straightforward: the states above have the same names as their corresponding CSS values).
It is possible to redefine the default CSS list styles used to implement this attribute in CSS user agents; doing so will affect how list items are rendered.
The reversed
and type IDL
attributes must reflect the respective content attributes of the same name.
The start IDL
attribute must reflect the content attribute of the same name, with a default value
of 1.
This means that the start IDL attribute does
not necessarily match the list's starting value, in cases
where the start content attribute is omitted and the reversed content attribute is specified.
The following markup shows a list where the order matters, and where the ol
element is therefore appropriate. Compare this list to the equivalent list in the ul
section to see an example of the same items using the ul element.
<p>I have lived in the following countries (given in the order of when
I first lived there):</p>
<ol>
<li>Switzerland
<li>United Kingdom
<li>United States
<li>Norway
</ol>Note how changing the order of the list changes the meaning of the document. In the following example, changing the relative order of the first two items has changed the birthplace of the author:
<p>I have lived in the following countries (given in the order of when
I first lived there):</p>
<ol>
<li>United Kingdom
<li>Switzerland
<li>United States
<li>Norway
</ol>ul elementSupport in all current engines.
Support in all current engines.
li element: Palpable content.li and script-supporting elements.[Exposed=Window]
interface HTMLUListElement : HTMLElement {
[HTMLConstructor] constructor();
// also has obsolete members
};The ul element represents a list of items, where the order of the
items is not important — that is, where changing the order would not materially change the
meaning of the document.
The items of the list are the li element child nodes of the ul
element.
The following markup shows a list where the order does not matter, and where the
ul element is therefore appropriate. Compare this list to the equivalent list in the
ol section to see an example of the same items using the ol
element.
<p>I have lived in the following countries:</p>
<ul>
<li>Norway
<li>Switzerland
<li>United Kingdom
<li>United States
</ul>Note that changing the order of the list does not change the meaning of the document. The items in the snippet above are given in alphabetical order, but in the snippet below they are given in order of the size of their current account balance in 2007, without changing the meaning of the document whatsoever:
<p>I have lived in the following countries:</p>
<ul>
<li>Switzerland
<li>Norway
<li>United Kingdom
<li>United States
</ul>menu elementSupport in all current engines.
Support in all current engines.
li element: Palpable content.li and script-supporting elements.[Exposed=Window]
interface HTMLMenuElement : HTMLElement {
[HTMLConstructor] constructor();
// also has obsolete members
};The menu element represents a toolbar consisting of its contents, in
the form of an unordered list of items (represented by li elements), each of
which represents a command that the user can perform or activate.
The menu element is simply a semantic alternative to ul
to express an unordered list of commands (a "toolbar").
In this example, a text-editing application uses a menu element to provide a
series of editing commands:
<menu>
<li><button onclick="copy()"><img src="copy.svg" alt="Copy"></button></li>
<li><button onclick="cut()"><img src="cut.svg" alt="Cut"></button></li>
<li><button onclick="paste()"><img src="paste.svg" alt="Paste"></button></li>
</menu>Note that the styling to make this look like a conventional toolbar menu is up to the application.
li elementSupport in all current engines.
Support in all current engines.
ol elements.ul elements.menu elements.li element's end tag can be omitted if the
li element is immediately followed by another li element or if there is
no more content in the parent element.ul or menu element: value — Ordinal value of the list item[Exposed=Window]
interface HTMLLIElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute long value;
// also has obsolete members
};The li element represents a list item. If its parent element is an
ol, ul, or menu element, then the element is an item of the
parent element's list, as defined for those elements. Otherwise, the list item has no defined
list-related relationship to any other li element.
The value attribute, if
present, must be a valid integer. It is used to determine the ordinal
value of the list item, when the li's list owner is an
ol element.
Any element whose computed value of 'display' is 'list-item' has a list owner, which is determined as follows:
If the element is not being rendered, return null; the element has no list owner.
Let ancestor be the element's parent.
If the element has an ol, ul, or menu ancestor, set
ancestor to the closest such ancestor element.
Return the closest inclusive ancestor of ancestor that produces a CSS box.
Such an element will always exist, as at the very least the document element will always produce a CSS box.
To determine the ordinal value of each element owned by a given list owner owner, perform the following steps:
Let i be 1.
If owner is an ol element, let numbering be
owner's starting value. Otherwise, let
numbering be 1.
Loop: If i is greater than the number of list items that owner owns, then return; all of owner's owned list items have been assigned ordinal values.
Let item be the ith of owner's owned list items, in tree order.
If item is an li element that has a value attribute, then:
Let parsed be the result of parsing the value of the attribute as an integer.
If parsed is not an error, then set numbering to parsed.
The ordinal value of item is numbering.
If owner is an ol element, and owner has a reversed attribute, decrement numbering by 1;
otherwise, increment numbering by 1.
Increment i by 1.
Go to the step labeled loop.
The value IDL
attribute must reflect the value of the value
content attribute.
The element's value IDL attribute does not directly
correspond to its ordinal value; it simply reflects
the content attribute. For example, given this list:
<ol>
<li>Item 1
<li value="3">Item 3
<li>Item 4
</ol>The ordinal values are 1, 3, and 4, whereas the value IDL attributes return 0, 3, 0 on getting.
The following example, the top ten movies are listed (in reverse order). Note the way the list
is given a title by using a figure element and its figcaption
element.
<figure>
<figcaption>The top 10 movies of all time</figcaption>
<ol>
<li value="10"><cite>Josie and the Pussycats</cite>, 2001</li>
<li value="9"><cite lang="sh">Црна мачка, бели мачор</cite>, 1998</li>
<li value="8"><cite>A Bug's Life</cite>, 1998</li>
<li value="7"><cite>Toy Story</cite>, 1995</li>
<li value="6"><cite>Monsters, Inc</cite>, 2001</li>
<li value="5"><cite>Cars</cite>, 2006</li>
<li value="4"><cite>Toy Story 2</cite>, 1999</li>
<li value="3"><cite>Finding Nemo</cite>, 2003</li>
<li value="2"><cite>The Incredibles</cite>, 2004</li>
<li value="1"><cite>Ratatouille</cite>, 2007</li>
</ol>
</figure>The markup could also be written as follows, using the reversed attribute on the ol element:
<figure>
<figcaption>The top 10 movies of all time</figcaption>
<ol reversed>
<li><cite>Josie and the Pussycats</cite>, 2001</li>
<li><cite lang="sh">Црна мачка, бели мачор</cite>, 1998</li>
<li><cite>A Bug's Life</cite>, 1998</li>
<li><cite>Toy Story</cite>, 1995</li>
<li><cite>Monsters, Inc</cite>, 2001</li>
<li><cite>Cars</cite>, 2006</li>
<li><cite>Toy Story 2</cite>, 1999</li>
<li><cite>Finding Nemo</cite>, 2003</li>
<li><cite>The Incredibles</cite>, 2004</li>
<li><cite>Ratatouille</cite>, 2007</li>
</ol>
</figure>While it is conforming to include heading elements (e.g. h1) inside
li elements, it likely does not convey the semantics that the author intended. A
heading starts a new section, so a heading in a list implicitly splits the list into spanning
multiple sections.
dl elementSupport in all current engines.
Support in all current engines.
dt elements followed by one or more dd elements, optionally intermixed with script-supporting elements.div elements, optionally intermixed with script-supporting elements.[Exposed=Window]
interface HTMLDListElement : HTMLElement {
[HTMLConstructor] constructor();
// also has obsolete members
};The dl element represents an association list consisting of zero or
more name-value groups (a description list). A name-value group consists of one or more names
(dt elements, possibly as children of a div element child) followed by
one or more values (dd elements, possibly as children of a div element
child), ignoring any nodes other than dt and dd element children, and
dt and dd elements that are children of div element
children. Within a single dl element, there should not be more than one
dt element for each name.
Name-value groups may be terms and definitions, metadata topics and values, questions and answers, or any other groups of name-value data.
The values within a group are alternatives; multiple paragraphs forming part of the same value
must all be given within the same dd element.
The order of the list of groups, and of the names and values within each group, may be significant.
In order to annotate groups with microdata attributes, or other global
attributes that apply to whole groups, or just for styling purposes, each group in a
dl element can be wrapped in a div element. This does not change the
semantics of the dl element.
The name-value groups of a dl element dl are determined using the
following algorithm. A name-value group has a name (a list of dt elements, initially
empty) and a value (a list of dd elements, initially empty).
Let groups be an empty list of name-value groups.
Let current be a new name-value group.
Let seenDd be false.
Let child be dl's first child.
Let grandchild be null.
While child is not null:
If child is a div element, then:
Let grandchild be child's first child.
While grandchild is not null:
Process dt or dd for
grandchild.
Set grandchild to grandchild's next sibling.
Otherwise, process dt or dd for
child.
Set child to child's next sibling.
If current is not empty, then append current to groups.
Return groups.
To process dt or dd for a node node means to
follow these steps:
Let groups, current, and seenDd be the same variables as those of the same name in the algorithm that invoked these steps.
If node is a dt element, then:
If seenDd is true, then append current to groups, set current to a new name-value group, and set seenDd to false.
Append node to current's name.
Otherwise, if node is a dd element, then append node to
current's value and set seenDd to true.
When a name-value group has an empty list as name or value, it is often due to
accidentally using dd elements in the place of dt elements and vice
versa. Conformance checkers can spot such mistakes and might be able to advise authors how to
correctly use the markup.
In the following example, one entry ("Authors") is linked to two values ("John" and "Luke").
<dl>
<dt> Authors
<dd> John
<dd> Luke
<dt> Editor
<dd> Frank
</dl>In the following example, one definition is linked to two terms.
<dl>
<dt lang="en-US"> <dfn>color</dfn> </dt>
<dt lang="en-GB"> <dfn>colour</dfn> </dt>
<dd> A sensation which (in humans) derives from the ability of
the fine structure of the eye to distinguish three differently
filtered analyses of a view. </dd>
</dl>The following example illustrates the use of the dl element to mark up metadata
of sorts. At the end of the example, one group has two metadata labels ("Authors" and "Editors")
and two values ("Robert Rothman" and "Daniel Jackson"). This example also uses the
div element around the groups of dt and dd element, to aid
with styling.
<dl>
<div>
<dt> Last modified time </dt>
<dd> 2004-12-23T23:33Z </dd>
</div>
<div>
<dt> Recommended update interval </dt>
<dd> 60s </dd>
</div>
<div>
<dt> Authors </dt>
<dt> Editors </dt>
<dd> Robert Rothman </dd>
<dd> Daniel Jackson </dd>
</div>
</dl>The following example shows the dl element used to give a set of instructions.
The order of the instructions here is important (in the other examples, the order of the blocks
was not important).
<p>Determine the victory points as follows (use the
first matching case):</p>
<dl>
<dt> If you have exactly five gold coins </dt>
<dd> You get five victory points </dd>
<dt> If you have one or more gold coins, and you have one or more silver coins </dt>
<dd> You get two victory points </dd>
<dt> If you have one or more silver coins </dt>
<dd> You get one victory point </dd>
<dt> Otherwise </dt>
<dd> You get no victory points </dd>
</dl>The following snippet shows a dl element being used as a glossary. Note the use
of dfn to indicate the word being defined.
<dl>
<dt><dfn>Apartment</dfn>, n.</dt>
<dd>An execution context grouping one or more threads with one or
more COM objects.</dd>
<dt><dfn>Flat</dfn>, n.</dt>
<dd>A deflated tire.</dd>
<dt><dfn>Home</dfn>, n.</dt>
<dd>The user's login directory.</dd>
</dl>This example uses microdata attributes in a dl element, together
with the div element, to annotate the ice cream desserts at a French restaurant.
<dl>
<div itemscope itemtype="http://schema.org/Product">
<dt itemprop="name">Café ou Chocolat Liégeois
<dd itemprop="offers" itemscope itemtype="http://schema.org/Offer">
<span itemprop="price">3.50</span>
<data itemprop="priceCurrency" value="EUR">€</data>
<dd itemprop="description">
2 boules Café ou Chocolat, 1 boule Vanille, sause café ou chocolat, chantilly
</div>
<div itemscope itemtype="http://schema.org/Product">
<dt itemprop="name">Américaine
<dd itemprop="offers" itemscope itemtype="http://schema.org/Offer">
<span itemprop="price">3.50</span>
<data itemprop="priceCurrency" value="EUR">€</data>
<dd itemprop="description">
1 boule Crème brûlée, 1 boule Vanille, 1 boule Caramel, chantilly
</div>
</dl>Without the div element the markup would need to use the itemref attribute to link the data in the dd elements
with the item, as follows.
<dl>
<dt itemscope itemtype="http://schema.org/Product" itemref="1-offer 1-description">
<span itemprop="name">Café ou Chocolat Liégeois</span>
<dd id="1-offer" itemprop="offers" itemscope itemtype="http://schema.org/Offer">
<span itemprop="price">3.50</span>
<data itemprop="priceCurrency" value="EUR">€</data>
<dd id="1-description" itemprop="description">
2 boules Café ou Chocolat, 1 boule Vanille, sause café ou chocolat, chantilly
<dt itemscope itemtype="http://schema.org/Product" itemref="2-offer 2-description">
<span itemprop="name">Américaine</span>
<dd id="2-offer" itemprop="offers" itemscope itemtype="http://schema.org/Offer">
<span itemprop="price">3.50</span>
<data itemprop="priceCurrency" value="EUR">€</data>
<dd id="2-description" itemprop="description">
1 boule Crème brûlée, 1 boule Vanille, 1 boule Caramel, chantilly
</dl>The dl element is inappropriate for marking up dialogue. See some examples of how to mark up dialogue.
dt elementSupport in all current engines.
dd or dt elements inside dl elements.dd or dt elements inside div elements that are children of a dl element.header, footer, sectioning content, or heading content descendants.dt element's end tag can be omitted if the
dt element is immediately followed by another dt element or a
dd element.HTMLElement.The dt element represents the term, or name, part of a
term-description group in a description list (dl element).
The dt element itself, when used in a dl element, does
not indicate that its contents are a term being defined, but this can be indicated using the
dfn element.
This example shows a list of frequently asked questions (a FAQ) marked up using the
dt element for questions and the dd element for answers.
<article>
<h1>FAQ</h1>
<dl>
<dt>What do we want?</dt>
<dd>Our data.</dd>
<dt>When do we want it?</dt>
<dd>Now.</dd>
<dt>Where is it?</dt>
<dd>We are not sure.</dd>
</dl>
</article>dd elementSupport in all current engines.
dt or dd elements inside dl elements.dt or dd elements inside div elements that are children of a dl element.dd element's end tag can be omitted if the
dd element is immediately followed by another dd element or a
dt element, or if there is no more content in the parent element.HTMLElement.The dd element represents the description, definition, or value, part
of a term-description group in a description list (dl element).
A dl can be used to define a vocabulary list, like in a dictionary. In the
following example, each entry, given by a dt with a dfn, has several
dds, showing the various parts of the definition.
<dl>
<dt><dfn>happiness</dfn></dt>
<dd class="pronunciation">/'hæ p. nes/</dd>
<dd class="part-of-speech"><i><abbr>n.</abbr></i></dd>
<dd>The state of being happy.</dd>
<dd>Good fortune; success. <q>Oh <b>happiness</b>! It worked!</q></dd>
<dt><dfn>rejoice</dfn></dt>
<dd class="pronunciation">/ri jois'/</dd>
<dd><i class="part-of-speech"><abbr>v.intr.</abbr></i> To be delighted oneself.</dd>
<dd><i class="part-of-speech"><abbr>v.tr.</abbr></i> To cause one to be delighted.</dd>
</dl>figure elementSupport in all current engines.
figcaption element followed by flow content.figcaption element.HTMLElement.The figure element represents some flow content,
optionally with a caption, that is self-contained (like a complete sentence) and is typically
referenced as a single unit from the main flow of the document.
"Self-contained" in this context does not necessarily mean independent. For
example, each sentence in a paragraph is self-contained; an image that is part of a sentence would
be inappropriate for figure, but an entire sentence made of images would be
fitting.
The element can thus be used to annotate illustrations, diagrams, photos, code listings, etc.
When a figure is referred to from the main content of the document by identifying
it by its caption (e.g., by figure number), it enables such content to be easily moved away from
that primary content, e.g., to the side of the page, to dedicated pages, or to an appendix,
without affecting the flow of the document.
If a figure element is referenced by its relative position, e.g.,
"in the photograph above" or "as the next figure shows", then moving the figure would disrupt the
page's meaning. Authors are encouraged to consider using labels to refer to figures, rather than
using such relative references, so that the page can easily be restyled without affecting the
page's meaning.
The first figcaption element child of the element, if any,
represents the caption of the figure element's contents. If there is no child
figcaption element, then there is no caption.
A figure element's contents are part of the surrounding flow. If the purpose of
the page is to display the figure, for example a photograph on an image sharing site, the
figure and figcaption elements can be used to explicitly provide a
caption for that figure. For content that is only tangentially related, or that serves a separate
purpose than the surrounding flow, the aside element should be used (and can itself
wrap a figure). For example, a pull quote that repeats content from an
article would be more appropriate in an aside than in a
figure, because it isn't part of the content, it's a repetition of the content for
the purposes of enticing readers or highlighting key topics.
This example shows the figure element to mark up a code listing.
<p>In <a href="#l4">listing 4</a> we see the primary core interface
API declaration.</p>
<figure id="l4">
<figcaption>Listing 4. The primary core interface API declaration.</figcaption>
<pre><code>interface PrimaryCore {
boolean verifyDataLine();
undefined sendData(sequence<byte> data);
undefined initSelfDestruct();
}</code></pre>
</figure>
<p>The API is designed to use UTF-8.</p>Here we see a figure element to mark up a photo that is the main content of the
page (as in a gallery).
<!DOCTYPE HTML>
<html lang="en">
<title>Bubbles at work — My Gallery™</title>
<figure>
<img src="bubbles-work.jpeg"
alt="Bubbles, sitting in his office chair, works on his
latest project intently.">
<figcaption>Bubbles at work</figcaption>
</figure>
<nav><a href="19414.html">Prev</a> — <a href="19416.html">Next</a></nav>In this example, we see an image that is not a figure, as well as an image and a
video that are. The first image is literally part of the example's second sentence, so it's not a
self-contained unit, and thus figure would be inappropriate.
<h2>Malinko's comics</h2>
<p>This case centered on some sort of "intellectual property"
infringement related to a comic (see Exhibit A). The suit started
after a trailer ending with these words:
<blockquote>
<img src="promblem-packed-action.png" alt="ROUGH COPY! Promblem-Packed Action!">
</blockquote>
<p>...was aired. A lawyer, armed with a Bigger Notebook, launched a
preemptive strike using snowballs. A complete copy of the trailer is
included with Exhibit B.
<figure>
<img src="ex-a.png" alt="Two squiggles on a dirty piece of paper.">
<figcaption>Exhibit A. The alleged <cite>rough copy</cite> comic.</figcaption>
</figure>
<figure>
<video src="ex-b.mov"></video>
<figcaption>Exhibit B. The <cite>Rough Copy</cite> trailer.</figcaption>
</figure>
<p>The case was resolved out of court.Here, a part of a poem is marked up using figure.
<figure>
<p>'Twas brillig, and the slithy toves<br>
Did gyre and gimble in the wabe;<br>
All mimsy were the borogoves,<br>
And the mome raths outgrabe.</p>
<figcaption><cite>Jabberwocky</cite> (first verse). Lewis Carroll, 1832-98</figcaption>
</figure>In this example, which could be part of a much larger work discussing a castle, nested
figure elements are used to provide both a group caption and individual captions for
each figure in the group:
<figure>
<figcaption>The castle through the ages: 1423, 1858, and 1999 respectively.</figcaption>
<figure>
<figcaption>Etching. Anonymous, ca. 1423.</figcaption>
<img src="castle1423.jpeg" alt="The castle has one tower, and a tall wall around it.">
</figure>
<figure>
<figcaption>Oil-based paint on canvas. Maria Towle, 1858.</figcaption>
<img src="castle1858.jpeg" alt="The castle now has two towers and two walls.">
</figure>
<figure>
<figcaption>Film photograph. Peter Jankle, 1999.</figcaption>
<img src="castle1999.jpeg" alt="The castle lies in ruins, the original tower all that remains in one piece.">
</figure>
</figure>The previous example could also be more succinctly written as follows (using title attributes in place of the nested
figure/figcaption pairs):
<figure>
<img src="castle1423.jpeg" title="Etching. Anonymous, ca. 1423."
alt="The castle has one tower, and a tall wall around it.">
<img src="castle1858.jpeg" title="Oil-based paint on canvas. Maria Towle, 1858."
alt="The castle now has two towers and two walls.">
<img src="castle1999.jpeg" title="Film photograph. Peter Jankle, 1999."
alt="The castle lies in ruins, the original tower all that remains in one piece.">
<figcaption>The castle through the ages: 1423, 1858, and 1999 respectively.</figcaption>
</figure>The figure is sometimes referenced only implicitly from the content:
<article>
<h1>Fiscal negotiations stumble in Congress as deadline nears</h1>
<figure>
<img src="obama-reid.jpeg" alt="Obama and Reid sit together smiling in the Oval Office.">
<figcaption>Barack Obama and Harry Reid. White House press photograph.</figcaption>
</figure>
<p>Negotiations in Congress to end the fiscal impasse sputtered on Tuesday, leaving both chambers
grasping for a way to reopen the government and raise the country's borrowing authority with a
Thursday deadline drawing near.</p>
...
</article>figcaption elementSupport in all current engines.
figure element.HTMLElement.The figcaption element represents a caption or legend for the rest of
the contents of the figcaption element's parent figure element, if any.
The element can contain additional information about the source:
<figcaption>
<p>A duck.</p>
<p><small>Photograph courtesy of 🌟 News.</small></p>
</figcaption><figcaption>
<p>Average rent for 3-room apartments, excluding non-profit apartments</p>
<p>Zürich’s Statistics Office — <time datetime=2017-11-14>14 November 2017</time></p>
</figcaption>main elementSupport in all current engines.
main element.HTMLElement.The main element represents the dominant contents of the
document.
A document must not have more than one main element that does not have the attribute specified.
A hierarchically correct main element is one whose ancestor elements
are limited to html, body, div, form without
an accessible name, and autonomous custom elements. Each main element must be a
hierarchically correct main element.
In this example, the author has used a presentation where each component of the page is
rendered in a box. To wrap the main content of the page (as opposed to the header, the footer,
the navigation bar, and a sidebar), the main element is used.
<!DOCTYPE html>
<html lang="en">
<title>RPG System 17</title>
<style>
header, nav, aside, main, footer {
margin: 0.5em; border: thin solid; padding: 0.5em;
background: #EFF; color: black; box-shadow: 0 0 0.25em #033;
}
h1, h2, p { margin: 0; }
nav, main { float: left; }
aside { float: right; }
footer { clear: both; }
</style>
<header>
<h1>System Eighteen</h1>
</header>
<nav>
<a href="../16/">← System 17</a>
<a href="../18/">RPXIX →</a>
</nav>
<aside>
<p>This system has no HP mechanic, so there's no healing.
</aside>
<main>
<h2>Character creation</h2>
<p>Attributes (magic, strength, agility) are purchased at the cost of one point per level.</p>
<h2>Rolls</h2>
<p>Each encounter, roll the dice for all your skills. If you roll more than the opponent, you win.</p>
</main>
<footer>
<p>Copyright © 2013
</footer>
</html>In the following example, multiple main elements are used and script is used to
make navigation work without a server roundtrip and to set the attribute on those that are not current:
<!doctype html>
<html lang=en-CA>
<meta charset=utf-8>
<title> … </title>
<link rel=stylesheet href=spa.css>
<script src=spa.js async></script>
<nav>
<a href=/>Home</a>
<a href=/about>About</a>
<a href=/contact>Contact</a>
</nav>
<main>
<h1>Home</h1>
…
</main>
<main hidden>
<h1>About</h1>
…
</main>
<main hidden>
<h1>Contact</h1>
…
</main>
<footer>Made with ❤️ by <a href=https://example.com/>Example 👻</a>.</footer>div elementSupport in all current engines.
Support in all current engines.
dl element.dl element: one or more dt elements followed by one or more dd elements, optionally intermixed with script-supporting elements.dl element: flow content.[Exposed=Window]
interface HTMLDivElement : HTMLElement {
[HTMLConstructor] constructor();
// also has obsolete members
};The div element has no special meaning at all. It represents its
children. It can be used with the class, lang, and title attributes to mark up
semantics common to a group of consecutive elements. It can also be used in a dl
element, wrapping groups of dt and dd elements.
Authors are strongly encouraged to view the div element as an element
of last resort, for when no other element is suitable. Use of more appropriate elements instead of
the div element leads to better accessibility for readers and easier maintainability
for authors.
For example, a blog post would be marked up using article, a chapter using
section, a page's navigation aids using nav, and a group of form
controls using fieldset.
On the other hand, div elements can be useful for stylistic purposes or to wrap
multiple paragraphs within a section that are all to be annotated in a similar way. In the
following example, we see div elements used as a way to set the language of two
paragraphs at once, instead of setting the language on the two paragraph elements separately:
<article lang="en-US">
<h1>My use of language and my cats</h1>
<p>My cat's behavior hasn't changed much since her absence, except
that she plays her new physique to the neighbors regularly, in an
attempt to get pets.</p>
<div lang="en-GB">
<p>My other cat, coloured black and white, is a sweetie. He followed
us to the pool today, walking down the pavement with us. Yesterday
he apparently visited our neighbours. I wonder if he recognises that
their flat is a mirror image of ours.</p>
<p>Hm, I just noticed that in the last paragraph I used British
English. But I'm supposed to write in American English. So I
shouldn't say "pavement" or "flat" or "colour"...</p>
</div>
<p>I should say "sidewalk" and "apartment" and "color"!</p>
</article>a elementSupport in all current engines.
Support in all current engines.
href attribute: Interactive content.a element descendant, or descendant with the tabindex attribute specified.href — Address of the hyperlinktarget — Browsing context for hyperlink navigationdownload — Whether to download the resource instead of navigating to it, and its file name if soping — URLs to pingrel — Relationship between the location in the document containing the hyperlink and the destination resourcehreflang — Language of the linked resourcetype — Hint for the type of the referenced resourcereferrerpolicy — Referrer policy for fetches initiated by the elementhref attribute: for authors; for implementers.[Exposed=Window]
interface HTMLAnchorElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString target;
[CEReactions] attribute DOMString download;
[CEReactions] attribute USVString ping;
[CEReactions] attribute DOMString rel;
[SameObject, PutForwards=value] readonly attribute DOMTokenList relList;
[CEReactions] attribute DOMString hreflang;
[CEReactions] attribute DOMString type;
[CEReactions] attribute DOMString text;
[CEReactions] attribute DOMString referrerPolicy;
// also has obsolete members
};
HTMLAnchorElement includes HTMLHyperlinkElementUtils;If the a element has an href attribute,
then it represents a hyperlink (a hypertext anchor) labeled by its
contents.
If the a element has no href attribute,
then the element represents a placeholder for where a link might otherwise have been
placed, if it had been relevant, consisting of just the element's contents.
The target, download, ping,
rel, hreflang, type,
and referrerpolicy attributes must be omitted
if the href attribute is not present.
If the itemprop attribute is specified on an a element,
then the href attribute must also be specified.
If a site uses a consistent navigation toolbar on every page, then the link that would
normally link to the page itself could be marked up using an a element:
<nav>
<ul>
<li> <a href="/">Home</a> </li>
<li> <a href="/news">News</a> </li>
<li> <a>Examples</a> </li>
<li> <a href="/legal">Legal</a> </li>
</ul>
</nav>The href, target, download, ping,
and referrerpolicy attributes affect what
happens when users follow hyperlinks or download hyperlinks created using the a
element. The rel, hreflang, and type
attributes may be used to indicate to the user the likely nature of the target resource before the
user follows the link.
The activation behavior of a elements that create hyperlinks is to run the following steps:
If the target of the click event is an img
element with an ismap attribute specified, then server-side
image map processing must be performed, as follows:
click event was a real pointing-device-triggered
click event on the img element, then set
x to the distance in CSS pixels from the left edge of the
image to the location of the click, and set y to the distance in CSS pixels from the top edge of the image to the location of the
click.Follow the hyperlink or download the hyperlink created by the a
element, as determined by the download attribute
and any expressed user preference, passing hyperlink suffix, if the steps above
defined it.
textSame as textContent.
Support in all current engines.
Support in all current engines.
The IDL attributes download, ping, target, rel, hreflang, and
type, must reflect the respective content
attributes of the same name.
Support in all current engines.
The IDL attribute relList must reflect the rel content attribute.
HTMLAnchorElement/referrerPolicy
Support in all current engines.
The IDL attribute referrerPolicy must
reflect the referrerpolicy
content attribute, limited to only known values.
The text
attribute's getter must return this element's descendant text content.
The text attribute's setter must string replace
all with the given value within this element.
The a element can be wrapped around entire paragraphs, lists, tables, and so
forth, even entire sections, so long as there is no interactive content within (e.g., buttons or
other links). This example shows how this can be used to make an entire advertising block into a
link:
<aside class="advertising">
<h1>Advertising</h1>
<a href="https://ad.example.com/?adid=1929&pubid=1422">
<section>
<h1>Mellblomatic 9000!</h1>
<p>Turn all your widgets into mellbloms!</p>
<p>Only $9.99 plus shipping and handling.</p>
</section>
</a>
<a href="https://ad.example.com/?adid=375&pubid=1422">
<section>
<h1>The Mellblom Browser</h1>
<p>Web browsing at the speed of light.</p>
<p>No other browser goes faster!</p>
</section>
</a>
</aside>The following example shows how a bit of script can be used to effectively make an entire row in a job listing table a hyperlink:
<table>
<tr>
<th>Position
<th>Team
<th>Location
<tr>
<td><a href="/jobs/manager">Manager</a>
<td>Remotees
<td>Remote
<tr>
<td><a href="/jobs/director">Director</a>
<td>Remotees
<td>Remote
<tr>
<td><a href="/jobs/astronaut">Astronaut</a>
<td>Architecture
<td>Remote
</table>
<script>
document.querySelector("table").onclick = ({ target }) => {
if (target.parentElement.localName === "tr") {
const link = target.parentElement.querySelector("a");
if (link) {
link.click();
}
}
}
</script>em elementSupport in all current engines.
HTMLElement.The em element represents stress emphasis of its contents.
The level of stress that a particular piece of content has is given by its number of ancestor
em elements.
The placement of stress emphasis changes the meaning of the sentence. The element thus forms an integral part of the content. The precise way in which stress is used in this way depends on the language.
These examples show how changing the stress emphasis changes the meaning. First, a general statement of fact, with no stress:
<p>Cats are cute animals.</p>By emphasizing the first word, the statement implies that the kind of animal under discussion is in question (maybe someone is asserting that dogs are cute):
<p><em>Cats</em> are cute animals.</p>Moving the stress to the verb, one highlights that the truth of the entire sentence is in question (maybe someone is saying cats are not cute):
<p>Cats <em>are</em> cute animals.</p>By moving it to the adjective, the exact nature of the cats is reasserted (maybe someone suggested cats were mean animals):
<p>Cats are <em>cute</em> animals.</p>Similarly, if someone asserted that cats were vegetables, someone correcting this might emphasize the last word:
<p>Cats are cute <em>animals</em>.</p>By emphasizing the entire sentence, it becomes clear that the speaker is fighting hard to get the point across. This kind of stress emphasis also typically affects the punctuation, hence the exclamation mark here.
<p><em>Cats are cute animals!</em></p>Anger mixed with emphasizing the cuteness could lead to markup such as:
<p><em>Cats are <em>cute</em> animals!</em></p>The em element isn't a generic "italics" element. Sometimes, text is intended to
stand out from the rest of the paragraph, as if it was in a different mood or voice. For this,
the i element is more appropriate.
The em element also isn't intended to convey importance; for that purpose, the
strong element is more appropriate.
strong elementSupport in all current engines.
HTMLElement.The strong element represents strong importance, seriousness, or
urgency for its contents.
Importance: the strong element can be used in a heading, caption,
or paragraph to distinguish the part that really matters from other parts that might be more
detailed, more jovial, or merely boilerplate. (This is distinct from marking up subheadings, for
which the hgroup element is appropriate.)
For example, the first word of the previous paragraph is marked up with
strong to distinguish it from the more detailed text in the rest of the
paragraph.
Seriousness: the strong element can be used to mark up a warning
or caution notice.
Urgency: the strong element can be used to denote contents that
the user needs to see sooner than other parts of the document.
The relative level of importance of a piece of content is given by its number of ancestor
strong elements; each strong element increases the importance of its
contents.
Changing the importance of a piece of text with the strong element does not change
the meaning of the sentence.
Here, the word "chapter" and the actual chapter number are mere boilerplate, and the actual
name of the chapter is marked up with strong:
<h1>Chapter 1: <strong>The Praxis</strong></h1>In the following example, the name of the diagram in the caption is marked up with
strong, to distinguish it from boilerplate text (before) and the description
(after):
<figcaption>Figure 1. <strong>Ant colony dynamics</strong>. The ants in this colony are
affected by the heat source (upper left) and the food source (lower right).</figcaption>In this example, the heading is really "Flowers, Bees, and Honey", but the author has added a
light-hearted addition to the heading. The strong element is thus used to mark up
the first part to distinguish it from the latter part.
<h1><strong>Flowers, Bees, and Honey</strong> and other things I don't understand</h1>Here is an example of a warning notice in a game, with the various parts marked up according to how important they are:
<p><strong>Warning.</strong> This dungeon is dangerous.
<strong>Avoid the ducks.</strong> Take any gold you find.
<strong><strong>Do not take any of the diamonds</strong>,
they are explosive and <strong>will destroy anything within
ten meters.</strong></strong> You have been warned.</p>In this example, the strong element is used to denote the part of the text that
the user is intended to read first.
<p>Welcome to Remy, the reminder system.</p>
<p>Your tasks for today:</p>
<ul>
<li><p><strong>Turn off the oven.</strong></p></li>
<li><p>Put out the trash.</p></li>
<li><p>Do the laundry.</p></li>
</ul>small elementSupport in all current engines.
HTMLElement.The small element represents side comments such as small print.
Small print typically features disclaimers, caveats, legal restrictions, or copyrights. Small print is also sometimes used for attribution, or for satisfying licensing requirements.
The small element does not "de-emphasize" or lower the importance of
text emphasized by the em element or marked as important with the strong
element. To mark text as not emphasized or important, simply do not mark it up with the
em or strong elements respectively.
The small element should not be used for extended spans of text, such as multiple
paragraphs, lists, or sections of text. It is only intended for short runs of text. The text of a
page listing terms of use, for instance, would not be a suitable candidate for the
small element: in such a case, the text is not a side comment, it is the main content
of the page.
The small element must not be used for subheadings; for that purpose, use the
hgroup element.
In this example, the small element is used to indicate that value-added tax is
not included in a price of a hotel room:
<dl>
<dt>Single room
<dd>199 € <small>breakfast included, VAT not included</small>
<dt>Double room
<dd>239 € <small>breakfast included, VAT not included</small>
</dl>In this second example, the small element is used for a side comment in an
article.
<p>Example Corp today announced record profits for the
second quarter <small>(Full Disclosure: Foo News is a subsidiary of
Example Corp)</small>, leading to speculation about a third quarter
merger with Demo Group.</p>This is distinct from a sidebar, which might be multiple paragraphs long and is removed from the main flow of text. In the following example, we see a sidebar from the same article. This sidebar also has small print, indicating the source of the information in the sidebar.
<aside>
<h1>Example Corp</h1>
<p>This company mostly creates small software and Web
sites.</p>
<p>The Example Corp company mission is "To provide entertainment
and news on a sample basis".</p>
<p><small>Information obtained from <a
href="https://example.com/about.html">example.com</a> home
page.</small></p>
</aside>In this last example, the small element is marked as being important
small print.
<p><strong><small>Continued use of this service will result in a kiss.</small></strong></p>s elementSupport in all current engines.
HTMLElement.The s element represents contents that are no longer accurate or no
longer relevant.
The s element is not appropriate when indicating document edits; to
mark a span of text as having been removed from a document, use the del element.
In this example a recommended retail price has been marked as no longer relevant as the product in question has a new sale price.
<p>Buy our Iced Tea and Lemonade!</p>
<p><s>Recommended retail price: $3.99 per bottle</s></p>
<p><strong>Now selling for just $2.99 a bottle!</strong></p>cite elementSupport in all current engines.
HTMLElement.The cite element represents the title of a work (e.g.
a book,
a paper,
an essay,
a poem,
a score,
a song,
a script,
a film,
a TV show,
a game,
a sculpture,
a painting,
a theatre production,
a play,
an opera,
a musical,
an exhibition,
a legal case report,
a computer program,
etc). This can be a work that is being quoted or referenced in detail (i.e. a
citation), or it can just be a work that is mentioned in passing.
A person's name is not the title of a work — even if people call that person a piece of
work — and the element must therefore not be used to mark up people's names. (In some cases,
the b element might be appropriate for names; e.g. in a gossip article where the
names of famous people are keywords rendered with a different style to draw attention to them. In
other cases, if an element is really needed, the span element can be
used.)
This next example shows a typical use of the cite element:
<p>My favorite book is <cite>The Reality Dysfunction</cite> by
Peter F. Hamilton. My favorite comic is <cite>Pearls Before
Swine</cite> by Stephan Pastis. My favorite track is <cite>Jive
Samba</cite> by the Cannonball Adderley Sextet.</p>This is correct usage:
<p>According to the Wikipedia article <cite>HTML</cite>, as it
stood in mid-February 2008, leaving attribute values unquoted is
unsafe. This is obviously an over-simplification.</p>The following, however, is incorrect usage, as the cite element here is
containing far more than the title of the work:
<!-- do not copy this example, it is an example of bad usage! -->
<p>According to <cite>the Wikipedia article on HTML</cite>, as it
stood in mid-February 2008, leaving attribute values unquoted is
unsafe. This is obviously an over-simplification.</p>The cite element is obviously a key part of any citation in a bibliography, but
it is only used to mark the title:
<p><cite>Universal Declaration of Human Rights</cite>, United Nations,
December 1948. Adopted by General Assembly resolution 217 A (III).</p>A citation is not a quote (for which the q element
is appropriate).
This is incorrect usage, because cite is not for quotes:
<p><cite>This is wrong!</cite>, said Ian.</p>This is also incorrect usage, because a person is not a work:
<p><q>This is still wrong!</q>, said <cite>Ian</cite>.</p>The correct usage does not use a cite element:
<p><q>This is correct</q>, said Ian.</p>As mentioned above, the b element might be relevant for marking names as being
keywords in certain kinds of documents:
<p>And then <b>Ian</b> said <q>this might be right, in a
gossip column, maybe!</q>.</p>q elementSupport in all current engines.
cite — Link to the source of the quotation or more information about the editHTMLQuoteElement.The q element represents some phrasing
content quoted from another source.
Quotation punctuation (such as quotation marks) that is quoting the contents of the element
must not appear immediately before, after, or inside q elements; they will be
inserted into the rendering by the user agent.
Content inside a q element must be quoted from another source, whose address, if
it has one, may be cited in the cite attribute. The source may be fictional, as when quoting
characters in a novel or screenplay.
If the cite attribute is present, it must be a valid
URL potentially surrounded by spaces. To obtain the corresponding citation
link, the value of the attribute must be parsed relative to the
element's node document. User agents may allow users to follow such citation
links, but they are primarily intended for private use (e.g., by server-side scripts collecting
statistics about a site's use of quotations), not for readers.
The q element must not be used in place of quotation marks that do not represent
quotes; for example, it is inappropriate to use the q element for marking up
sarcastic statements.
The use of q elements to mark up quotations is entirely optional; using explicit
quotation punctuation without q elements is just as correct.
Here is a simple example of the use of the q element:
<p>The man said <q>Things that are impossible just take
longer</q>. I disagreed with him.</p>Here is an example with both an explicit citation link in the q element, and an
explicit citation outside:
<p>The W3C page <cite>About W3C</cite> says the W3C's
mission is <q cite="https://www.w3.org/Consortium/">To lead the
World Wide Web to its full potential by developing protocols and
guidelines that ensure long-term growth for the Web</q>. I
disagree with this mission.</p>In the following example, the quotation itself contains a quotation:
<p>In <cite>Example One</cite>, he writes <q>The man
said <q>Things that are impossible just take longer</q>. I
disagreed with him</q>. Well, I disagree even more!</p>In the following example, quotation marks are used instead of the q element:
<p>His best argument was ❝I disagree❞, which
I thought was laughable.</p>In the following example, there is no quote — the quotation marks are used to name a
word. Use of the q element in this case would be inappropriate.
<p>The word "ineffable" could have been used to describe the disaster
resulting from the campaign's mismanagement.</p>dfn elementSupport in all current engines.
dfn element descendants.title attribute has special semantics on this element: Full term or expansion of abbreviation.HTMLElement.The dfn element represents the defining instance of a term. The paragraph, description list group, or section that is the nearest ancestor of the dfn
element must also contain the definition(s) for the term given
by the dfn element.
Defining term: if the dfn element has a title attribute, then the exact value of that attribute
is the term being defined. Otherwise, if it contains exactly one element child node and no child
Text nodes, and that child element is an abbr element with a title attribute, then the exact value of that attribute
is the term being defined. Otherwise, it is the descendant text content of the
dfn element that gives the term being defined.
If the title attribute of the dfn element is
present, then it must contain only the term being defined.
The title attribute of ancestor elements does not
affect dfn elements.
An a element that links to a dfn element represents an instance of
the term defined by the dfn element.
In the following fragment, the term "Garage Door Opener" is first defined in the first paragraph, then used in the second. In both cases, its abbreviation is what is actually displayed.
<p>The <dfn><abbr title="Garage Door Opener">GDO</abbr></dfn>
is a device that allows off-world teams to open the iris.</p>
<!-- ... later in the document: -->
<p>Teal'c activated his <abbr title="Garage Door Opener">GDO</abbr>
and so Hammond ordered the iris to be opened.</p>With the addition of an a element, the reference
can be made explicit:
<p>The <dfn id=gdo><abbr title="Garage Door Opener">GDO</abbr></dfn>
is a device that allows off-world teams to open the iris.</p>
<!-- ... later in the document: -->
<p>Teal'c activated his <a href=#gdo><abbr title="Garage Door Opener">GDO</abbr></a>
and so Hammond ordered the iris to be opened.</p>abbr elementSupport in all current engines.
title attribute has special semantics on this element: Full term or expansion of abbreviation.HTMLElement.The abbr element represents an abbreviation or acronym, optionally
with its expansion. The title attribute may be used to provide an expansion of the
abbreviation. The attribute, if specified, must contain an expansion of the abbreviation, and
nothing else.
The paragraph below contains an abbreviation marked up with the abbr element.
This paragraph defines the term "Web Hypertext Application
Technology Working Group".
<p>The <dfn id=whatwg><abbr
title="Web Hypertext Application Technology Working Group">WHATWG</abbr></dfn>
is a loose unofficial collaboration of web browser manufacturers and
interested parties who wish to develop new technologies designed to
allow authors to write and deploy Applications over the World Wide
Web.</p>An alternative way to write this would be:
<p>The <dfn id=whatwg>Web Hypertext Application Technology
Working Group</dfn> (<abbr
title="Web Hypertext Application Technology Working Group">WHATWG</abbr>)
is a loose unofficial collaboration of web browser manufacturers and
interested parties who wish to develop new technologies designed to
allow authors to write and deploy Applications over the World Wide
Web.</p>This paragraph has two abbreviations. Notice how only one is defined; the other, with no
expansion associated with it, does not use the abbr element.
<p>The
<abbr title="Web Hypertext Application Technology Working Group">WHATWG</abbr>
started working on HTML5 in 2004.</p>This paragraph links an abbreviation to its definition.
<p>The <a href="#whatwg"><abbr
title="Web Hypertext Application Technology Working Group">WHATWG</abbr></a>
community does not have much representation from Asia.</p>This paragraph marks up an abbreviation without giving an expansion, possibly as a hook to apply styles for abbreviations (e.g. smallcaps).
<p>Philip` and Dashiva both denied that they were going to
get the issue counts from past revisions of the specification to
backfill the <abbr>WHATWG</abbr> issue graph.</p>If an abbreviation is pluralized, the expansion's grammatical number (plural vs singular) must match the grammatical number of the contents of the element.
Here the plural is outside the element, so the expansion is in the singular:
<p>Two <abbr title="Working Group">WG</abbr>s worked on
this specification: the <abbr>WHATWG</abbr> and the
<abbr>HTMLWG</abbr>.</p>Here the plural is inside the element, so the expansion is in the plural:
<p>Two <abbr title="Working Groups">WGs</abbr> worked on
this specification: the <abbr>WHATWG</abbr> and the
<abbr>HTMLWG</abbr>.</p>Abbreviations do not have to be marked up using this element. It is expected to be useful in the following cases:
abbr element with a title attribute is an
alternative to including the expansion inline (e.g. in parentheses).abbr element with a title attribute or include the expansion inline in the text the first
time the abbreviation is used.abbr element
can be used without a title attribute.Providing an expansion in a title attribute once
will not necessarily cause other abbr elements in the same document with the same
contents but without a title attribute to behave as if they had
the same expansion. Every abbr element is independent.
ruby elementSupport in all current engines.
HTMLElement.The ruby element allows one or more spans of phrasing content to be marked with
ruby annotations. Ruby annotations are short runs of text presented alongside base text, primarily
used in East Asian typography as a guide for pronunciation or to include other annotations. In
Japanese, this form of typography is also known as furigana.
The content model of ruby elements consists of one or more of the following
sequences:
ruby elements and with no ruby element descendants
ruby element that itself has no ruby element descendants
The ruby and rt elements can be used for a variety of kinds of
annotations, including in particular (though by no means limited to) those described below. For
more details on Japanese Ruby in particular, and how to render Ruby for Japanese, see
Requirements for Japanese Text Layout. [JLREQ]
At the time of writing, CSS does not yet provide a way to fully control the
rendering of the HTML ruby element. It is hoped that CSS will be extended to support
the styles described below in due course.
One or more hiragana or katakana characters (the ruby annotation) are placed with each ideographic character (the base text). This is used to provide readings of kanji characters.
<ruby>B<rt>annotation</ruby>In this example, notice how each annotation corresponds to a single base character.
<ruby>君<rt>くん</ruby><ruby>子<rt>し</ruby>は<ruby>和<rt>わ</ruby>して<ruby>同<rt>どう</ruby>ぜず。君子は和して同ぜず。
This example can also be written as follows, using one ruby element with two
segments of base text and two annotations (one for each) rather than two back-to-back
ruby elements each with one base text segment and annotation (as in the markup
above):
<ruby>君<rt>くん</rt>子<rt>し</ruby>は<ruby>和<rt>わ</ruby>して<ruby>同<rt>どう</ruby>ぜず。This is similar to the previous case: each ideographic character in the compound word (the base text) has its reading given in hiragana or katakana characters (the ruby annotation). The difference is that the base text segments form a compound word rather than being separate from each other.
<ruby>B<rt>annotation</rt>B<rt>annotation</ruby>In this example, notice again how each annotation corresponds to a single base character. In this example, each compound word (jukugo) corresponds to a single ruby element.
The rendering here is expected to be that each annotation be placed over (or next to, in vertical text) the corresponding base character, with the annotations not overhanging any of the adjacent characters.
<ruby>鬼<rt>き</rt>門<rt>もん</rt></ruby>の<ruby>方<rt>ほう</rt>角<rt>がく</rt></ruby>を<ruby>凝<rt>ぎょう</rt>視<rt>し</rt></ruby>する鬼門の方角を凝視する
This is semantically identical to the previous case (each individual ideographic character in the base compound word has its reading given in an annotation in hiragana or katakana characters), but the rendering is the more complicated Jukugo Ruby rendering.
This is the same example as above for mono-ruby for compound words. The different rendering is expected to be achieved using different styling (e.g. in CSS), and is not shown here.
<ruby>鬼<rt>き</rt>門<rt>もん</rt></ruby>の<ruby>方<rt>ほう</rt>角<rt>がく</rt></ruby>を<ruby>凝<rt>ぎょう</rt>視<rt>し</rt></ruby>するFor more details on Jukugo Ruby rendering, see Appendix F in the Requirements for Japanese Text Layout. [JLREQ]
The annotation describes the meaning of the base text, rather than (or in addition to) the pronunciation. As such, both the base text and the annotation can be multiple characters long.
<ruby>BASE<rt>annotation</ruby>Here a compound ideographic word has its corresponding katakana given as an annotation.
<ruby>境界面<rt>インターフェース</ruby>境界面
Here a compound ideographic word has its translation in English provided as an annotation.
<ruby lang="ja">編集者<rt lang="en">editor</ruby>編集者
A phonetic reading that corresponds to multiple base characters, because a one-to-one mapping would be difficult. (In English, the words "Colonel" and "Lieutenant" are examples of words where a direct mapping of pronunciation to individual letters is, in some dialects, rather unclear.)
In this example, the name of a species of flowers has a phonetic reading provided using group ruby:
<ruby>紫陽花<rt>あじさい</ruby>紫陽花
Sometimes, ruby styles described above are combined.
If this results in two annotations covering the same single base segment, then the annotations can just be placed back to back.
<ruby>BASE<rt>annotation 1<rt>annotation 2</ruby><ruby>B<rt>a<rt>a</ruby><ruby>A<rt>a<rt>a</ruby><ruby>S<rt>a<rt>a</ruby><ruby>E<rt>a<rt>a</ruby>In this contrived example, some symbols are given names in English and French.
<ruby>
♥ <rt> Heart <rt lang=fr> Cœur </rt>
☘ <rt> Shamrock <rt lang=fr> Trèfle </rt>
✶ <rt> Star <rt lang=fr> Étoile </rt>
</ruby>In more complication situations such as following examples, a nested ruby
element is used to give the inner annotations, and then that whole ruby is then
given an annotation at the "outer" level.
<ruby><ruby>B<rt>a</rt>A<rt>n</rt>S<rt>t</rt>E<rt>n</rt></ruby><rt>annotation</ruby>Here both a phonetic reading and the meaning are given in ruby annotations. The annotation on the nested ruby element gives a mono-ruby phonetic annotation for each base character, while the annotation in the rt element that is a child of the outer ruby element gives the meaning using hiragana.
<ruby><ruby>東<rt>とう</rt>南<rt>なん</rt></ruby><rt>たつみ</rt></ruby>の方角東南の方角
This is the same example, but the meaning is given in English instead of Japanese:
<ruby><ruby>東<rt>とう</rt>南<rt>なん</rt></ruby><rt lang=en>Southeast</rt></ruby>の方角東南の方角
Within a ruby element that does not have a ruby element ancestor,
content is segmented and segments are placed into three categories: base text segments, annotation
segments, and ignored segments. Ignored segments do not form part of the document's semantics
(they consist of some inter-element whitespace and rp elements, the
latter of which are used for legacy user agents that do not support ruby at all). Base text
segments can overlap (with a limit of two segments overlapping any one position in the DOM, and
with any segment having an earlier start point than an overlapping segment also having an equal or
later end point, and any segment have a later end point than an overlapping segment also having an
equal or earlier start point). Annotation segments correspond to rt elements. Each annotation
segment can be associated with a base text segment, and each base text segment can have annotation
segments associated with it. (In a conforming document, each base text segment is associated with
at least one annotation segment, and each annotation segment is associated with one base text
segment.) A ruby element represents the union of the segments of base
text it contains, along with the mapping from those base text segments to annotation segments.
Segments are described in terms of DOM ranges; annotation segment ranges always
consist of exactly one element. [DOM]
At any particular time, the segmentation and categorization of content of a ruby
element is the result that would be obtained from running the following algorithm:
Let base text segments be an empty list of base text segments, each potentially with a list of base text subsegments.
Let annotation segments be an empty list of annotation segments, each potentially being associated with a base text segment or subsegment.
Let root be the ruby element for which the algorithm is
being run.
If root has a ruby element ancestor, then jump to the
step labeled end.
Let current parent be root.
Let index be 0.
Let start index be null.
Let parent start index be null.
Let current base text be null.
Start mode: If index is equal to or greater than the number of child nodes in current parent, then jump to the step labeled end mode.
If the indexth node in current parent is an
rt or rp element, jump to the step labeled annotation
mode.
Set start index to the value of index.
Base mode: If the indexth node in current
parent is a ruby element, and if current parent is the
same element as root, then push a ruby level and then jump to
the step labeled start mode.
If the indexth node in current parent is an
rt or rp element, then set the current base text and then
jump to the step labeled annotation mode.
Increment index by one.
Base mode post-increment: If index is equal to or greater than the number of child nodes in current parent, then jump to the step labeled end mode.
Jump back to the step labeled base mode.
Annotation mode: If the indexth node in current
parent is an rt element, then push a ruby annotation and jump to
the step labeled annotation mode increment.
If the indexth node in current parent is an
rp element, jump to the step labeled annotation mode increment.
If the indexth node in current parent is not a
Text node, or is a Text node that is not inter-element
whitespace, then jump to the step labeled base mode.
Annotation mode increment: Let lookahead index be index plus one.
Annotation mode white-space skipper: If lookahead index is equal to the number of child nodes in current parent then jump to the step labeled end mode.
If the lookahead indexth node in current parent is
an rt element or an rp element, then set index to
lookahead index and jump to the step labeled annotation mode.
If the lookahead indexth node in current parent is
not a Text node, or is a Text node that is not inter-element
whitespace, then jump to the step labeled base mode (without further incrementing
index, so the inter-element whitespace seen so far becomes part
of the next base text segment).
Increment lookahead index by one.
Jump to the step labeled annotation mode white-space skipper.
End mode: If current parent is not the same element as root, then pop a ruby level and jump to the step labeled base mode post-increment.
End: Return base text segments and annotation
segments. Any content of the ruby element not described by segments in either
of those lists is implicitly in an ignored segment.
When the steps above say to set the current base text, it means to run the following steps at that point in the algorithm:
Let text range be a DOM range whose start is the boundary point (current parent, start index) and whose end is the boundary point (current parent, index).
Let new text segment be a base text segment described by the range annotation range.
Add new text segment to base text segments.
Let current base text be new text segment.
Let start index be null.
When the steps above say to push a ruby level, it means to run the following steps at that point in the algorithm:
Let current parent be the indexth node in current parent.
Let index be 0.
Set saved start index to the value of start index.
Let start index be null.
When the steps above say to pop a ruby level, it means to run the following steps at that point in the algorithm:
Let index be the position of current parent in root.
Let current parent be root.
Increment index by one.
Set start index to the value of saved start index.
Let saved start index be null.
When the steps above say to push a ruby annotation, it means to run the following steps at that point in the algorithm:
Let rt be the rt element that is the indexth node of current parent.
Let annotation range be a DOM range whose start is the boundary point (current parent, index) and whose end is the boundary point (current parent, index plus one) (i.e. that contains only rt).
Let new annotation segment be an annotation segment described by the range annotation range.
If current base text is not null, associate new annotation segment with current base text.
Add new annotation segment to annotation segments.
In this example, each ideograph in the Japanese text 漢字 is annotated with its reading in hiragana.
...
<ruby>漢<rt>かん</rt>字<rt>じ</rt></ruby>
...This might be rendered as:
In this example, each ideograph in the traditional Chinese text 漢字 is annotated with its bopomofo reading.
<ruby>漢<rt>ㄏㄢˋ</rt>字<rt>ㄗˋ</rt></ruby>This might be rendered as:
In this example, each ideograph in the simplified Chinese text 汉字 is annotated with its pinyin reading.
...<ruby>汉<rt>hàn</rt>字<rt>zì</rt></ruby>...This might be rendered as:
In this more contrived example, the acronym "HTML" has four annotations: one for the whole acronym, briefly describing what it is, one for the letters "HT" expanding them to "Hypertext", one for the letter "M" expanding it to "Markup", and one for the letter "L" expanding it to "Language".
<ruby>
<ruby>HT<rt>Hypertext</rt>M<rt>Markup</rt>L<rt>Language</rt></ruby>
<rt>An abstract language for describing documents and applications
</ruby>rt elementSupport in all current engines.
ruby element.rt element's end tag can be omitted if the
rt element is immediately followed by an rt or rp element,
or if there is no more content in the parent element.HTMLElement.The rt element marks the ruby text component of a ruby annotation. When it is the
child of a ruby element, it doesn't represent
anything itself, but the ruby element uses it as part of determining what it
represents.
An rt element that is not a child of a ruby element
represents the same thing as its children.
rp elementSupport in all current engines.
ruby element, either immediately before or immediately after an rt element.rp element's end tag can be omitted if the
rp element is immediately followed by an rt or rp element,
or if there is no more content in the parent element.HTMLElement.The rp element can be used to provide parentheses or other content around a ruby
text component of a ruby annotation, to be shown by user agents that don't support ruby
annotations.
An rp element that is a child of a ruby
element represents nothing. An rp element
whose parent element is not a ruby element represents its
children.
The example above, in which each ideograph in the text 漢字 is annotated with its phonetic reading, could be expanded to
use rp so that in legacy user agents the readings are in parentheses:
...
<ruby>漢<rp>（</rp><rt>かん</rt><rp>）</rp>字<rp>（</rp><rt>じ</rt><rp>）</rp></ruby>
...In conforming user agents the rendering would be as above, but in user agents that do not support ruby, the rendering would be:
... 漢（かん）字（じ）...
When there are multiple annotations for a segment, rp elements can also be placed
between the annotations. Here is another copy of an earlier contrived example showing some
symbols with names given in English and French, but this time with rp elements as
well:
<ruby>
♥<rp>: </rp><rt>Heart</rt><rp>, </rp><rt lang=fr>Cœur</rt><rp>.</rp>
☘<rp>: </rp><rt>Shamrock</rt><rp>, </rp><rt lang=fr>Trèfle</rt><rp>.</rp>
✶<rp>: </rp><rt>Star</rt><rp>, </rp><rt lang=fr>Étoile</rt><rp>.</rp>
</ruby>This would make the example render as follows in non-ruby-capable user agents:
♥: Heart, Cœur. ☘: Shamrock, Trèfle. ✶: Star, Étoile.
data elementSupport in all current engines.
Support in all current engines.
value — Machine-readable value[Exposed=Window]
interface HTMLDataElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString value;
};The data element represents its contents, along with a
machine-readable form of those contents in the value
attribute.
The value attribute
must be present. Its value must be a representation of the element's contents in a
machine-readable format.
When the value is date- or time-related, the more specific time
element can be used instead.
The element can be used for several purposes.
When combined with microformats or the microdata attributes defined in
this specification, the element serves to provide both a machine-readable value for the purposes
of data processors, and a human-readable value for the purposes of rendering in a web browser. In
this case, the format to be used in the value attribute is
determined by the microformats or microdata vocabulary in use.
The element can also, however, be used in conjunction with scripts in the page, for when a
script has a literal value to store alongside a human-readable value. In such cases, the format to
be used depends only on the needs of the script. (The data-*
attributes can also be useful in such situations.)
Support in all current engines.
The value IDL
attribute must reflect the content attribute of the same name.
Here, a short table has its numeric values encoded using the data element so
that the table sorting JavaScript library can provide a sorting mechanism on each column
despite the numbers being presented in textual form in one column and in a decomposed form in
another.
<script src="sortable.js"></script>
<table class="sortable">
<thead> <tr> <th> Game <th> Corporations <th> Map Size
<tbody>
<tr> <td> 1830 <td> <data value="8">Eight</data> <td> <data value="93">19+74 hexes (93 total)</data>
<tr> <td> 1856 <td> <data value="11">Eleven</data> <td> <data value="99">12+87 hexes (99 total)</data>
<tr> <td> 1870 <td> <data value="10">Ten</data> <td> <data value="149">4+145 hexes (149 total)</data>
</table>time elementSupport in all current engines.
Support in all current engines.
datetime attribute: Phrasing content.datetime — Machine-readable value[Exposed=Window]
interface HTMLTimeElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString dateTime;
};The time element represents its contents, along with a
machine-readable form of those contents in the datetime
attribute. The kind of content is limited to various kinds of dates, times, time-zone offsets, and
durations, as described below.
The datetime
attribute may be present. If present, its value must be a representation of the element's contents
in a machine-readable format.
A time element that does not have a datetime content attribute must not have any element
descendants.
The datetime value of a time element is the value of the element's
datetime content attribute, if it has one, otherwise the
child text content of the time element.
The datetime value of a time element must match one of the following
syntaxes.
<time>2011-11</time><time>2011-11-18</time><time>11-18</time><time>14:54</time><time>14:54:39</time><time>14:54:39.929</time><time>2011-11-18T14:54</time><time>2011-11-18T14:54:39</time><time>2011-11-18T14:54:39.929</time><time>2011-11-18 14:54</time><time>2011-11-18 14:54:39</time><time>2011-11-18 14:54:39.929</time>Times with dates but without a time zone offset are useful for specifying events that are observed at the same specific time in each time zone, throughout a day. For example, the 2020 new year is celebrated at 2020-01-01 00:00 in each time zone, not at the same precise moment across all time zones. For events that occur at the same time across all time zones, for example a videoconference meeting, a valid global date and time string is likely more useful.
<time>Z</time><time>+0000</time><time>+00:00</time><time>-0800</time><time>-08:00</time>For times without dates (or times referring to events that recur on multiple dates), specifying the geographic location that controls the time is usually more useful than specifying a time zone offset, because geographic locations change time zone offsets with daylight saving time. In some cases, geographic locations even change time zone, e.g. when the boundaries of those time zones are redrawn, as happened with Samoa at the end of 2011. There exists a time zone database that describes the boundaries of time zones and what rules apply within each such zone, known as the time zone database. [TZDATABASE]
<time>2011-11-18T14:54Z</time><time>2011-11-18T14:54:39Z</time><time>2011-11-18T14:54:39.929Z</time><time>2011-11-18T14:54+0000</time><time>2011-11-18T14:54:39+0000</time><time>2011-11-18T14:54:39.929+0000</time><time>2011-11-18T14:54+00:00</time><time>2011-11-18T14:54:39+00:00</time><time>2011-11-18T14:54:39.929+00:00</time><time>2011-11-18T06:54-0800</time><time>2011-11-18T06:54:39-0800</time><time>2011-11-18T06:54:39.929-0800</time><time>2011-11-18T06:54-08:00</time><time>2011-11-18T06:54:39-08:00</time><time>2011-11-18T06:54:39.929-08:00</time><time>2011-11-18 14:54Z</time><time>2011-11-18 14:54:39Z</time><time>2011-11-18 14:54:39.929Z</time><time>2011-11-18 14:54+0000</time><time>2011-11-18 14:54:39+0000</time><time>2011-11-18 14:54:39.929+0000</time><time>2011-11-18 14:54+00:00</time><time>2011-11-18 14:54:39+00:00</time><time>2011-11-18 14:54:39.929+00:00</time><time>2011-11-18 06:54-0800</time><time>2011-11-18 06:54:39-0800</time><time>2011-11-18 06:54:39.929-0800</time><time>2011-11-18 06:54-08:00</time><time>2011-11-18 06:54:39-08:00</time><time>2011-11-18 06:54:39.929-08:00</time>Times with dates and a time zone offset are useful for specifying specific events, or recurring virtual events where the time is not anchored to a specific geographic location. For example, the precise time of an asteroid impact, or a particular meeting in a series of meetings held at 1400 UTC every day, regardless of whether any particular part of the world is observing daylight saving time or not. For events where the precise time varies by the local time zone offset of a specific geographic location, a valid local date and time string combined with that geographic location is likely more useful.
<time>2011-W47</time><time>2011</time><time>0001</time><time>PT4H18M3S</time><time>4h 18m 3s</time>The machine-readable equivalent of the element's contents must be obtained from the element's datetime value by using the following algorithm:
If parsing a month string from the element's datetime value returns a month, that is the machine-readable equivalent; return.
If parsing a date string from the element's datetime value returns a date, that is the machine-readable equivalent; return.
If parsing a yearless date string from the element's datetime value returns a yearless date, that is the machine-readable equivalent; return.
If parsing a time string from the element's datetime value returns a time, that is the machine-readable equivalent; return.
If parsing a local date and time string from the element's datetime value returns a local date and time, that is the machine-readable equivalent; return.
If parsing a time-zone offset string from the element's datetime value returns a time-zone offset, that is the machine-readable equivalent; return.
If parsing a global date and time string from the element's datetime value returns a global date and time, that is the machine-readable equivalent; return.
If parsing a week string from the element's datetime value returns a week, that is the machine-readable equivalent; return.
If the element's datetime value consists of only ASCII digits, at least one of which is not U+0030 DIGIT ZERO (0), then the machine-readable equivalent is the base-ten interpretation of those digits, representing a year; return.
If parsing a duration string from the element's datetime value returns a duration, that is the machine-readable equivalent; return.
There is no machine-readable equivalent.
The algorithms referenced above are intended to be designed such that for any arbitrary string s, only one of the algorithms returns a value. A more efficient approach might be to create a single algorithm that parses all these data types in one pass; developing such an algorithm is left as an exercise to the reader.
Support in all current engines.
The dateTime IDL attribute must
reflect the element's datetime content
attribute.
The time element can be used to encode dates, for example in microformats. The
following shows a hypothetical way of encoding an event using a variant on hCalendar that uses
the time element:
<div class="vevent">
<a class="url" href="http://www.web2con.com/">http://www.web2con.com/</a>
<span class="summary">Web 2.0 Conference</span>:
<time class="dtstart" datetime="2005-10-05">October 5</time> -
<time class="dtend" datetime="2005-10-07">7</time>,
at the <span class="location">Argent Hotel, San Francisco, CA</span>
</div>Here, a fictional microdata vocabulary based on the Atom vocabulary is used with the
time element to mark up a blog post's publication date.
<article itemscope itemtype="https://n.example.org/rfc4287">
<h1 itemprop="title">Big tasks</h1>
<footer>Published <time itemprop="published" datetime="2009-08-29">two days ago</time>.</footer>
<p itemprop="content">Today, I went out and bought a bike for my kid.</p>
</article>In this example, another article's publication date is marked up using time, this
time using the schema.org microdata vocabulary:
<article itemscope itemtype="http://schema.org/BlogPosting">
<h1 itemprop="headline">Small tasks</h1>
<footer>Published <time itemprop="datePublished" datetime="2009-08-30">yesterday</time>.</footer>
<p itemprop="articleBody">I put a bike bell on her bike.</p>
</article>In the following snippet, the time element is used to encode a date in the
ISO8601 format, for later processing by a script:
<p>Our first date was <time datetime="2006-09-23">a Saturday</time>.</p>In this second snippet, the value includes a time:
<p>We stopped talking at <time datetime="2006-09-24T05:00-07:00">5am the next morning</time>.</p>A script loaded by the page (and thus privy to the page's internal convention of marking up
dates and times using the time element) could scan through the page and look at all
the time elements therein to create an index of dates and times.
For example, this element conveys the string "Friday" with the additional semantic that the 18th of November 2011 is the meaning that corresponds to "Friday":
Today is <time datetime="2011-11-18">Friday</time>.In this example, a specific time in the Pacific Standard Time timezone is specified:
Your next meeting is at <time datetime="2011-11-18T15:00-08:00">3pm</time>.code elementSupport in all current engines.
HTMLElement.The code element represents a fragment of computer code. This could
be an XML element name, a file name, a computer program, or any other string that a computer would
recognize.
There is no formal way to indicate the language of computer code being marked up. Authors who
wish to mark code elements with the language used, e.g. so that syntax highlighting
scripts can use the right rules, can use the class attribute, e.g.
by adding a class prefixed with "language-" to the element.
The following example shows how the element can be used in a paragraph to mark up element names and computer code, including punctuation.
<p>The <code>code</code> element represents a fragment of computer
code.</p>
<p>When you call the <code>activate()</code> method on the
<code>robotSnowman</code> object, the eyes glow.</p>
<p>The example below uses the <code>begin</code> keyword to indicate
the start of a statement block. It is paired with an <code>end</code>
keyword, which is followed by the <code>.</code> punctuation character
(full stop) to indicate the end of the program.</p>The following example shows how a block of code could be marked up using the pre
and code elements.
<pre><code class="language-pascal">var i: Integer;
begin
i := 1;
end.</code></pre>A class is used in that example to indicate the language used.
See the pre element for more details.
var elementSupport in all current engines.
HTMLElement.The var element represents a variable. This could be an actual
variable in a mathematical expression or programming context, an identifier representing a
constant, a symbol identifying a physical quantity, a function parameter, or just be a term used
as a placeholder in prose.
In the paragraph below, the letter "n" is being used as a variable in prose:
<p>If there are <var>n</var> pipes leading to the ice
cream factory then I expect at <em>least</em> <var>n</var>
flavors of ice cream to be available for purchase!</p>For mathematics, in particular for anything beyond the simplest of expressions, MathML is more
appropriate. However, the var element can still be used to refer to specific
variables that are then mentioned in MathML expressions.
In this example, an equation is shown, with a legend that references the variables in the
equation. The expression itself is marked up with MathML, but the variables are mentioned in the
figure's legend using var.
<figure>
<math>
<mi>a</mi>
<mo>=</mo>
<msqrt>
<msup><mi>b</mi><mn>2</mn></msup>
<mi>+</mi>
<msup><mi>c</mi><mn>2</mn></msup>
</msqrt>
</math>
<figcaption>
Using Pythagoras' theorem to solve for the hypotenuse <var>a</var> of
a triangle with sides <var>b</var> and <var>c</var>
</figcaption>
</figure>Here, the equation describing mass-energy equivalence is used in a sentence, and the
var element is used to mark the variables and constants in that equation:
<p>Then she turned to the blackboard and picked up the chalk. After a few moment's
thought, she wrote <var>E</var> = <var>m</var> <var>c</var><sup>2</sup>. The teacher
looked pleased.</p>samp elementSupport in all current engines.
HTMLElement.The samp element represents sample or quoted output from another
program or computing system.
See the pre and kbd elements for more details.
This element can be contrasted with the output element, which can be
used to provide immediate output in a web application.
This example shows the samp element being used
inline:
<p>The computer said <samp>Too much cheese in tray
two</samp> but I didn't know what that meant.</p>This second example shows a block of sample output from a console program. Nested
samp and kbd elements allow for the styling of specific elements
of the sample output using a style sheet. There's also a few parts of the samp that
are annotated with even more detailed markup, to enable very precise styling. To achieve this,
span elements are used.
<pre><samp><span class="prompt">jdoe@mowmow:~$</span> <kbd>ssh demo.example.com</kbd>
Last login: Tue Apr 12 09:10:17 2005 from mowmow.example.com on pts/1
Linux demo 2.6.10-grsec+gg3+e+fhs6b+nfs+gr0501+++p3+c4a+gr2b-reslog-v6.189 #1 SMP Tue Feb 1 11:22:36 PST 2005 i686 unknown
<span class="prompt">jdoe@demo:~$</span> <span class="cursor">_</span></samp></pre>This third example shows a block of input and its respective output. The example uses
both code and samp elements.
<pre>
<code class="language-javascript">console.log(2.3 + 2.4)</code>
<samp>4.699999999999999</samp>
</pre>kbd elementSupport in all current engines.
HTMLElement.The kbd element represents user input (typically keyboard input,
although it may also be used to represent other input, such as voice commands).
When the kbd element is nested inside a samp element, it represents
the input as it was echoed by the system.
When the kbd element contains a samp element, it represents
input based on system output, for example invoking a menu item.
When the kbd element is nested inside another kbd element, it
represents an actual key or other single unit of input as appropriate for the input mechanism.
Here the kbd element is used to indicate keys to press:
<p>To make George eat an apple, press <kbd><kbd>Shift</kbd>+<kbd>F3</kbd></kbd></p>In this second example, the user is told to pick a particular menu item. The outer
kbd element marks up a block of input, with the inner kbd elements
representing each individual step of the input, and the samp elements inside them
indicating that the steps are input based on something being displayed by the system, in this
case menu labels:
<p>To make George eat an apple, select
<kbd><kbd><samp>File</samp></kbd>|<kbd><samp>Eat Apple...</samp></kbd></kbd>
</p>Such precision isn't necessary; the following is equally fine:
<p>To make George eat an apple, select <kbd>File | Eat Apple...</kbd></p>sub and sup elementsSupport in all current engines.
Support in all current engines.
sub element: for authors; for implementers.sup element: for authors; for implementers.HTMLElement.The sup element represents a superscript and the sub
element represents a subscript.
These elements must be used only to mark up typographical conventions with specific meanings,
not for typographical presentation for presentation's sake. For example, it would be inappropriate
for the sub and sup elements to be used in the name of the LaTeX
document preparation system. In general, authors should use these elements only if the
absence of those elements would change the meaning of the content.
In certain languages, superscripts are part of the typographical conventions for some abbreviations.
<p>Their names are
<span lang="fr"><abbr>M<sup>lle</sup></abbr> Gwendoline</span> and
<span lang="fr"><abbr>M<sup>me</sup></abbr> Denise</span>.</p>The sub element can be used inside a var element, for variables that
have subscripts.
Here, the sub element is used to represent the subscript that identifies the
variable in a family of variables:
<p>The coordinate of the <var>i</var>th point is
(<var>x<sub><var>i</var></sub></var>, <var>y<sub><var>i</var></sub></var>).
For example, the 10th point has coordinate
(<var>x<sub>10</sub></var>, <var>y<sub>10</sub></var>).</p>Mathematical expressions often use subscripts and superscripts. Authors are encouraged to use
MathML for marking up mathematics, but authors may opt to use sub and
sup if detailed mathematical markup is not desired. [MATHML]
<var>E</var>=<var>m</var><var>c</var><sup>2</sup>f(<var>x</var>, <var>n</var>) = log<sub>4</sub><var>x</var><sup><var>n</var></sup>i elementSupport in all current engines.
HTMLElement.The i element represents a span of text in an alternate voice or
mood, or otherwise offset from the normal prose in a manner indicating a different quality of
text, such as a taxonomic designation, a technical term, an idiomatic phrase from another
language, transliteration, a thought, or a ship name in Western texts.
Terms in languages different from the main text should be annotated with lang attributes (or, in XML, lang attributes in the XML namespace).
The examples below show uses of the i element:
<p>The <i class="taxonomy">Felis silvestris catus</i> is cute.</p>
<p>The term <i>prose content</i> is defined above.</p>
<p>There is a certain <i lang="fr">je ne sais quoi</i> in the air.</p>In the following example, a dream sequence is marked up using
i elements.
<p>Raymond tried to sleep.</p>
<p><i>The ship sailed away on Thursday</i>, he
dreamt. <i>The ship had many people aboard, including a beautiful
princess called Carey. He watched her, day-in, day-out, hoping she
would notice him, but she never did.</i></p>
<p><i>Finally one night he picked up the courage to speak with
her—</i></p>
<p>Raymond woke with a start as the fire alarm rang out.</p>Authors can use the class attribute on the i
element to identify why the element is being used, so that if the style of a particular use (e.g.
dream sequences as opposed to taxonomic terms) is to be changed at a later date, the author
doesn't have to go through the entire document (or series of related documents) annotating each
use.
Authors are encouraged to consider whether other elements might be more applicable than the
i element, for instance the em element for marking up stress emphasis,
or the dfn element to mark up the defining instance of a term.
Style sheets can be used to format i elements, just like any other
element can be restyled. Thus, it is not the case that content in i elements will
necessarily be italicized.
b elementSupport in all current engines.
HTMLElement.The b element represents a span of text to which attention is being
drawn for utilitarian purposes without conveying any extra importance and with no implication of
an alternate voice or mood, such as key words in a document abstract, product names in a review,
actionable words in interactive text-driven software, or an article lede.
The following example shows a use of the b element to highlight key words without
marking them up as important:
<p>The <b>frobonitor</b> and <b>barbinator</b> components are fried.</p>In the following example, objects in a text adventure are highlighted as being special by use
of the b element.
<p>You enter a small room. Your <b>sword</b> glows
brighter. A <b>rat</b> scurries past the corner wall.</p>Another case where the b element is appropriate is in marking up the lede (or
lead) sentence or paragraph. The following example shows how a BBC article about
kittens adopting a rabbit as their own could be marked up:
<article>
<h2>Kittens 'adopted' by pet rabbit</h2>
<p><b class="lede">Six abandoned kittens have found an
unexpected new mother figure — a pet rabbit.</b></p>
<p>Veterinary nurse Melanie Humble took the three-week-old
kittens to her Aberdeen home.</p>
[...]As with the i element, authors can use the class
attribute on the b element to identify why the element is being used, so that if the
style of a particular use is to be changed at a later date, the author doesn't have to go through
annotating each use.
The b element should be used as a last resort when no other element is more
appropriate. In particular, headings should use the h1 to h6 elements,
stress emphasis should use the em element, importance should be denoted with the
strong element, and text marked or highlighted should use the mark
element.
The following would be incorrect usage:
<p><b>WARNING!</b> Do not frob the barbinator!</p>In the previous example, the correct element to use would have been strong, not
b.
Style sheets can be used to format b elements, just like any other
element can be restyled. Thus, it is not the case that content in b elements will
necessarily be boldened.
u elementSupport in all current engines.
HTMLElement.The u element represents a span of text with an unarticulated, though
explicitly rendered, non-textual annotation, such as labeling the text as being a proper name in
Chinese text (a Chinese proper name mark), or labeling the text as being misspelt.
In most cases, another element is likely to be more appropriate: for marking stress emphasis,
the em element should be used; for marking key words or phrases either the
b element or the mark element should be used, depending on the context;
for marking book titles, the cite element should be used; for labeling text with explicit textual annotations, the
ruby element should be used; for technical terms, taxonomic designation,
transliteration, a thought, or for labeling ship names in Western texts, the i
element should be used.
The default rendering of the u element in visual presentations
clashes with the conventional rendering of hyperlinks (underlining). Authors are encouraged to
avoid using the u element where it could be confused for a hyperlink.
In this example, a u element is used to mark a word as misspelt:
<p>The <u>see</u> is full of fish.</p>mark elementSupport in all current engines.
HTMLElement.The mark element represents a run of text in one document marked or
highlighted for reference purposes, due to its relevance in
another context. When used in a quotation or other block of text referred to from the prose, it
indicates a highlight that was not originally present but which has been added to bring the
reader's attention to a part of the text that might not have been considered important by the
original author when the block was originally written, but which is now under previously
unexpected scrutiny. When used in the main prose of a document, it indicates a part of the
document that has been highlighted due to its likely relevance to the user's current activity.
This example shows how the mark element can be used to bring attention to a
particular part of a quotation:
<p lang="en-US">Consider the following quote:</p>
<blockquote lang="en-GB">
<p>Look around and you will find, no-one's really
<mark>colour</mark> blind.</p>
</blockquote>
<p lang="en-US">As we can tell from the <em>spelling</em> of the word,
the person writing this quote is clearly not American.</p>(If the goal was to mark the element as misspelt, however, the u element,
possibly with a class, would be more appropriate.)
Another example of the mark element is highlighting parts of a document that are
matching some search string. If someone looked at a document, and the server knew that the user
was searching for the word "kitten", then the server might return the document with one paragraph
modified as follows:
<p>I also have some <mark>kitten</mark>s who are visiting me
these days. They're really cute. I think they like my garden! Maybe I
should adopt a <mark>kitten</mark>.</p>In the following snippet, a paragraph of text refers to a specific part of a code fragment.
<p>The highlighted part below is where the error lies:</p>
<pre><code>var i: Integer;
begin
i := <mark>1.1</mark>;
end.</code></pre>This is separate from syntax highlighting, for which span is more
appropriate. Combining both, one would get:
<p>The highlighted part below is where the error lies:</p>
<pre><code><span class=keyword>var</span> <span class=ident>i</span>: <span class=type>Integer</span>;
<span class=keyword>begin</span>
<span class=ident>i</span> := <span class=literal><mark>1.1</mark></span>;
<span class=keyword>end</span>.</code></pre>This is another example showing the use of mark to highlight a part of quoted
text that was originally not emphasized. In this example, common typographic conventions have led
the author to explicitly style mark elements in quotes to render in italics.
<style>
blockquote mark, q mark {
font: inherit; font-style: italic;
text-decoration: none;
background: transparent; color: inherit;
}
.bubble em {
font: inherit; font-size: larger;
text-decoration: underline;
}
</style>
<article>
<h1>She knew</h1>
<p>Did you notice the subtle joke in the joke on panel 4?</p>
<blockquote>
<p class="bubble">I didn't <em>want</em> to believe. <mark>Of course
on some level I realized it was a known-plaintext attack.</mark> But I
couldn't admit it until I saw for myself.</p>
</blockquote>
<p>(Emphasis mine.) I thought that was great. It's so pedantic, yet it
explains everything neatly.</p>
</article>Note, incidentally, the distinction between the em element in this example, which
is part of the original text being quoted, and the mark element, which is
highlighting a part for comment.
The following example shows the difference between denoting the importance of a span
of text (strong) as opposed to denoting the relevance of a span of text
(mark). It is an extract from a textbook, where the extract has had the parts
relevant to the exam highlighted. The safety warnings, important though they may be, are
apparently not relevant to the exam.
<h3>Wormhole Physics Introduction</h3>
<p><mark>A wormhole in normal conditions can be held open for a
maximum of just under 39 minutes.</mark> Conditions that can increase
the time include a powerful energy source coupled to one or both of
the gates connecting the wormhole, and a large gravity well (such as a
black hole).</p>
<p><mark>Momentum is preserved across the wormhole. Electromagnetic
radiation can travel in both directions through a wormhole,
but matter cannot.</mark></p>
<p>When a wormhole is created, a vortex normally forms.
<strong>Warning: The vortex caused by the wormhole opening will
annihilate anything in its path.</strong> Vortexes can be avoided when
using sufficiently advanced dialing technology.</p>
<p><mark>An obstruction in a gate will prevent it from accepting a
wormhole connection.</mark></p>bdi elementSupport in all current engines.
dir global attribute has special semantics on this element.HTMLElement.The bdi element represents a span of text that is to be isolated from
its surroundings for the purposes of bidirectional text formatting. [BIDI]
The dir global attribute defaults to auto on this element (it never inherits from the parent element like
with other elements).
This element has rendering requirements involving the bidirectional algorithm.
This element is especially useful when embedding user-generated content with an unknown directionality.
In this example, usernames are shown along with the number of posts that the user has
submitted. If the bdi element were not used, the username of the Arabic user would
end up confusing the text (the bidirectional algorithm would put the colon and the number "3"
next to the word "User" rather than next to the word "posts").
<ul>
<li>User <bdi>jcranmer</bdi>: 12 posts.
<li>User <bdi>hober</bdi>: 5 posts.
<li>User <bdi>إيان</bdi>: 3 posts.
</ul>
bdi element, the username acts as expected.
bdi element were to be replaced by a b element, the username would confuse the bidirectional algorithm and the third bullet would end up saying "User 3 :", followed by the Arabic name (right-to-left), followed by "posts" and a period.bdo elementSupport in all current engines.
dir global attribute has special semantics on this element.HTMLElement.The bdo element represents explicit text directionality formatting
control for its children. It allows authors to override the Unicode bidirectional algorithm by
explicitly specifying a direction override. [BIDI]
Authors must specify the dir attribute on this element, with the
value ltr to specify a left-to-right override and with the value rtl to
specify a right-to-left override. The auto value must not be specified.
This element has rendering requirements involving the bidirectional algorithm.
span elementSupport in all current engines.
Support in all current engines.
[Exposed=Window]
interface HTMLSpanElement : HTMLElement {
[HTMLConstructor] constructor();
};The span element doesn't mean anything on its own, but can be useful when used
together with the global attributes, e.g. class,
lang, or dir. It
represents its children.
In this example, a code fragment is marked up using span elements and class attributes so that its keywords and identifiers can be
color-coded from CSS:
<pre><code class="lang-c"><span class="keyword">for</span> (<span class="ident">j</span> = 0; <span class="ident">j</span> < 256; <span class="ident">j</span>++) {
<span class="ident">i_t3</span> = (<span class="ident">i_t3</span> & 0x1ffff) | (<span class="ident">j</span> << 17);
<span class="ident">i_t6</span> = (((((((<span class="ident">i_t3</span> >> 3) ^ <span class="ident">i_t3</span>) >> 1) ^ <span class="ident">i_t3</span>) >> 8) ^ <span class="ident">i_t3</span>) >> 5) & 0xff;
<span class="keyword">if</span> (<span class="ident">i_t6</span> == <span class="ident">i_t1</span>)
<span class="keyword">break</span>;
}</code></pre>br elementSupport in all current engines.
Support in all current engines.
[Exposed=Window]
interface HTMLBRElement : HTMLElement {
[HTMLConstructor] constructor();
// also has obsolete members
};The br element represents a line break.
While line breaks are usually represented in visual media by physically moving subsequent text to a new line, a style sheet or user agent would be equally justified in causing line breaks to be rendered in a different manner, for instance as green dots, or as extra spacing.
br elements must be used only for line breaks that are actually part of the
content, as in poems or addresses.
The following example is correct usage of the br element:
<p>P. Sherman<br>
42 Wallaby Way<br>
Sydney</p>br elements must not be used for separating thematic groups in a paragraph.
The following examples are non-conforming, as they abuse the br element:
<p><a ...>34 comments.</a><br>
<a ...>Add a comment.</a></p><p><label>Name: <input name="name"></label><br>
<label>Address: <input name="address"></label></p>Here are alternatives to the above, which are correct:
<p><a ...>34 comments.</a></p>
<p><a ...>Add a comment.</a></p><p><label>Name: <input name="name"></label></p>
<p><label>Address: <input name="address"></label></p>If a paragraph consists of nothing but a single br element, it
represents a placeholder blank line (e.g. as in a template). Such blank lines must not be used for
presentation purposes.
Any content inside br elements must not be considered part of the surrounding
text.
This element has rendering requirements involving the bidirectional algorithm.
wbr elementSupport in all current engines.
HTMLElement.The wbr element represents a line break opportunity.
In the following example, someone is quoted as saying something which, for effect, is written
as one long word. However, to ensure that the text can be wrapped in a readable fashion, the
individual words in the quote are separated using a wbr element.
<p>So then she pointed at the tiger and screamed
"there<wbr>is<wbr>no<wbr>way<wbr>you<wbr>are<wbr>ever<wbr>going<wbr>to<wbr>catch<wbr>me"!</p>Any content inside wbr elements must not be considered part of the surrounding
text.
var wbr = document.createElement("wbr");
wbr.textContent = "This is wrong";
document.body.appendChild(wbr);This element has rendering requirements involving the bidirectional algorithm.
This section is non-normative.
| Element | Purpose | Example |
|---|---|---|
a
| Hyperlinks | |
em
| Stress emphasis | |
strong
| Importance | |
small
| Side comments | |
s
| Inaccurate text | |
cite
| Titles of works | |
q
| Quotations | |
dfn
| Defining instance | |
abbr
| Abbreviations | |
ruby, rt, rp
| Ruby annotations | |
data
| Machine-readable equivalent | |
time
| Machine-readable equivalent of date- or time-related data | |
code
| Computer code | |
var
| Variables | |
samp
| Computer output | |
kbd
| User input | |
sub
| Subscripts | |
sup
| Superscripts | |
i
| Alternative voice | |
b
| Keywords | |
u
| Annotations | |
mark
| Highlight | |
bdi
| Text directionality isolation | |
bdo
| Text directionality formatting | |
span
| Other | |
br
| Line break | |
wbr
| Line breaking opportunity | |
链接是一种概念构造，由 a, area 和 link 元素创建，
表示 两个资源之间的联系，其中一个是当前 Document。
在 HTML 中有两种链接：
这些链接指向的资源用于增强当前文档，通常由用户代理自动处理。
这些指向其他文档的链接通常通过用户代理暴露给用户，这样用户可以让用户代理 导航 到那些资源，例如在浏览器中访问或者下载它们。
对于有 href 属性和
rel 属性的 link 元素，
必须为 rel 属性的关键字按照
链接类型 部分的定义创建一个链接。
类似地，对于有 href 属性和
rel 属性的 a 和 area 元素，
必须为 rel 属性的关键字按照
链接类型 部分的定义创建一个链接。
与 link 元素不同，对于有 href 属性的
a 和 area 元素，如果没有
rel 属性或
rel 属性的关键字的定义中没有一个是用来指定
超链接 的，也必须创建一个 超链接。
这个暗示的超链接没有特殊含义（它也没有 链接类型），只是把该元素的
节点文档 链接到了该元素的 href 属性给出的资源。
超链接 可以有一个或更多的 超链接注释，用来修改这个超链接的处理语义。
a and area elementsThe href
attribute on a and area elements must have a value that is a valid
URL potentially surrounded by spaces.
The href attribute on a and
area elements is not required; when those elements do not have href attributes they do not create hyperlinks.
The target
attribute, if present, must be a valid browsing context name or keyword. It gives the
name of the browsing context that will be used. User agents use this
name when following hyperlinks.
When an a or area element's activation behavior is
invoked, the user agent may allow the user to indicate a preference regarding whether the
hyperlink is to be used for navigation or whether the resource it
specifies is to be downloaded.
In the absence of a user preference, the default should be navigation if the element has no
download attribute, and should be to download the
specified resource if it does.
Whether determined by the user's preferences or via the presence or absence of the attribute, if the decision is to use the hyperlink for navigation then the user agent must follow the hyperlink, and if the decision is to use the hyperlink to download a resource, the user agent must download the hyperlink. These terms are defined in subsequent sections below.
The download
attribute, if present, indicates that the author intends the hyperlink to be used for downloading a resource. The attribute may have a value; the
value, if any, specifies the default file name that the author recommends for use in labeling the
resource in a local file system. There are no restrictions on allowed values, but authors are
cautioned that most file systems have limitations with regard to what punctuation is supported in
file names, and user agents are likely to adjust file names accordingly.
Support in all current engines.
The ping attribute, if present,
gives the URLs of the resources that are interested in being notified if the user follows the
hyperlink. The value must be a set of space-separated tokens, each of which must be a
valid non-empty URL whose scheme is an
HTTP(S) scheme. The value is used by the user agent for hyperlink
auditing.
The rel attribute on a and area
elements controls what kinds of links the elements create. The attribute's value must be a
unordered set of unique space-separated tokens. The allowed
keywords and their meanings are defined below.
rel's supported tokens are the keywords defined in HTML link types which are allowed on a and area
elements, impact the processing model, and are supported by the user agent. The possible supported tokens are noreferrer, noopener, and opener. rel's supported tokens must only include the tokens from this
list that the user agent implements the processing model for.
The rel attribute has no default value. If the
attribute is omitted or if none of the values in the attribute are recognized by the user agent,
then the document has no particular relationship with the destination resource other than there
being a hyperlink between the two.
The hreflang
attribute on a elements that create hyperlinks, if
present, gives the language of the linked resource. It is purely advisory. The value must be a
valid BCP 47 language tag. [BCP47] User agents must not consider this
attribute authoritative — upon fetching the resource, user agents must use only language
information associated with the resource to determine its language, not metadata included in the
link to the resource.
The type
attribute, if present, gives the MIME type of the linked resource. It is purely
advisory. The value must be a valid MIME type string. User agents must
not consider the type attribute authoritative —
upon fetching the resource, user agents must not use metadata included in the link to the resource
to determine its type.
The referrerpolicy attribute is a referrer
policy attribute. Its purpose is to set the referrer policy used when
following hyperlinks. [REFERRERPOLICY]
a and area elementsinterface mixin HTMLHyperlinkElementUtils {
[CEReactions] stringifier attribute USVString href;
readonly attribute USVString origin;
[CEReactions] attribute USVString protocol;
[CEReactions] attribute USVString username;
[CEReactions] attribute USVString password;
[CEReactions] attribute USVString host;
[CEReactions] attribute USVString hostname;
[CEReactions] attribute USVString port;
[CEReactions] attribute USVString pathname;
[CEReactions] attribute USVString search;
[CEReactions] attribute USVString hash;
};toString()hrefSupport in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Returns the hyperlink's URL.
Can be set, to change the URL.
originSupport in all current engines.
Support in all current engines.
Returns the hyperlink's URL's origin.
protocolSupport in all current engines.
Support in all current engines.
Returns the hyperlink's URL's scheme.
Can be set, to change the URL's scheme.
usernameSupport in all current engines.
Support in all current engines.
Returns the hyperlink's URL's username.
Can be set, to change the URL's username.
passwordSupport in all current engines.
Support in all current engines.
Returns the hyperlink's URL's password.
Can be set, to change the URL's password.
hostSupport in all current engines.
Support in all current engines.
Returns the hyperlink's URL's host and port (if different from the default port for the scheme).
Can be set, to change the URL's host and port.
hostnameSupport in all current engines.
Support in all current engines.
Returns the hyperlink's URL's host.
Can be set, to change the URL's host.
portSupport in all current engines.
Support in all current engines.
Returns the hyperlink's URL's port.
Can be set, to change the URL's port.
pathnameSupport in all current engines.
Support in all current engines.
Returns the hyperlink's URL's path.
Can be set, to change the URL's path.
searchSupport in all current engines.
Support in all current engines.
Returns the hyperlink's URL's query (includes leading "?" if
non-empty).
Can be set, to change the URL's query (ignores leading "?").
hashSupport in all current engines.
Support in all current engines.
Returns the hyperlink's URL's fragment (includes leading "#" if
non-empty).
Can be set, to change the URL's fragment (ignores leading "#").
An element implementing the HTMLHyperlinkElementUtils mixin has an associated url (null or a URL). It is initially null.
An element implementing the HTMLHyperlinkElementUtils mixin has an associated set the url algorithm, which runs these steps:
If this element's href content attribute is
absent, set this element's url to null.
Otherwise, parse this element's href content
attribute value relative to this element's node document. If parsing is successful, set this element's url to the result; otherwise, set this element's
url to null.
When elements implementing the HTMLHyperlinkElementUtils mixin are created, and
whenever those elements have their href content
attribute set, changed, or removed, the user agent must set the url.
This is only observable for blob: URLs as
parsing them involves a Blob URL Store lookup.
An element implementing the HTMLHyperlinkElementUtils mixin has an associated
reinitialize url algorithm, which runs these
steps:
If element's url is non-null, its scheme is "blob", and its
cannot-be-a-base-URL flag is set, terminate these steps.
To update href, set the element's href content attribute's value to the element's url, serialized.
The href attribute's getter must run these steps:
Let url be this element's url.
If url is null and this element has no href content attribute, return the empty string.
Otherwise, if url is null, return this element's href content attribute's value.
Return url, serialized.
The href attribute's setter must set this element's
href content attribute's value to the given value.
The origin attribute's getter must run these steps:
If this element's url is null, return the empty string.
Return the serialization of this element's url's origin.
The protocol attribute's getter must run these steps:
If this element's url is null, return ":".
The protocol attribute's setter must run these
steps:
If this element's url is null, terminate these steps.
Basic URL parse the given value, followed by
":", with this element's url
as url and scheme start state as state override.
Because the URL parser ignores multiple consecutive colons, providing a value
of "https:" (or even "https::::") is the same as
providing a value of "https".
The username attribute's getter must run these steps:
If this element's url is null, return the empty string.
The username attribute's setter must run these
steps:
Let url be this element's url.
If url is null or url cannot have a username/password/port, then return.
Set the username, given url and the given value.
The password attribute's getter must run these steps:
The password attribute's setter must run these
steps:
Let url be this element's url.
If url is null or url cannot have a username/password/port, then return.
Set the password, given url and the given value.
The host attribute's getter must run these steps:
Let url be this element's url.
If url or url's host is null, return the empty string.
If url's port is null, return url's host, serialized.
Return url's host, serialized, followed by ":" and url's port, serialized.
The host attribute's setter must run these steps:
Let url be this element's url.
If url is null or url's cannot-be-a-base-URL flag is set, terminate these steps.
Basic URL parse the given value, with url as url and host state as state override.
The hostname attribute's getter must run these steps:
Let url be this element's url.
If url or url's host is null, return the empty string.
Return url's host, serialized.
The hostname attribute's setter must run these
steps:
Let url be this element's url.
If url is null or url's cannot-be-a-base-URL flag is set, terminate these steps.
Basic URL parse the given value, with url as url and hostname state as state override.
The port attribute's getter must run these steps:
Let url be this element's url.
If url or url's port is null, return the empty string.
Return url's port, serialized.
The port attribute's setter must run these steps:
Let url be this element's url.
If url is null or url cannot have a username/password/port, then return.
If the given value is the empty string, then set url's port to null.
Otherwise, basic URL parse the given value, with url as url and port state as state override.
The pathname attribute's getter must run these steps:
Let url be this element's url.
If url is null, return the empty string.
If url's cannot-be-a-base-URL flag is set, return the first string in url's path.
If url's path is empty, then return the empty string.
Return "/", followed by the strings in url's path (including empty strings), separated from each other by
"/".
The pathname attribute's setter must run these
steps:
Let url be this element's url.
If url is null or url's cannot-be-a-base-URL flag is set, terminate these steps.
Set url's path to the empty list.
Basic URL parse the given value, with url as url and path start state as state override.
The search attribute's getter must run these steps:
Let url be this element's url.
If url is null, or url's query is either null or the empty string, return the empty string.
Return "?", followed by url's query.
The search attribute's setter must run these
steps:
Let url be this element's url.
If url is null, terminate these steps.
If the given value is the empty string, set url's query to null.
Otherwise:
Let input be the given value with a single leading "?"
removed, if any.
Set url's query to the empty string.
Basic URL parse input, with url as url and query state as state override, and this element's node document's document's character encoding as encoding override.
The hash attribute's getter must run these steps:
Let url be this element's url.
If url is null, or url's fragment is either null or the empty string, return the empty string.
Return "#", followed by url's fragment.
The hash attribute's setter must run these steps:
Let url be this element's url.
If url is null, then return.
If the given value is the empty string, set url's fragment to null.
Otherwise:
Let input be the given value with a single leading "#"
removed, if any.
Set url's fragment to the empty string.
Basic URL parse input, with url as url and fragment state as state override.
An element element cannot navigate if one of the following is true:
a element and is not connected.This is also used by form submission for
the form element. The exception for a elements is for compatibility with
web content.
To get an element's noopener, given an a, area, or
form element element and a string target, run these steps:
If element's link types include the noopener or noreferrer
keyword, then return true.
If element's link types
do not include the opener keyword and target is an
ASCII case-insensitive match for "_blank", then return
true.
Return false.
When a user follows a hyperlink created by an element subject, optionally with a hyperlink suffix, the user agent must run the following steps:
If subject cannot navigate, then return.
Let replace be false.
Let source be subject's node document's browsing context.
Let targetAttributeValue be the empty string.
If subject is an a or area element, then set
targetAttributeValue to the result of getting
an element's target given subject.
Let noopener be the result of getting an element's noopener with subject and targetAttributeValue.
Let target and windowType be the result of applying the rules for choosing a browsing context given targetAttributeValue, source, and noopener.
If target is null, then return.
Parse the URL given by subject's
href attribute, relative to subject's
node document.
If that is successful, let URL be the resulting URL string.
Otherwise, if parsing the URL failed, the user agent may report the error to the user in a user-agent-specific manner, may queue an element task on the DOM manipulation task source given subject to navigate the target browsing context to an error page to report the error, or may ignore the error and do nothing. In any case, the user agent must then return.
If there is a hyperlink suffix, append it to URL.
Let request be a new request whose url is URL and whose referrer policy is the current state of
subject's referrerpolicy content attribute.
If subject's link
types includes the noreferrer keyword, then set
request's referrer to "no-referrer".
Let historyHandling be "replace" if
windowType is not "existing or none"; otherwise, "default".
Queue an element task on the DOM manipulation task source given subject to navigate target to request with historyHandling set to historyHandling and the source browsing context set to source.
Support in all current engines.
In some cases, resources are intended for later use rather than immediate viewing. To indicate
that a resource is intended to be downloaded for use later, rather than immediately used, the
download attribute can be specified on the
a or area element that creates the hyperlink to that
resource.
The attribute can furthermore be given a value, to specify the file name that user agents are
to use when storing the resource in a file system. This value can be overridden by the `Content-Disposition` HTTP header's filename parameters.
[RFC6266]
In cross-origin situations, the download
attribute has to be combined with the `Content-Disposition` HTTP header, specifically with the
attachment disposition type, to avoid the user being warned of possibly
nefarious activity. (This is to protect users from being made to download sensitive personal or
confidential information without their full understanding.)
The following allowed to download algorithm takes an initiator browsing context and an instantiator browsing context, and returns a boolean indicating whether or not downloading is allowed:
If the initiator browsing context's sandboxing flags has the sandboxed downloads browsing context flag set, then return false.
If the instantiator browsing context is non-null, and its sandboxing flags has the sandboxed downloads browsing context flag set, then return false.
Optionally, the user agent may return false, if it believes doing so would safeguard the user from a potentially hostile download.
Return true.
When a user downloads a hyperlink created by an element subject, optionally with a hyperlink suffix, the user agent must run the following steps:
If subject cannot navigate, then return.
Run the allowed to download algorithm with the subject's node document's browsing context and null. If the algorithm returns false, then return.
Parse the URL given by
subject's href attribute, relative to
subject's node document.
If parsing the URL fails, the user agent may report the error to the user in a user-agent-specific manner, may navigate to an error page to report the error, or may ignore the error and do nothing. In either case, the user agent must return.
Otherwise, let URL be the resulting URL string.
If there is a hyperlink suffix, append it to URL.
Run these steps in parallel:
Let request be a new request whose
url is URL,
client is entry settings object,
initiator is "download",
destination is the empty string, and whose
synchronous flag and use-URL-credentials flag are set.
Handle the result of fetching request as a download.
When a user agent is to handle a resource obtained from a fetch as a download, it should provide the user with a way to save the resource for later use, if a resource is successfully obtained. Otherwise, it should report any problems downloading the file to the user.
If the user agent needs a file name for a resource being handled as a download, it should select one using the following algorithm.
This algorithm is intended to mitigate security dangers involved in downloading files from untrusted sites, and user agents are strongly urged to follow it.
Let filename be the undefined value.
If the resource has a `Content-Disposition`
header, that header specifies the attachment disposition type, and the
header includes file name information, then let filename have the value
specified by the header, and jump to the step labeled sanitize below. [RFC6266]
Let interface origin be the origin of the
Document in which the download or
navigate action resulting in the download was initiated, if any.
Let resource origin be the origin of the URL of the
resource being downloaded, unless that URL's scheme
component is data, in which case let resource origin be
the same as the interface origin, if any.
If there is no interface origin, then let trusted operation be true. Otherwise, let trusted operation be true if resource origin is the same origin as interface origin, and false otherwise.
If trusted operation is true and the resource has a `Content-Disposition` header and that header includes file
name information, then let filename have the value specified by the header,
and jump to the step labeled sanitize below. [RFC6266]
If the download was not initiated from a hyperlink created by an
a or area element, or if the element of the hyperlink from
which it was initiated did not have a download
attribute when the download was initiated, or if there was such an attribute but its value when
the download was initiated was the empty string, then jump to the step labeled no proposed
file name.
Let proposed filename have the value of the download attribute of the element of the
hyperlink that initiated the download at the time the download was
initiated.
If trusted operation is true, let filename have the value of proposed filename, and jump to the step labeled sanitize below.
If the resource has a `Content-Disposition`
header and that header specifies the attachment disposition type, let filename have the value of proposed filename, and jump to the
step labeled sanitize below. [RFC6266]
No proposed file name: If trusted operation is true, or if the user indicated a preference for having the resource in question downloaded, let filename have a value derived from the URL of the resource in an implementation-defined manner, and jump to the step labeled sanitize below.
Let filename be set to the user's preferred file name or to a file name selected by the user agent, and jump to the step labeled sanitize below.
If the algorithm reaches this step, then a download was begun from a different origin than
the resource being downloaded, and the origin did not mark the file as suitable for
downloading, and the download was not initiated by the user. This could be because a download attribute was used to trigger the download, or
because the resource in question is not of a type that the user agent supports.
This could be dangerous, because, for instance, a hostile server could be trying to get a user to unknowingly download private information and then re-upload it to the hostile server, by tricking the user into thinking the data is from the hostile server.
Thus, it is in the user's interests that the user be somehow notified that the resource in question comes from quite a different source, and to prevent confusion, any suggested file name from the potentially hostile interface origin should be ignored.
Sanitize: Optionally, allow the user to influence filename. For example, a user agent could prompt the user for a file name, potentially providing the value of filename as determined above as a default value.
Adjust filename to be suitable for the local file system.
For example, this could involve removing characters that are not legal in file names, or trimming leading and trailing whitespace.
If the platform conventions do not in any way use extensions to determine the types of file on the file system, then return filename as the file name.
Let claimed type be the type given by the resource's Content-Type metadata, if any is known. Let named type be the type given by filename's extension, if any is known. For the purposes of this step, a type is a mapping of a MIME type to an extension.
If named type is consistent with the user's preferences (e.g. because the value of filename was determined by prompting the user), then return filename as the file name.
If claimed type and named type are the same type (i.e. the type given by the resource's Content-Type metadata is consistent with the type given by filename's extension), then return filename as the file name.
If the claimed type is known, then alter filename to add an extension corresponding to claimed type.
Otherwise, if named type is known to be potentially dangerous (e.g. it
will be treated by the platform conventions as a native executable, shell script, HTML
application, or executable-macro-capable document) then optionally alter filename to add a known-safe extension
(e.g. ".txt").
This last step would make it impossible to download executables, which might not be desirable. As always, implementers are forced to balance security and usability in this matter.
Return filename as the file name.
For the purposes of this algorithm, a file extension
consists of any part of the file name that platform conventions dictate will be used for
identifying the type of the file. For example, many operating systems use the part of the file
name following the last dot (".") in the file name to determine the type of
the file, and from that the manner in which the file is to be opened or executed.
User agents should ignore any directory or path information provided by the resource itself,
its URL, and any download attribute, in
deciding where to store the resulting file in the user's file system.
If a hyperlink created by an a or area element has a
ping attribute, and the user follows the hyperlink, and
the value of the element's href attribute can be parsed, relative to the element's node document, without
failure, then the user agent must take the ping
attribute's value, split that string on ASCII
whitespace, parse each resulting token relative to the
element's node document, and then run these steps for each resulting URL
record ping URL, ignoring tokens that fail to parse:
If ping URL's scheme is not an HTTP(S) scheme, then return.
Optionally, return. (For example, the user agent might wish to ignore any or all ping URLs in accordance with the user's expressed preferences.)
Let request be a new request whose
url is ping URL, method is `POST`, body is `PING`, client is the environment settings object of
the Document containing the hyperlink, destination is the empty string,
credentials mode is "include", referrer is "no-referrer", and whose use-URL-credentials flag is set.
Let target URL be the resulting URL string obtained from parsing the value of the element's href attribute and then:
Document object
containing the hyperlink being audited and ping URL have the same
originDocument containing the
hyperlink being audited is not "https"Ping-From` header
with, as its value, the URL of the document
containing the hyperlink, and a `Ping-To` HTTP header with,
as its value, the target URL.Ping-To` HTTP header
with, as its value, target URL. request does not
include a `Ping-From` header.Fetch request.
This may be done in parallel with the primary fetch, and is independent of the result of that fetch.
User agents should allow the user to adjust this behavior, for example in conjunction with a
setting that disables the sending of HTTP `Referer` (sic)
headers. Based on the user's preferences, UAs may either ignore the ping attribute altogether, or selectively ignore URLs in the
list (e.g. ignoring any third-party URLs); this is explicitly accounted for in the steps
above.
User agents must ignore any entity bodies returned in the responses. User agents may close the connection prematurely once they start receiving a response body.
When the ping attribute is present, user agents
should clearly indicate to the user that following the hyperlink will also cause secondary
requests to be sent in the background, possibly including listing the actual target URLs.
For example, a visual user agent could include the hostnames of the target ping URLs along with the hyperlink's actual URL in a status bar or tooltip.
The ping attribute is redundant with pre-existing
technologies like HTTP redirects and JavaScript in allowing web pages to track which off-site
links are most popular or allowing advertisers to track click-through rates.
However, the ping attribute provides these advantages
to the user over those alternatives:
Thus, while it is possible to track users without this feature, authors are encouraged to use
the ping attribute so that the user agent can make the
user experience more transparent.
Support in all current engines.
The following table summarizes the link types that are defined by this specification, by their corresponding keywords. This table is non-normative; the actual definitions for the link types are given in the next few sections.
In this section, the term referenced document refers to the resource identified by the element representing the link, and the term current document refers to the resource within which the element representing the link finds itself.
To determine which link types apply to a link, a, area,
or form element, the element's rel attribute must be split on ASCII whitespace. The resulting tokens
are the keywords for the link types that apply to that element.
Except where otherwise specified, a keyword must not be specified more than once per rel attribute.
Some of the sections that follow the table below list synonyms for certain keywords. The
indicated synonyms are to be handled as specified by user agents, but must
not be used in documents (for example, the keyword "copyright").
Keywords are always ASCII case-insensitive, and must be compared as such.
Thus, rel="next" is the same as rel="NEXT".
Keywords that are body-ok affect whether link elements are
allowed in the body. The body-ok keywords are
dns-prefetch,
modulepreload,
pingback,
preconnect,
prefetch,
preload,
prerender, and
stylesheet.
New link types that are to be implemented by web browsers are to be added to this standard. The remainder can be registered as extensions.
| Link type | Effect on... | body-ok | Brief description | ||
|---|---|---|---|---|---|
link | a and area | form | |||
alternate | Hyperlink | not allowed | · | Gives alternate representations of the current document. | |
canonical | Hyperlink | not allowed | · | Gives the preferred URL for the current document. | |
author | Hyperlink | not allowed | · | Gives a link to the author of the current document or article. | |
bookmark | not allowed | Hyperlink | not allowed | · | Gives the permalink for the nearest ancestor section. |
dns-prefetch | External Resource | not allowed | Yes | Specifies that the user agent should preemptively perform DNS resolution for the target resource's origin. | |
external | not allowed | Annotation | · | Indicates that the referenced document is not part of the same site as the current document. | |
help | Hyperlink | · | Provides a link to context-sensitive help. | ||
icon | External Resource | not allowed | · | Imports an icon to represent the current document. | |
manifest | External Resource | not allowed | · | Imports or links to an application manifest. [MANIFEST] | |
modulepreload | External Resource | not allowed | Yes | Specifies that the user agent must preemptively fetch the module script and store it in the document's module map for later evaluation. Optionally, the module's dependencies can be fetched as well. | |
license | Hyperlink | · | Indicates that the main content of the current document is covered by the copyright license described by the referenced document. | ||
next | Hyperlink | · | Indicates that the current document is a part of a series, and that the next document in the series is the referenced document. | ||
nofollow | not allowed | Annotation | · | Indicates that the current document's original author or publisher does not endorse the referenced document. | |
noopener | not allowed | Annotation | · | Creates a top-level browsing context that is not an auxiliary
browsing context if the hyperlink would create either of those to begin with (i.e., has
an appropriate target attribute value). | |
noreferrer | not allowed | Annotation | · | No `Referer` (sic) header will be included.
Additionally, has the same effect as noopener. | |
opener | not allowed | Annotation | · | Creates an auxiliary browsing context if the hyperlink would otherwise create
a top-level browsing context that is not an auxiliary browsing
context (i.e., has "_blank" as target attribute value). | |
pingback | External Resource | not allowed | Yes | Gives the address of the pingback server that handles pingbacks to the current document. | |
preconnect | External Resource | not allowed | Yes | Specifies that the user agent should preemptively connect to the target resource's origin. | |
prefetch | External Resource | not allowed | Yes | Specifies that the user agent should preemptively fetch and cache the target resource as it is likely to be required for a followup navigation. | |
preload | External Resource | not allowed | Yes | Specifies that the user agent must preemptively fetch and cache the target resource for current navigation according to the potential destination given by the as attribute (and the priority associated with the corresponding destination). | |
prerender | External Resource | not allowed | Yes | Specifies that the user agent should preemptively fetch the target resource and process it in a way that helps deliver a faster response in the future. | |
prev | Hyperlink | · | Indicates that the current document is a part of a series, and that the previous document in the series is the referenced document. | ||
search | Hyperlink | · | Gives a link to a resource that can be used to search through the current document and its related pages. | ||
stylesheet | External Resource | not allowed | Yes | Imports a style sheet. | |
tag | not allowed | Hyperlink | not allowed | · | Gives a tag (identified by the given address) that applies to the current document. |
alternate"Support in one engine only.
alternate 关键字可用于 link, a 和 area 元素。
该关键的含义取决于其他属性的值。
link 元素且 rel 属性包含关键字 stylesheetalternate 关键字会更改 stylesheet 关键字的含义（见它的说明）。
alternate 关键字自己不会创建链接。
下面的这些 link 元素提供了一些样式表：
<!-- a persistent style sheet -->
<link rel="stylesheet" href="default.css">
<!-- the preferred alternate style sheet -->
<link rel="stylesheet" href="green.css" title="Green styles">
<!-- some alternate style sheets -->
<link rel="alternate stylesheet" href="contrast.css" title="High contrast">
<link rel="alternate stylesheet" href="big.css" title="Big fonts">
<link rel="alternate stylesheet" href="wide.css" title="Wide screen">alternate 关键字用于 type 属性值为 application/rss+xml 或 application/atom+xml该关键字创建了一个 超链接 指向聚合 feed（不一定需要发布和当前页面同样的内容）。
为了 feed 自动发现，用户代理应该考虑文档里所有使用了 alternate 关键字，
并且 type 属性值为 application/rss+xml 或 application/atom+xml 的 link 元素。
如果用户代理有默认 feed 聚合的概念，应该默认使用（按照树序）第一个这样的元素。
下面的 link 元素给出了一个博客的 feed 聚合：
<link rel="alternate" type="application/atom+xml" href="posts.xml" title="Cool Stuff Blog">
<link rel="alternate" type="application/atom+xml" href="posts.xml?category=robots" title="Cool Stuff Blog: robots category">
<link rel="alternate" type="application/atom+xml" href="comments.xml" title="Cool Stuff Blog: Comments">这样的 link 元素应该用于用户代理自动发现 feed，且第一个（如果可用的话）为默认值。
下面的例子使用 a 元素给用户提供了多种不同的 feed 聚合：
<p>You can access the planets database using Atom feeds:</p>
<ul>
<li><a href="recently-visited-planets.xml" rel="alternate" type="application/atom+xml">Recently Visited Planets</a></li>
<li><a href="known-bad-planets.xml" rel="alternate" type="application/atom+xml">Known Bad Planets</a></li>
<li><a href="unexplored-planets.xml" rel="alternate" type="application/atom+xml">Unexplored Planets</a></li>
</ul>这些链接不会被用于 feed 自动发现。
该关键字将创建一个 超链接 指向当前文档的另一种表示。
被引用文档的特征由 hreflang 和 type 属性给出。
如果 alternate 关键字和
hreflang 属性一起使用，
且该属性的值与 文档元素 的 语言 不同，
它表示被引用文档是一个翻译版本。
如果 alternate 关键字和
type 属性一起使用，它表示被引用文档是当前文档的另一种格式的版本。
hreflang 和 type 属性可以和 alternate 关键字一起使用。
下面的例子展示了如何指定当前页面的其他格式版本，其他语言版本，以及为其他媒体准备的版本：
<link rel=alternate href="/en/html" hreflang=en type=text/html title="English HTML">
<link rel=alternate href="/fr/html" hreflang=fr type=text/html title="French HTML">
<link rel=alternate href="/en/html/print" hreflang=en type=text/html media=print title="English HTML (for printing)">
<link rel=alternate href="/fr/html/print" hreflang=fr type=text/html media=print title="French HTML (for printing)">
<link rel=alternate href="/en/pdf" hreflang=en type=application/pdf title="English PDF">
<link rel=alternate href="/fr/pdf" hreflang=fr type=application/pdf title="French PDF">这一关系具有传递性 — 也就是说如果一篇文档使用 "alternate" 链接到另外两篇文档，
那么意味着那两篇文档是第一篇文档的替代表示，也意味着那两篇文档互相为替代表示。
author" 链接类型author 关键字可用于 link，a 和
area 元素。该关键字会创建一个 超链接。
对于 a 和 area 元素，author
关键字表示被引用的文档提供了离定义该超链接的元素最近的祖先 article 元素的作者的更多信息，
如果没有这样的祖先元素，就是指整个页面的作者。
对于 link 元素，author 关键字表示被引用文档提供了
关于整个页面的作者的更多信息。
"被引用文档" 可以是（也经常是）一个 mailto: URL 给出了作者的 e-mail 地址。[MAILTO]
同义词：由于历史原因，用户代理必须把
rev 属性值为 "made" 的
link, a, 和 area 元素也当作有一个
author 关键字来处理。
bookmark" 链接类型bookmark 关键字可用于 link，a 和
area 元素。该关键字会创建一个 超链接。
bookmark 关键字给出了这个链接元素最近的祖先 article
元素的永久链接。如果没有祖先 article 元素的话，就是指
这个链接元素关联最紧密的 section 。
下面的片段有三个永久链接。用户代理通过给出永久链接的位置来确定每个部分对应的永久链接。
...
<body>
<h1>Example of permalinks</h1>
<div id="a">
<h2>First example</h2>
<p><a href="a.html" rel="bookmark">This permalink applies to
only the content from the first H2 to the second H2</a>. The DIV isn't
exactly that section, but it roughly corresponds to it.</p>
</div>
<h2>Second example</h2>
<article id="b">
<p><a href="b.html" rel="bookmark">This permalink applies to
the outer ARTICLE element</a> (which could be, e.g., a blog post).</p>
<article id="c">
<p><a href="c.html" rel="bookmark">This permalink applies to
the inner ARTICLE element</a> (which could be, e.g., a blog comment).</p>
</article>
</article>
</body>
...
canonical"The canonical keyword may be used with link
element. This keyword creates a hyperlink.
The canonical keyword indicates that URL given by the href attribute is the preferred URL for the current document. That
helps search engines reduce duplicate content, as described in more detail in The Canonical
Link Relation. [RFC6596]
dns-prefetch" 链接类型dns-prefetch 关键字可用于
link 元素。该关键字会创建
外部资源链接。
这个关键字是 body-ok 的。
dns-prefetch 关键字表示最好为指定资源的
域 事先执行 DNS 解析，因为用户很可能会要位于那个
域 的资源，避免了 DNS 解析相关的延迟会提升用户体验。
用户代理必须实现 Resource Hints 所描述的
dns-prefetch 关键字的处理模型。
[RESOURCEHINTS]
dns-prefetch 关键字没有默认的资源类型。
external" 链接类型external 关键字可用于 a 和
area 元素。该元素不会创建 超链接，但可以 注释 该元素创建的任何其他超链接
（如果其他关键字没有创建超链接的话就是注释暗示的超链接）。
external 关键字表示该链接指向的文档不是当前文档所属站点的一部分。
help" 链接类型help 关键字可用于 link，a 和
area 元素。该关键字会创建一个 超链接。
对于 a 和 area 元素，help
关键字表示被引用文档为定义了该超链接的父元素及其子元素提供了更多帮助信息。
在下面的例子中，表单控件关联了上下文相关的帮助。用户代理可以使用这个信息， 比如用户按下 “Help” 或 “F1” 键时可以显示被引用的文档。
<p><label> Topic: <input name=topic> <a href="help/topic.html" rel="help">(Help)</a></label></p>
对于 link 元素，这个 help 关键字表示
被引用的文档为整个页面提供了帮助。
对于 a 和 area 元素，在有些浏览器中 help 关键字会使链接显示不同的光标。
icon"Support in all current engines.
The icon keyword may be used with link elements.
This keyword creates an external resource link.
The specified resource is an icon representing the page or site, and should be used by the user agent when representing the page in the user interface.
Icons could be auditory icons, visual icons, or other kinds of icons. If
multiple icons are provided, the user agent must select the most appropriate icon according to the
type, media, and sizes attributes. If there are multiple equally appropriate icons,
user agents must use the last one declared in tree order at the time that the user
agent collected the list of icons. If the user agent tries to use an icon but that icon is
determined, upon closer examination, to in fact be inappropriate (e.g. because it uses an
unsupported format), then the user agent must try the next-most-appropriate icon as determined by
the attributes.
User agents are not required to update icons when the list of icons changes, but are encouraged to do so.
There is no default type for resources given by the icon keyword.
However, for the purposes of determining the type of the
resource, user agents must expect the resource to be an image.
The sizes keywords represent icon sizes in raw pixels (as
opposed to CSS pixels).
An icon that is 50 CSS pixels wide intended for displays with a device pixel density of two device pixels per CSS pixel (2x, 192dpi) would have a width of 100 raw pixels. This feature does not support indicating that a different resource is to be used for small high-resolution icons vs large low-resolution icons (e.g. 50×50 2x vs 100×100 1x).
To parse and process the attribute's value, the user agent must first split the attribute's value on ASCII whitespace, and must then parse each resulting keyword to determine what it represents.
The any keyword represents that the
resource contains a scalable icon, e.g. as provided by an SVG image.
Other keywords must be further parsed as follows to determine what they represent:
If the keyword doesn't contain exactly one U+0078 LATIN SMALL LETTER X or U+0058 LATIN CAPITAL LETTER X character, then this keyword doesn't represent anything. Return for that keyword.
Let width string be the string before the "x" or
"X".
Let height string be the string after the "x" or
"X".
If either width string or height string start with a U+0030 DIGIT ZERO (0) character or contain any characters other than ASCII digits, then this keyword doesn't represent anything. Return for that keyword.
Apply the rules for parsing non-negative integers to width string to obtain width.
Apply the rules for parsing non-negative integers to height string to obtain height.
The keyword represents that the resource contains a bitmap icon with a width of width device pixels and a height of height device pixels.
The keywords specified on the sizes attribute must not
represent icon sizes that are not actually available in the linked resource.
The linked resource fetch setup steps for this type of linked resource, given a
link element el and request
request, are:
Set request's destination to
"image".
Return true.
In the absence of a link with the icon keyword, for
Document objects whose URL's
scheme is an HTTP(S) scheme, user agents may
instead run these steps in parallel:
Let request be a new request whose
url is the URL record obtained by
resolving the URL "/favicon.ico" against the
Document object's URL, client is the Document object's
relevant settings object, destination is "image",
synchronous flag is set, credentials
mode is "include", and whose use-URL-credentials flag
is set.
Let response be the result of fetching request.
Use response's unsafe response as an icon as if it had been
declared using the icon keyword.
The following snippet shows the top part of an application with several icons.
<!DOCTYPE HTML>
<html lang="en">
<head>
<title>lsForums — Inbox</title>
<link rel=icon href=favicon.png sizes="16x16" type="image/png">
<link rel=icon href=windows.ico sizes="32x32 48x48" type="image/vnd.microsoft.icon">
<link rel=icon href=mac.icns sizes="128x128 512x512 8192x8192 32768x32768">
<link rel=icon href=iphone.png sizes="57x57" type="image/png">
<link rel=icon href=gnome.svg sizes="any" type="image/svg+xml">
<link rel=stylesheet href=lsforums.css>
<script src=lsforums.js></script>
<meta name=application-name content="lsForums">
</head>
<body>
...For historical reasons, the icon keyword may be preceded by the
keyword "shortcut". If the "shortcut" keyword is
present, the rel attribute's entire value must be an
ASCII case-insensitive match for the string "shortcut icon" (with a single U+0020 SPACE character between the tokens and
no other ASCII whitespace).
license" 链接类型license 关键字可用于 link，a 和
area 元素。该关键字会创建一个 超链接。
license
关键字表示被引用文档提供了当前文档中主要内容的版权许可条款。
本规范未指定如何区分文档中的主要内容和不被视为该主要内容的部分。 应该让用户能够明确这个区分。
考虑一个相片分享网站。这个站点的一个页面可能会描述并显示一张照片，这个页面可能是这样写的：
<!DOCTYPE HTML> <html lang="en"> <head> <title>Exampl Pictures: Kissat</title> <link rel="stylesheet" href="/style/default"> </head> <body> <h1>Kissat</h1> <nav> <a href="../">Return to photo index</a> </nav> <figure> <img src="/pix/39627052_fd8dcd98b5.jpg"> <figcaption>Kissat</figcaption> </figure> <p>One of them has six toes!</p> <p><small><a rel="license" href="http://www.opensource.org/licenses/mit-license.php">MIT Licensed</a></small></p> <footer> <a href="/">Home</a> | <a href="../">Photo index</a> <p><small>© copyright 2009 Exampl Pictures. All Rights Reserved.</small></p> </footer> </body> </html>
这个例子中 license 只适用于这张照片
（该文档的主要内容）而不是整个文档。尤其不是页面的设计本身，它的版权在文档底部给出。
这可以通过样式来表达清楚（例如让许可链接贴着照片，同时把页面的版权以细小的文字放在页底）。
同义词：由于历史原因，用户代理必须把 "copyright"
当作 license 关键字处理。
manifest"Support in one engine only.
The manifest keyword may be used with link elements.
This keyword creates an external resource link.
The manifest keyword indicates the manifest file that provides
metadata associated with the current document.
There is no default type for resources given by the manifest
keyword.
The appropriate time to fetch and process the linked resource for this link type
is when the user agent deems it necessary. For example, when the user chooses to install the web application. In that case, only the
first link element in tree order whose rel attribute contains the token manifest may be used.
A user agent must not delay the load event for this link type.
The linked resource fetch setup steps for this type of linked resource, given a
link element el and request
request, are:
Let context be el's node document's browsing context.
If context is null, then return false.
If context is not a top-level browsing context, then return false.
Set request's initiator to
"manifest".
Set request's destination to
"manifest".
Set request's mode to "cors".
Set request's credentials
mode to the CORS settings attribute credentials mode for el's
crossorigin content attribute.
Return true.
To process this type of linked resource given
a link element el, boolean success, and response response:
If response's Content-Type metadata is not a JSON MIME type, then set success to false.
If success is true, then process the manifest given el and response. [MANIFEST]
modulepreload"Support in one engine only.
The modulepreload keyword may be used with
link elements. This keyword creates an external resource link. This
keyword is body-ok.
The modulepreload keyword is a specialized alternative
to the preload keyword, with a processing model geared toward
preloading module scripts. In particular, it uses the specific
fetch behavior for module scripts (including, e.g., a different interpretation of the crossorigin attribute), and places the result into the
appropriate module map for later evaluation. In
contrast, a similar external resource link using the preload keyword would place the result in the preload cache, without
affecting the document's module map.
Additionally, implementations can take advantage of the fact that module scripts declare their dependencies in order to fetch the specified module's
dependency as well. This is intended as an optimization opportunity, since the user agent knows
that, in all likelihood, those dependencies will also be needed later. It will not generally be
observable without using technology such as service workers, or monitoring on the server side.
Notably, the appropriate load or error events will occur after the specified module is fetched, and
will not wait for any dependencies.
The appropriate times to fetch and process the linked resource for such a link are:
When the external resource link is created on a link element
that is already browsing-context connected.
When the external resource link's link element becomes
browsing-context connected.
When the href attribute of the link
element of an external resource link that is already browsing-context
connected is changed.
Unlike some other link relations, changing the relevant attributes (such as as, crossorigin, and
referrerpolicy) of such a link
does not trigger a new fetch. This is because the document's module map has already been populated by a previous
fetch, and so re-fetching would be pointless.
The fetch and process the linked resource algorithm for modulepreload links, given a link element
el, is as follows:
If the href attribute's value is the empty string,
then return.
Let destination be the current state of the as attribute (a destination), or "script" if
it is in no state.
If destination is not script-like, then queue an element
task on the networking task source given the link element to
fire an event named error at the link element, and return.
Parse the URL given by the href attribute, relative to the element's node
document. If that fails, then return. Otherwise, let url be the resulting
URL record.
Let settings object be the link element's node
document's relevant settings object.
Let credentials mode be the CORS settings attribute credentials
mode for the crossorigin attribute.
Let cryptographic nonce be the current value of the element's [[CryptographicNonce]] internal slot.
Let integrity metadata be the value of the integrity attribute, if it is specified, or the empty string
otherwise.
Let referrer policy be the current state of the element's referrerpolicy attribute.
Let options be a script fetch options whose cryptographic nonce is cryptographic
nonce, integrity metadata is
integrity metadata, parser
metadata is "not-parser-inserted", credentials mode is credentials
mode, and referrer
policy is referrer policy.
Fetch a modulepreload module script graph given url, destination, settings object, and options. Wait until the algorithm asynchronously completes with result.
If result is null, then fire an event
named error at the link element, and
return.
Fire an event named load at the link element.
The following snippet shows the top part of an application with several modules preloaded:
<!DOCTYPE html>
<html lang="en">
<title>IRCFog</title>
<link rel="modulepreload" href="app.mjs">
<link rel="modulepreload" href="helpers.mjs">
<link rel="modulepreload" href="irc.mjs">
<link rel="modulepreload" href="fog-machine.mjs">
<script type="module" src="app.mjs">
...Assume that the module graph for the application is as follows:
Here we see the application developer has used modulepreload to declare all of the modules in their module graph,
ensuring that the user agent initiates fetches for them all. Without such preloading, the user
agent might need to go through multiple network roundtrips before discovering helpers.mjs, if technologies such as HTTP/2 Server Push are not in play. In
this way, modulepreload link elements can be
used as a sort of "manifest" of the application's modules.
The following code shows how modulepreload links can
be used in conjunction with import() to ensure network fetching is done ahead of
time, so that when import() is called, the module is already ready (but not
evaluated) in the module map:
<link rel="modulepreload" href="awesome-viewer.mjs">
<button onclick="import('./awesome-viewer.mjs').then(m => m.view())">
View awesome thing
</button>nofollow" 链接类型nofollow 关键字可用于 a 和
area 元素。这个关键字不会创建 超链接，但是可以 注释 任何其他该元素创建的超链接。
（如果没有其他关键字创建链接，那就注释其隐含的超链接）。
nofollow 关键字表示该链接不被该网页的原始作者或发布者所认可，
或者这个指向被引用文档的链接主要是为了这两个页面的所有者之间的商业关系。
noopener"Support in all current engines.
Support in all current engines.
The noopener keyword may be used with a,
area, and form elements. This keyword does not create a
hyperlink, but annotates any other
hyperlinks created by the element (the implied hyperlink, if no other keywords create one).
The keyword indicates that any newly created top-level browsing context which
results from following the hyperlink will not be an auxiliary browsing
context. E.g., its window.opener attribute will be
null.
See also the processing model where the branching between an auxiliary browsing context and a top-level browsing context is defined.
This typically creates an auxiliary browsing context (assuming there is no
existing browsing context whose browsing context name is
"example"):
<a href=help.html target=example>Help!</a>This creates a top-level browsing context that is not an auxiliary browsing context (assuming the same thing):
<a href=help.html target=example rel=noopener>Help!</a>These are equivalent and only navigate the parent browsing context:
<a href=index.html target=_parent>Home</a><a href=index.html target=_parent rel=noopener>Home</a>noreferrer"Support in all current engines.
Support in all current engines.
The noreferrer keyword may be used with a,
area, and form elements. This keyword does not create a
hyperlink, but annotates any other
hyperlinks created by the element (the implied hyperlink, if no other keywords create one).
It indicates that no referrer information is to be leaked when following the link and also
implies the noopener keyword behavior under the same
conditions.
See also the processing model where referrer is directly manipulated.
<a href="..." rel="noreferrer" target="_blank">
has the same behavior as <a href="..." rel="noreferrer noopener" target="_blank">.
opener"The opener keyword may be used with a,
area, and form elements. This keyword does not create a
hyperlink, but annotates any other
hyperlinks created by the element (the implied hyperlink, if no other keywords create one).
The keyword indicates that any newly created top-level browsing context which results from following the hyperlink will be an auxiliary browsing context.
See also the processing model.
In the following example the opener is used to allow the help
page popup to navigate its opener, e.g., in case what the user is looking for can be found
elsewhere. An alternative might be to use a named target, rather than _blank, but this has the potential to clash with existing names.
<a href="..." rel=opener target=_blank>Help!</a>pingback" 链接类型pingback 关键字可以用于 link
元素。该关键字会创建一个 外部资源链接。
这个关键字是 body-ok 的。
关于 pingback 关键字的语义，参考 Pingback
1.0 规范。 [PINGBACK]
preconnect 链接类型"Support in all current engines.
preconnect 关键字可用于 link 元素。
该关键字会创建一个 外部资源链接。
这个关键字是 body-ok 的。
preconnect 关键字表示最好事先初始化一个到指定资源的
域 的连接。因为用户很可能会访问处于这个
域 的资源，这省去了建立连接的延迟，所以会提升用户体验。
用户代理必须实现 Resource Hints 中描述的
preconnect 关键字的处理模型。 [RESOURCEHINTS]
preconnect 关键字给出的资源没有默认类型。
prefetch" 链接类型prefetch 关键字可用于 link
元素。该关键字会创建 外部资源链接。
这个关键在是 body-ok 的。
prefetch 关键字表示最好事先
获取 并缓存指定的资源。
因为用户在将来的导航中很可能会要这个资源。
用户代理必须实现 Resource Hints 所描述的
prefetch 关键字的处理模型。
[RESOURCEHINTS]
prefetch 关键字没有默认的资源类型。
preload"Support in one engine only.
The preload keyword may be used with link
elements. This keyword creates an external resource
link. This keyword is body-ok.
The preload keyword indicates that the user agent must
preemptively fetch and cache the specified resource according
to the potential destination given by the
as attribute (and the priority associated with the corresponding destination), as it is highly likely that the user
will require this resource for the current navigation. User agents must implement
the processing model of the preload keyword described in
Preload, as well as in this specification's fetch and process the linked
resource algorithm. [PRELOAD]
There is no default type for resources given by the preload
keyword.
The linked resource fetch setup steps for this type of linked resource, given a
link element el and request
request, are:
Let as be the current state of el's as attribute.
If as does not represent a state, return false.
Set request's destination to the result of translating as.
If as is "image", then:
Let selected source and selected pixel density be the URL and pixel density that results from selecting an image source given el, respectively.
If selected source is null, then return false.
Parse selected source, relative to el's node document. If that fails, then return false. Otherwise, let url be the resulting URL record.
Set request's url to url.
Return true.
prerender" 链接类型Support in one engine only.
prerender 关键字可用于 link
元素。该关键字会创建 外部资源链接。
这个关键在是 body-ok 的。
prerender 关键字表示指定的资源可能在下次导航时用到。
所以最好先去 获取 这个资源并处理它，
比如 获取 它的子资源或执行一些渲染。用户代理必须实现 Resource Hints 中描述的
prerender 关键字的处理模型。
[RESOURCEHINTS]
prerender 关键字没有默认的资源类型。
search"The search keyword may be used with link,
a, area, and form elements. This keyword creates a
hyperlink.
The search keyword indicates that the referenced document
provides an interface specifically for searching the document and its related resources.
OpenSearch description documents can be used with link elements and
the search link type to enable user agents to autodiscover search
interfaces. [OPENSEARCH]
stylesheet"The stylesheet keyword may be used with link
elements. This keyword creates an external resource
link that contributes to the styling processing model. This keyword is
body-ok.
The specified resource is a CSS style sheet that describes how to present the document.
If the alternate keyword is also specified on the
link element, then the link is an
alternative style sheet; in this case, the title attribute
must be specified on the link element, with a non-empty value.
The default type for resources given by the stylesheet
keyword is text/css.
The appropriate times to fetch and process this type of link are:
When the external resource link is created on a link element
that is already browsing-context connected.
When the external resource link's link element becomes
browsing-context connected.
When the href attribute of the link
element of an external resource link that is already browsing-context
connected is changed.
When the disabled attribute of the
link element of an external resource link that is already
browsing-context connected is set, changed, or removed.
When the crossorigin attribute of the
link element of an external resource
link that is already browsing-context connected is set, changed, or
removed.
When the type attribute of the link
element of an external resource link that is already browsing-context
connected is set or changed to a value that does not or no longer matches the Content-Type metadata of the previous obtained external resource, if
any.
When the type attribute of the link
element of an external resource link that is already browsing-context
connected, but was previously not obtained due to the type attribute specifying an unsupported type, is set, removed, or
changed.
When the external resource link that is already browsing-context connected changes from being an alternative style sheet to not being one, or vice versa.
Quirk: If the document has been set to quirks mode, has the
same origin as the URL of the external resource,
and the Content-Type metadata of the external resource is not a
supported style sheet type, the user agent must instead assume it to be text/css.
The linked resource fetch setup steps for this type of linked resource, given a
link element el (ignoring the request) are:
If el's disabled attribute is set,
then return false.
If el contributes a script-blocking style sheet, increment el's node document's script-blocking style sheet counter by 1.
Return true.
See issue #968 for plans to use the CSSOM fetch a CSS style sheet algorithm instead of the default fetch and process the linked resource algorithm.
To process this type of linked resource
given a link element el, boolean success, and response response, the user agent must run these
steps:
If the resource's Content-Type metadata is not
text/css, then set success to false.
If el no longer creates an external resource link that contributes to the styling processing model, or if, since the resource in question was fetched, it has become appropriate to fetch it again, then return.
If el has an associated CSS style sheet, remove the CSS style sheet.
If success is true, then:
Create a CSS style sheet with the following properties:
The resulting URL string determined during the fetch and process the linked resource algorithm.
This is before any redirects get applied.
element
The media attribute of element.
This is a reference to the (possibly absent at this time) attribute, rather than a copy of the attribute's current value. CSSOM defines what happens when the attribute is dynamically set, changed, or removed.
The title attribute of element, if
element is in a document tree, or the empty string otherwise.
This is similarly a reference to the attribute, rather than a copy of the attribute's current value.
Set if the link is an alternative style sheet and element's explicitly enabled is false; unset otherwise.
Set if the resource is CORS-same-origin; unset otherwise.
null
Left at its default value.
Left uninitialized.
This doesn't seem right. Presumably we should be using the response body? Tracked as issue #2997.
The CSS environment encoding is the result of running the following steps: [CSSSYNTAX]
If the element has a charset attribute, get an encoding from that attribute's value. If that
succeeds, return the resulting encoding. [ENCODING]
Otherwise, return the document's character encoding. [DOM]
Fire an event named load at el.
Otherwise, fire an event named error at el.
If el contributes a script-blocking style sheet, then:
Assert: el's node document's script-blocking style sheet counter is greater than 0.
Decrement el's node document's script-blocking style sheet counter by 1.
tag" 链接类型tag 关键字可用于 a 和
area 元素。该关键字会创建一个 超链接。
tag 关键字表示被引用的文档表示的 tag
适用于当前文档。
由于它表示该 tag 适用于当前文档, 在 tag 云 的标记中用这个关键字是不合适的，因为 tag 云列出了很多页面的 tag。
本文档是关于宝石的，所以它有一个
"https://en.wikipedia.org/wiki/Gemstone"
tag 来把它归类到“珠宝”，而不是美国的城镇、Ruby 包格式，或瑞士机车分级：
<!DOCTYPE HTML> <html lang="en"> <head> <title>My Precious</title> </head> <body> <header><h1>My precious</h1> <p>Summer 2012</p></header> <p>Recently I managed to dispose of a red gem that had been bothering me. I now have a much nicer blue sapphire.</p> <p>The red gem had been found in a bauxite stone while I was digging out the office level, but nobody was willing to haul it away. The same red gem stayed there for literally years.</p> <footer> Tags: <a rel=tag href="https://en.wikipedia.org/wiki/Gemstone">Gemstone</a> </footer> </body> </html>
在 本 文档中有两篇文章。但 "tag" 链接适用于整个页面
（不论它放在了哪里，以及是否在 article 元素中）。
<!DOCTYPE HTML>
<html lang="en">
<head>
<title>Gem 4/4</title>
</head>
<body>
<article>
<h1>801: Steinbock</h1>
<p>The number 801 Gem 4/4 electro-diesel has an ibex and was rebuilt in 2002.</p>
</article>
<article>
<h1>802: Murmeltier</h1>
<figure>
<img src="https://upload.wikimedia.org/wikipedia/commons/b/b0/Trains_de_la_Bernina_en_hiver_2.jpg"
alt="The 802 was red with pantographs and tall vents on the side.">
<figcaption>The 802 in the 1980s, above Lago Bianco.</figcaption>
</figure>
<p>The number 802 Gem 4/4 electro-diesel has a marmot and was rebuilt in 2003.</p>
</article>
<p class="topic"><a rel=tag href="https://en.wikipedia.org/wiki/Rhaetian_Railway_Gem_4/4">Gem 4/4</a></p>
</body>
</html>
有些文档会组成一个文档序列。
文档序列是指，每个文档可以有一个 前驱 和一个 后继。 没有前驱的文档是其所在序列的开始文档， 没有后继的文档是其所在序列的结束文档。
一个文档可能属于多个序列。
next" 链接类型next 关键字可用于 link,
a 和 area 元素。该关键字会创建 超链接。
next 关键字表示该文档是一个文档序列的一部分，
而且该链接指向逻辑上位于当前文档之后的那个文档。
当 next 关键字用于 link 元素时，
用户代理应该实现一个 Resource Hints 中描述的处理模型，
也就是说这样的链接应该被当做使用了
dns-prefetch, preconnect,
prefetch 或 prerender
关键字来处理。用户代理希望使用哪个资源暗示是实现相关的；
例如用户代理为了节省数据、电池或计算量可能会希望使用代价较小的
preconnect 暗示，
或者希望基于对类似场景中以前的用户行为的启发式分析来选取一个资源暗示。[RESOURCEHINTS]
prev" 链接类型prev 关键字可用于 link,
a 和 area 元素。该关键字会创建 超链接。
prev 关键字表示该文档是一个文档序列的一部分，
而且该链接指向逻辑上位于当前文档之前的那个文档。
同义词：由于历史原因，用户代理必须把
"previous" 关键字当作 prev 关键字处理。
预定义链接类型的扩展 可以注册在 microformats Wiki 已有的 rel 值页面。 [MFREL]
任何人在任何时候都可以编辑 microformats Wiki 已有的 rel 值页面 来增加一个类型。扩展类型必须指定以下信息：
要定义的实际关键字。这个值不应与其他已定义的值相似以避免混淆。 （例如只有大小写不同）
如果值包含 U+003A COLON 字符（:），它就必须是一个 绝对 URL。
link下面之一：
link 元素上指定。link 元素上指定；它会创建一个
超链接。link 元素上指定，它会创建一个
外部资源链接。a 和 area下面之一：
a 和 area 元素上指定。a 和 area 元素上指定；它会创建一个
超链接。a 和 area 元素上指定；它会创建一个
外部资源链接。a 和 area 元素上指定；它用来 注释 该元素创建的其他 超链接。对这个关键字的含义进行简短的非形式化的描述。
指向对该关键字的语义和要求的更细节的描述的链接。 它可以是 Wiki 上的另一个页面，或者一个外部页面。
有着同样处理要求的其他关键字值的列表。 作者不应该使用已经定义为同义词的值，它们只是为了让用户代理支持旧的内容。 任何人都可以移除一个实践中已经不再使用的同义词 只有用于兼容旧内容的同义词才应该注册在这里。
以下之一：
如果一个关键字与其他现存的值冗余，那么应该把它移除并列为那个现存值的同义词。
如果一个关键字已经添加到 “已提议” 状态经过一个多月的时间并未被使用或标准化， 那么应该把它从注册中移除。
如果一个关键字已经添加到 “已提议” 状态但却与现存的值冗余， 就应该把它移除并列为该现存值的同义词。 如果一个名称被添加到 “已提议” 状态但被发现有害，则应该把它改为 “已终止” 状态。
任何人都可以随时更新状态，但应该根据上述定义进行。
一致性检查工具必须使用 microformats wiki 已有的 rel 值页面 给出的信息来决定值是否被允许： 对于本规范中定义的值或标记为 “已提议” 或 “已批准” 的值，当用于 “作用于...” 字段描述的元素时， 工具必须接受它，但标记为 “已终止” 的值，或者本标准和上述页面都没有列出的值，就必须视为不合法并拒绝它。 一致性检查工具可以缓存这个信息（例如，为了性能原因或为了避免使用不可靠的网络连接）。
当作者使用本规范和 Wiki 页面没有定义的新类型时，一致性检查工具应该可以提示把这个值添加到 Wiki， 使用上面描述的细节，并设为 “已提议” 状态。
在
microformats wiki 已有 rel 值页面 中定义为扩展的类型，
如果状态是 “已提议” 或 “已批准” 就可以根据 “作用于...” 字段用于
link 的 rel 属性, a, 和 area
元素。[MFREL]
The ins and del elements represent edits to the document.
ins elementSupport in all current engines.
cite — Link to the source of the quotation or more information about the editdatetime — Date and (optionally) time of the changeHTMLModElement.The ins element represents an addition to the document.
The following represents the addition of a single paragraph:
<aside>
<ins>
<p> I like fruit. </p>
</ins>
</aside>As does the following, because everything in the aside element here counts as
phrasing content and therefore there is just one paragraph:
<aside>
<ins>
Apples are <em>tasty</em>.
</ins>
<ins>
So are pears.
</ins>
</aside>ins elements should not cross implied paragraph
boundaries.
The following example represents the addition of two paragraphs, the second of which was
inserted in two parts. The first ins element in this example thus crosses a
paragraph boundary, which is considered poor form.
<aside>
<!-- don't do this -->
<ins datetime="2005-03-16 00:00Z">
<p> I like fruit. </p>
Apples are <em>tasty</em>.
</ins>
<ins datetime="2007-12-19 00:00Z">
So are pears.
</ins>
</aside>Here is a better way of marking this up. It uses more elements, but none of the elements cross implied paragraph boundaries.
<aside>
<ins datetime="2005-03-16 00:00Z">
<p> I like fruit. </p>
</ins>
<ins datetime="2005-03-16 00:00Z">
Apples are <em>tasty</em>.
</ins>
<ins datetime="2007-12-19 00:00Z">
So are pears.
</ins>
</aside>del elementSupport in all current engines.
cite — Link to the source of the quotation or more information about the editdatetime — Date and (optionally) time of the changeHTMLModElement.The del element represents a removal from the document.
del elements should not cross implied paragraph
boundaries.
The following shows a "to do" list where items that have been done are crossed-off with the date and time of their completion.
<h1>To Do</h1>
<ul>
<li>Empty the dishwasher</li>
<li><del datetime="2009-10-11T01:25-07:00">Watch Walter Lewin's lectures</del></li>
<li><del datetime="2009-10-10T23:38-07:00">Download more tracks</del></li>
<li>Buy a printer</li>
</ul>ins and del elementsThe cite attribute
may be used to specify the URL of a document that
explains the change. When that document is long, for instance the minutes of a meeting, authors
are encouraged to include a fragment pointing to the
specific part of that document that discusses the change.
If the cite attribute is present, it must be a valid
URL potentially surrounded by spaces that explains the change. To obtain
the corresponding citation link, the value of the attribute must be parsed relative to the element's node document. User agents may
allow users to follow such citation links, but they are primarily intended for private use (e.g.,
by server-side scripts collecting statistics about a site's edits), not for readers.
The datetime
attribute may be used to specify the time and date of the change.
If present, the datetime attribute's value must be a
valid date string with optional time.
User agents must parse the datetime attribute according
to the parse a date or time string algorithm. If that doesn't return a date or a global date and time,
then the modification has no associated timestamp (the value is non-conforming; it is not a
valid date string with optional time). Otherwise, the modification is marked as
having been made at the given date or global date and time. If the given value is a global date and time then user agents should use the associated
time-zone offset information to determine which time zone to present the given datetime in.
This value may be shown to the user, but it is primarily intended for private use.
The ins and del elements must implement the
HTMLModElement interface:
Support in all current engines.
[Exposed=Window]
interface HTMLModElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute USVString cite;
[CEReactions] attribute DOMString dateTime;
};The cite IDL
attribute must reflect the element's cite content
attribute. The dateTime IDL attribute must
reflect the element's datetime content
attribute.
This section is non-normative.
由于 ins 和 del 元素不影响 分段，在一些情况下段落可能是 隐式的
（没有显式的 p 元素），这样 ins 或 del
元素就可以跨越一整个段落和其他非 短语内容 元素，
以及另一个段落的一部分。例如：
<section> <ins> <p> This is a paragraph that was inserted. </p> This is another paragraph whose first sentence was inserted at the same time as the paragraph above. </ins> This is a second sentence, which was there all along. </section>
通过只包含 p 元素的一部分内容，还可以把一个段落的尾部，加一整个段落，再加第三个段落的开始，
包含到同一个 ins 或 del 元素中（但这很费解，也不是最佳实践）：
<section> This is the first paragraph. <ins>This sentence was inserted. <p>This second paragraph was inserted.</p> This sentence was inserted too.</ins> This is the third paragraph in this example. <!-- (don't do this) --> </section>
但是根据 隐式段落 的定义，
不可能用同一个 ins 或 del 元素包含一个段落尾加紧接着的段落开始部分。
你必须使用一个（或两个）p 元素和两个 ins 或 del 元素，
例如：
<section> <p>This is the first paragraph. <del>This sentence was deleted.</del></p> <p><del>This sentence was deleted too.</del> That sentence needed a separate <del> element.</p> </section>
由于上面提到的困惑，强烈建议作者总是把所有段落标记为 p 元素，
而不是让 ins 或 del 元素穿越
隐式段落 的边界。
This section is non-normative.
ol 和 ul 元素的内容模型不允许
ins 和 del 元素作为子节点。
列表总是表示它的所有列表项，包括应该被标记为删除的列表项。
为了表示一项被插入或删除，可以用 ins 或 del
元素包含 li 元素的内容。为了表示一个元素被替换，
一个 li 元素可以有一个或更多的 del 元素，
再加一个或更多的 ins 元素。
在以下示例中，开始为空的列表随着时间的推移添加或删除了项目。 示例中强调了列表中属于“当前”状态的部分。尽管如此，列表项目编号并未考虑编辑。
<h1>Stop-ship bugs</h1> <ol> <li><ins datetime="2008-02-12T15:20Z">Bug 225: Rain detector doesn't work in snow</ins></li> <li><del datetime="2008-03-01T20:22Z"><ins datetime="2008-02-14T12:02Z">Bug 228: Water buffer overflows in April</ins></del></li> <li><ins datetime="2008-02-16T13:50Z">Bug 230: Water heater doesn't use renewable fuels</ins></li> <li><del datetime="2008-02-20T21:15Z"><ins datetime="2008-02-16T14:25Z">Bug 232: Carbon dioxide emissions detected after startup</ins></del></li> </ol>
下面的示例中，一个开始只包含水果的列表被替换为只包含颜色的列表。
<h1>List of <del>fruits</del><ins>colors</ins></h1> <ul> <li><del>Lime</del><ins>Green</ins></li> <li><del>Apple</del></li> <li>Orange</li> <li><del>Pear</del></li> <li><ins>Teal</ins></li> <li><del>Lemon</del><ins>Yellow</ins></li> <li>Olive</li> <li><ins>Purple</ins></li> </ul>
This section is non-normative.
组成表格模型的元素有着复杂的内容模型要求，不允许使用 ins 和 del 元素，
所以很难表示对图片的编辑。
为了表示整行的添加和移除，可以（分别）用 ins 或 del
元素包含该行的每个单元格的整个内容。
这个表格中有一整行的添加：
<table> <thead> <tr> <th> Game name <th> Game publisher <th> Verdict <tbody> <tr> <td> Diablo 2 <td> Blizzard <td> 8/10 <tr> <td> Portal <td> Valve <td> 10/10 <tr> <td> <ins>Portal 2</ins> <td> <ins>Valve</ins> <td> <ins>10/10</ins> </table>
这里有一列被移除（移除时间也给出了，还给了一个页面的链接来解释原因）：
<table> <thead> <tr> <th> Game name <th> Game publisher <th> <del cite="/edits/r192" datetime="2011-05-02 14:23Z">Verdict</del> <tbody> <tr> <td> Diablo 2 <td> Blizzard <td> <del cite="/edits/r192" datetime="2011-05-02 14:23Z">8/10</del> <tr> <td> Portal <td> Valve <td> <del cite="/edits/r192" datetime="2011-05-02 14:23Z">10/10</del> <tr> <td> Portal 2 <td> Valve <td> <del cite="/edits/r192" datetime="2011-05-02 14:23Z">10/10</del> </table>
通常不应该表示更复杂的编辑（例如删除了一个单元格，把后续所有单元格上移或左移）。
picture elementSupport in all current engines.
Support in all current engines.
source elements, followed by one img element,
optionally intermixed with script-supporting elements.[Exposed=Window]
interface HTMLPictureElement : HTMLElement {
[HTMLConstructor] constructor();
};The picture element is a container
which provides multiple sources to its contained img element
to allow authors to declaratively control or give hints to the user agent about which image resource to use,
based on the screen pixel density, viewport size, image format, and other factors.
It represents its children.
The picture element is somewhat different from the similar-looking
video and audio elements. While all of them contain source
elements, the source element's src attribute
has no meaning when the element is nested within a picture element, and the resource
selection algorithm is different. Also, the picture element itself does not display
anything; it merely provides a context for its contained img element that enables it
to choose from multiple URLs.
source elementSupport in all current engines.
Support in all current engines.
picture element, before the img element.track elements.src — Address of the resourcetype — Type of embedded resourcesrcset — Images to use in different situations, e.g., high-resolution displays, small monitors, etc.sizes — Image sizes for different page layoutsmedia — Applicable media[Exposed=Window]
interface HTMLSourceElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute USVString src;
[CEReactions] attribute DOMString type;
[CEReactions] attribute USVString srcset;
[CEReactions] attribute DOMString sizes;
[CEReactions] attribute DOMString media;
};The source element allows authors to specify multiple alternative
source sets for img elements or multiple alternative
media resources for media
elements. It does not represent anything on its own.
The type attribute may be present. If
present, the value must be a valid MIME type string.
The remainder of the requirements depend on whether the parent is a picture
element or a media element:
source element's parent is a picture elementThe srcset attribute must be present, and
is a srcset attribute.
The srcset attribute contributes the image sources to the source set, if the
source element is selected.
If the srcset attribute has any image candidate strings using a width descriptor, the sizes attribute must also be present, and is a
sizes attribute. The sizes attribute
contributes the source size to the source set, if the
source element is selected.
The media attributes may also be present.
If present, the value must contain a valid media query list. The user agent will
skip to the next source element if the value does not match the environment.
The type attribute gives the type of the images in the
source set, to allow the user agent to skip to the next source element
if it does not support the given type.
If the type attribute is not
specified, the user agent will not select a different source element if it finds
that it does not support the image format after fetching it.
When a source element has a following sibling source element or
img element with a srcset attribute
specified, it must have at least one of the following:
A media attribute specified with a value that,
after stripping leading and trailing
ASCII whitespace, is not the empty string and is not an ASCII
case-insensitive match for the string "all".
A type attribute specified.
The src attribute must not be present.
source element's parent is a media elementThe src attribute
gives the URL of the media resource. The value must be a valid
non-empty URL potentially surrounded by spaces. This attribute must be present.
Dynamically modifying a source element and its attribute when the
element is already inserted in a video or audio element will have no
effect. To change what is playing, just use the src
attribute on the media element directly, possibly making use of the canPlayType() method to pick from amongst available
resources. Generally, manipulating source elements manually after the document has
been parsed is an unnecessarily complicated approach.
The type attribute gives the type of the media
resource, to help the user agent determine if it can play this media
resource before fetching it. The codecs parameter, which certain
MIME types define, might be necessary to specify exactly how the resource is encoded. [RFC6381]
The following list shows some examples of how to use the codecs= MIME
parameter in the type attribute.
<source src='video.mp4' type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'><source src='video.mp4' type='video/mp4; codecs="avc1.58A01E, mp4a.40.2"'><source src='video.mp4' type='video/mp4; codecs="avc1.4D401E, mp4a.40.2"'><source src='video.mp4' type='video/mp4; codecs="avc1.64001E, mp4a.40.2"'><source src='video.mp4' type='video/mp4; codecs="mp4v.20.8, mp4a.40.2"'><source src='video.mp4' type='video/mp4; codecs="mp4v.20.240, mp4a.40.2"'><source src='video.3gp' type='video/3gpp; codecs="mp4v.20.8, samr"'><source src='video.ogv' type='video/ogg; codecs="theora, vorbis"'><source src='video.ogv' type='video/ogg; codecs="theora, speex"'><source src='audio.ogg' type='audio/ogg; codecs=vorbis'><source src='audio.spx' type='audio/ogg; codecs=speex'><source src='audio.oga' type='audio/ogg; codecs=flac'><source src='video.ogv' type='video/ogg; codecs="dirac, vorbis"'>The srcset, sizes, and media
attributes must not be present.
If a source element is inserted as a
child of a media element that has no src
attribute and whose networkState has the value NETWORK_EMPTY, the user agent must invoke the media
element's resource selection
algorithm.
The IDL attributes src, type,
srcset, sizes and media must reflect the respective
content attributes of the same name.
If the author isn't sure if user agents will all be able to render the media resources
provided, the author can listen to the error event on the last
source element and trigger fallback behavior:
<script>
function fallback(video) {
// replace <video> with its contents
while (video.hasChildNodes()) {
if (video.firstChild instanceof HTMLSourceElement)
video.removeChild(video.firstChild);
else
video.parentNode.insertBefore(video.firstChild, video);
}
video.parentNode.removeChild(video);
}
</script>
<video controls autoplay>
<source src='video.mp4' type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'>
<source src='video.ogv' type='video/ogg; codecs="theora, vorbis"'
onerror="fallback(parentNode)">
...
</video>img elementSupport in all current engines.
Support in all current engines.
usemap attribute: Interactive content.alt — Replacement text for use when images are not availablesrc — Address of the resourcesrcset — Images to use in different situations, e.g., high-resolution displays, small monitors, etc.sizes — Image sizes for different page layoutscrossorigin — How the element handles crossorigin requestsusemap — Name of image map to useismap — Whether the image is a server-side image mapwidth — Horizontal dimensionheight — Vertical dimensionreferrerpolicy — Referrer policy for fetches initiated by the elementdecoding — Decoding hint to use when processing this image for presentationloading — Used when determining loading deferralalt attribute: for authors; for implementers.[Exposed=Window,
LegacyFactoryFunction=Image(optional unsigned long width, optional unsigned long height)]
interface HTMLImageElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString alt;
[CEReactions] attribute USVString src;
[CEReactions] attribute USVString srcset;
[CEReactions] attribute DOMString sizes;
[CEReactions] attribute DOMString? crossOrigin;
[CEReactions] attribute DOMString useMap;
[CEReactions] attribute boolean isMap;
[CEReactions] attribute unsigned long width;
[CEReactions] attribute unsigned long height;
readonly attribute unsigned long naturalWidth;
readonly attribute unsigned long naturalHeight;
readonly attribute boolean complete;
readonly attribute USVString currentSrc;
[CEReactions] attribute DOMString referrerPolicy;
[CEReactions] attribute DOMString decoding;
[CEReactions] attribute DOMString loading;
Promise<undefined> decode();
// also has obsolete members
};An img element represents an image.
Support in all current engines.
The image given by the src and srcset attributes, and any previous sibling
source elements' srcset attributes if the
parent is a picture element, is the embedded content; the value of the alt attribute provides equivalent content for those who
cannot process images or who have image loading disabled (i.e. it is the img
element's fallback content).
The requirements on the alt attribute's value are described
in a separate section.
The src attribute must be present, and must contain a
valid non-empty URL potentially surrounded by spaces referencing a non-interactive,
optionally animated, image resource that is neither paged nor scripted.
The requirements above imply that images can be static bitmaps (e.g. PNGs, GIFs, JPEGs), single-page vector documents (single-page PDFs, XML files with an SVG document element), animated bitmaps (APNGs, animated GIFs), animated vector graphics (XML files with an SVG document element that use declarative SMIL animation), and so forth. However, these definitions preclude SVG files with script, multipage PDF files, interactive MNG files, HTML documents, plain text documents, and so forth. [PNG] [GIF] [JPEG] [PDF] [XML] [APNG] [SVG] [MNG]
The srcset attribute may also be present, and is a
srcset attribute.
The srcset attribute and the src attribute (if width
descriptors are not used) contribute the image sources
to the source set (if no source element was selected).
If the srcset attribute is present and has any image candidate strings using a width
descriptor, the sizes attribute must also
be present, and is a sizes attribute. The sizes
attribute contributes the source size to the source set (if no
source element was selected).
Support in all current engines.
The crossorigin attribute is a CORS
settings attribute. Its purpose is to allow images from third-party sites that allow
cross-origin access to be used with canvas.
The referrerpolicy attribute is a
referrer policy attribute. Its purpose is to set the referrer policy
used when fetching the image. [REFERRERPOLICY]
The decoding attribute indicates the
preferred method to decode this image. The attribute,
if present, must be an image decoding hint. This attribute's missing value default and invalid value
default are both the auto state.
The loading attribute is a lazy
loading attribute. Its purpose is to indicate the policy for loading images that are
outside the viewport.
When the loading attribute's state is changed to the
Eager state, the user agent must run these
steps:
Let resumptionSteps be the img element's lazy load
resumption steps.
If resumptionSteps is null, then return.
Set the img's lazy load resumption steps to null.
Invoke resumptionSteps.
<img src="1.jpeg" alt="1">
<img src="2.jpeg" loading=eager alt="2">
<img src="3.jpeg" loading=lazy alt="3">
<div id=very-large></div> <!-- Everything after this div is below the viewport -->
<img src="4.jpeg" alt="4">
<img src="5.jpeg" loading=lazy alt="5">In the example above, the images load as follows:
1.jpeg, 2.jpeg,
4.jpegThe images load eagerly and delay the window's load event.
3.jpegThe image loads when layout is known, due to being in the viewport, however it does not delay the window's load event.
5.jpegThe image loads only once scrolled into the viewport, and does not delay the window's load event.
Developers are encouraged to specify an intrinsic aspect ratio via width and height attributes
on lazy loaded images, even if CSS sets the image's width and height properties, to prevent the
page layout from shifting around after the image loads.
The img element must not be used as a layout tool. In particular, img
elements should not be used to display transparent images, as such images rarely convey meaning and
rarely add anything useful to the document.
What an img element represents depends on the src attribute and the alt
attribute.
src attribute is set and the alt attribute is set to the empty stringThe image is either decorative or supplemental to the rest of the content, redundant with some other information in the document.
If the image is available and the user agent is configured to display that image, then the element represents the element's image data.
Otherwise, the element represents nothing, and may be omitted completely from the rendering. User agents may provide the user with a notification that an image is present but has been omitted from the rendering.
src attribute is set and the alt attribute is set to a value that isn't emptyThe image is a key part of the content; the alt attribute
gives a textual equivalent or replacement for the image.
If the image is available and the user agent is configured to display that image, then the element represents the element's image data.
Otherwise, the element represents the text given by the alt attribute. User agents may provide the user with a notification
that an image is present but has been omitted from the rendering.
src attribute is set and the alt attribute is notThe image might be a key part of the content, and there is no textual equivalent of the image available.
In a conforming document, the absence of the alt attribute indicates that the image is a key part of the content
but that a textual replacement for the image was not available when the image was generated.
If the image is available and the user agent is configured to display that image, then the element represents the element's image data.
If the image has a src attribute whose value is
the empty string, then the element represents nothing.
Otherwise, the user agent should display some sort of indicator that there is an image that is not being rendered, and may, if requested by the user, or if so configured, or when required to provide contextual information in response to navigation, provide caption information for the image, derived as follows:
If the image has a title attribute whose value is not
the empty string, then return the value of that attribute.
If the image is a descendant of a figure element that has a child
figcaption element, and, ignoring the figcaption element and its
descendants, the figure element has no flow content descendants other
than inter-element whitespace and the img element, then return the
contents of the first such figcaption element.
Return nothing. (There is no caption information.)
src attribute is not set and either the alt attribute is set to the empty string or the alt attribute is not set at allThe element represents nothing.
The element represents the text given by the alt attribute.
The alt attribute does not represent advisory information.
User agents must not present the contents of the alt attribute
in the same way as content of the title attribute.
User agents may always provide the user with the option to display any image, or to prevent any image from being displayed. User agents may also apply heuristics to help the user make use of the image when the user is unable to see it, e.g. due to a visual disability or because they are using a text terminal with no graphics capabilities. Such heuristics could include, for instance, optical character recognition (OCR) of text found within the image.
While user agents are encouraged to repair cases of missing alt attributes, authors must not rely on such behavior. Requirements for providing text to act as an alternative for images are described
in detail below.
The contents of img elements, if any, are ignored for the purposes of
rendering.
The usemap attribute,
if present, can indicate that the image has an associated
image map.
The ismap
attribute, when used on an element that is a descendant of an
a element with an href attribute, indicates by its
presence that the element provides access to a server-side image
map. This affects how events are handled on the corresponding
a element.
The ismap attribute is a
boolean attribute. The attribute must not be specified
on an element that does not have an ancestor a element
with an href attribute.
The usemap and ismap attributes can result in confusing behavior when used
together with source elements with the media
attribute specified in a picture element.
The img element supports dimension
attributes.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
The alt, src, srcset and sizes IDL attributes must reflect the
respective content attributes of the same name.
Support in all current engines.
The crossOrigin IDL attribute must
reflect the crossorigin content attribute,
limited to only known values.
Support in all current engines.
The useMap IDL attribute must
reflect the usemap content attribute.
Support in all current engines.
The isMap IDL attribute must reflect
the ismap content attribute.
HTMLImageElement/referrerPolicy
Support in all current engines.
The referrerPolicy IDL attribute must
reflect the referrerpolicy
content attribute, limited to only known values.
Support in all current engines.
The decoding
IDL attribute must reflect the decoding
content attribute, limited to only known values.
Support in all current engines.
The loading
IDL attribute must reflect the loading content
attribute, limited to only known values.
width [ = value ]Support in all current engines.
height [ = value ]Support in all current engines.
These attributes return the actual rendered dimensions of the image, or zero if the dimensions are not known.
They can be set, to change the corresponding content attributes.
naturalWidthSupport in all current engines.
naturalHeightHTMLImageElement/naturalHeight
Support in all current engines.
These attributes return the intrinsic dimensions of the image, or zero if the dimensions are not known.
completeSupport in all current engines.
Returns true if the image has been completely downloaded or if no image is specified; otherwise, returns false.
currentSrcSupport in all current engines.
Returns the image's absolute URL.
decode()Support in all current engines.
This method causes the user agent to decode the image in parallel, returning a promise that fulfills when decoding is complete.
The promise will be rejected with an "EncodingError"
DOMException if the image cannot be decoded.
Image( [ width [, height ] ] )Support in all current engines.
Returns a new img element, with the width and height attributes set to the values
passed in the relevant arguments, if applicable.
The IDL attributes width and height must return the rendered width and height of the
image, in CSS pixels, if the image is being rendered, and
is being rendered to a visual medium; or else the density-corrected intrinsic width and
height of the image, in CSS pixels, if the image has
intrinsic dimensions and is available but not being
rendered to a visual medium; or else 0, if the image is not available or does not have intrinsic dimensions. [CSS]
On setting, they must act as if they reflected the respective content attributes of the same name.
The IDL attributes naturalWidth and
naturalHeight must return the
density-corrected intrinsic width and height of the image, in CSS pixels, if the image has intrinsic dimensions and is available, or else 0. [CSS]
Since the intrinsic dimensions of an image take into account any
orientation specified in its metadata, naturalWidth and
naturalHeight reflect the dimensions after applying
any rotation needed to correctly orient the image, regardless of the value of the
'image-orientation' property.
The IDL attribute complete must return true if
any of the following conditions is true:
src attribute and the srcset attribute are omitted.
srcset attribute is omitted and the src attribute's value is the empty string.
img element's current request's state is completely available and its
pending request is null.
img element's current request's state is broken and its
pending request is null.
Otherwise, the attribute must return false.
The currentSrc IDL attribute
must return the img element's current request's current URL.
The decode() method, when invoked, must perform
the following steps:
Let promise be a new promise.
Queue a microtask to perform the following steps:
This is done because updating the image data takes place in a microtask as well. Thus, to make code such as
img.src = "stars.jpg";
img.decode();properly decode stars.jpg, we need to delay any processing by one
microtask.
If any of the following conditions are true about this img element:
its node document is not an active document;
its current request's state is broken,
then reject promise with an "EncodingError"
DOMException.
Otherwise, in parallel, wait for one of the following cases to occur, and perform the corresponding actions:
img element's node document stops being an active
documentimg element's current request changes or is mutatedimg element's current request's state becomes brokenReject promise with an "EncodingError"
DOMException.
img element's current request's state becomes completely
availableDecode the image.
If decoding does not need to be performed for this image (for example because it is a vector graphic), resolve promise with undefined.
If decoding fails (for example due to invalid image data), reject promise with
an "EncodingError" DOMException.
If the decoding process completes successfully, resolve promise with undefined.
User agents should ensure that the decoded media data stays readily available until at least the end of the next successful update the rendering step in the event loop. This is an important part of the API contract, and should not be broken if at all possible. (Typically, this would only be violated in low-memory situations that require evicting decoded image data, or when the image is too large to keep in decoded form for this period of time.)
Animated images will become completely available only after all their frames are loaded. Thus, even though an implementation could decode the first frame before that point, the above steps will not do so, instead waiting until all frames are available.
Return promise.
Without the decode() method, the process of loading an
img element and then displaying it might look like the following:
const img = new Image();
img.src = "nebula.jpg";
img.onload = () => {
document.body.appendChild(img);
};
img.onerror = () => {
document.body.appendChild(new Text("Could not load the nebula :("));
};However, this can cause notable dropped frames, as the paint that occurs after inserting the image into the DOM causes a synchronous decode on the main thread.
This can instead be rewritten using the decode()
method:
const img = new Image();
img.src = "nebula.jpg";
img.decode().then(() => {
document.body.appendChild(img);
}).catch(() => {
document.body.appendChild(new Text("Could not load the nebula :("));
});This latter form avoids the dropped frames of the original, by allowing the user agent to decode the image in parallel, and only inserting it into the DOM (and thus causing it to be painted) once the decoding process is complete.
Because the decode() method attempts to ensure that the
decoded image data is available for at least one frame, it can be combined with the requestAnimationFrame() API.
This means it can be used with coding styles or frameworks that ensure that all DOM modifications
are batched together as animation frame
callbacks:
const container = document.querySelector("#container");
const { containerWidth, containerHeight } = computeDesiredSize();
requestAnimationFrame(() => {
container.style.width = containerWidth;
container.style.height = containerHeight;
});
// ...
const img = new Image();
img.src = "supernova.jpg";
img.decode().then(() => {
requestAnimationFrame(() => container.appendChild(img));
});A legacy factory function is provided for creating HTMLImageElement objects (in
addition to the factory methods from DOM such as createElement()): Image(width, height). When invoked,
the legacy factory function must perform the following steps:
Let document be the current global object's associated Document.
Let img be the result of creating an
element given document, img, and the HTML
namespace.
If width is given, then set
an attribute value for img using "width"
and width.
If height is given, then set an attribute value for img
using "height" and height.
Return img.
A single image can have different appropriate alternative text depending on the context.
In each of the following cases, the same image is used, yet the alt text is different each time. The image is the coat of arms of the
Carouge municipality in the canton Geneva in Switzerland.
Here it is used as a supplementary icon:
<p>I lived in <img src="carouge.svg" alt=""> Carouge.</p>Here it is used as an icon representing the town:
<p>Home town: <img src="carouge.svg" alt="Carouge"></p>Here it is used as part of a text on the town:
<p>Carouge has a coat of arms.</p>
<p><img src="carouge.svg" alt="The coat of arms depicts a lion, sitting in front of a tree."></p>
<p>It is used as decoration all over the town.</p>Here it is used as a way to support a similar text where the description is given as well as, instead of as an alternative to, the image:
<p>Carouge has a coat of arms.</p>
<p><img src="carouge.svg" alt=""></p>
<p>The coat of arms depicts a lion, sitting in front of a tree.
It is used as decoration all over the town.</p>Here it is used as part of a story:
<p>She picked up the folder and a piece of paper fell out.</p>
<p><img src="carouge.svg" alt="Shaped like a shield, the paper had a
red background, a green tree, and a yellow lion with its tongue
hanging out and whose tail was shaped like an S."></p>
<p>She stared at the folder. S! The answer she had been looking for all
this time was simply the letter S! How had she not seen that before? It all
came together now. The phone call where Hector had referred to a lion's tail,
the time Maria had stuck her tongue out...</p>Here it is not known at the time of publication what the image will be, only that it will be a
coat of arms of some kind, and thus no replacement text can be provided, and instead only a brief
caption for the image is provided, in the title attribute:
<p>The last user to have uploaded a coat of arms uploaded this one:</p>
<p><img src="last-uploaded-coat-of-arms.cgi" title="User-uploaded coat of arms."></p>Ideally, the author would find a way to provide real replacement text even in this case, e.g. by asking the previous user. Not providing replacement text makes the document more difficult to use for people who are unable to view images, e.g. blind users, or users or very low-bandwidth connections or who pay by the byte, or users who are forced to use a text-only web browser.
Here are some more examples showing the same picture used in different contexts, with different appropriate alternate texts each time.
<article>
<h1>My cats</h1>
<h2>Fluffy</h2>
<p>Fluffy is my favorite.</p>
<img src="fluffy.jpg" alt="She likes playing with a ball of yarn.">
<p>She's just too cute.</p>
<h2>Miles</h2>
<p>My other cat, Miles just eats and sleeps.</p>
</article><article>
<h1>Photography</h1>
<h2>Shooting moving targets indoors</h2>
<p>The trick here is to know how to anticipate; to know at what speed and
what distance the subject will pass by.</p>
<img src="fluffy.jpg" alt="A cat flying by, chasing a ball of yarn, can be
photographed quite nicely using this technique.">
<h2>Nature by night</h2>
<p>To achieve this, you'll need either an extremely sensitive film, or
immense flash lights.</p>
</article><article>
<h1>About me</h1>
<h2>My pets</h2>
<p>I've got a cat named Fluffy and a dog named Miles.</p>
<img src="fluffy.jpg" alt="Fluffy, my cat, tends to keep itself busy.">
<p>My dog Miles and I like go on long walks together.</p>
<h2>music</h2>
<p>After our walks, having emptied my mind, I like listening to Bach.</p>
</article><article>
<h1>Fluffy and the Yarn</h1>
<p>Fluffy was a cat who liked to play with yarn. She also liked to jump.</p>
<aside><img src="fluffy.jpg" alt="" title="Fluffy"></aside>
<p>She would play in the morning, she would play in the evening.</p>
</article>This section is non-normative.
在 HTML 中嵌入图片时，如果只有一个图片资源，就使用 img
元素和它的 src 属性。
<h2>From today's featured article</h2> <img src="/uploads/100-marie-lloyd.jpg" alt="" width="100" height="150"> <p><b><a href="/wiki/Marie_Lloyd">Marie Lloyd</a></b> (1870–1922) was an English <a href="/wiki/Music_hall">music hall</a> singer, ...
但是有些情况下作者希望提供多张图片让用户代理选择：
不同用户可能有不同的环境特征：
用户的物理屏幕大小可能是不一样的。
手机屏幕可能是 4 英寸，但笔记本屏幕可能是 14 英寸。
这只在图片渲染大小依赖 视口 大小时有用。
用户的屏幕像素密度可能是不一样的。
手机屏幕之间每英寸的物理像素可能有三倍之差，与物理屏幕大小无关。
用户的缩放级别可能是不一样的，同一用户在不同时间也可能不一样。
用户可能会放大某个图片来查看细节。
缩放级别和屏幕像素密度（上一点）都会影响单个 CSS pixel 上的物理屏幕像素。这个比例通常称为 设备像素比。
用户的屏幕方向也可能不同，同一用户在不同时间也可能不一样。
平板电脑可以直立或旋转 90 度，所以屏幕可能是 "portrait" 或 "landscape"。
用户的网络速度、网络延迟和带宽价格也可能不一样，同一用户不同时间也可能不一样。
用户可能工作时在一个快速的低延迟的单一价格的连接上， 在家时在一个低速的低延迟的单一价格的连接上， 其他地方时在一个速度变化的高延迟的价格变化的连接上。
作者可能要显示同一张图片，但渲染大小根据 viewport 有所不同。 这通常称为 基于视口的选择。
网页上方可能有一个横幅，始终横跨整个 视口 宽度。 这种情况下，图片的渲染大小取决于屏幕的物理大小（假设浏览器窗口是最大化的）。
另一个网页可能会分列显示图片，在物理大小较小的屏幕上显示一列， 中等物理大小的屏幕上显示两列，在物理大小较大的屏幕上显示三列。 图片以可变大小渲染来充满 视口。 这种情况下，单列布局下图片的渲染大小可能比两列布局下 大，虽然屏幕更小。
作者可能会根据图片的渲染大小显示不同的图片内容。这通常称为 美术设计。
当网页在较大的物理像素的屏幕上查看时（假设浏览器窗口最大化了）， 作者可能希望在图片中重要的部分周围包含一些不太相关的部分。 当同样的网页在较小的物理像素的屏幕上查看时，作者可能希望只显示图片中重要的部分。
作者可能希望显示同样的图片内容但根据用户代理的支持情况使用不同的图片格式。 这通常被称为 基于图片格式的选择。
一个网页可能有 JPEG, WebP 和 JPEG XR 等各种格式的图片， 后面两个相比 JPEG 有更好的压缩能力。由于不同的用户代理可能支持不同的图片格式， 有些格式提供了更好的压缩比，作者可能希望给支持的浏览器提供这些格式，不支持的用户代理提供 JPEG。
上述各种情形不是互斥的。例如，结合不同的 设备像素比 和不同的 美术设计 来提供不同的资源也是合理的。
虽然可以使用脚本来解决这些问题，但这样会引起一些其他问题：
有些用户代理会先下载 HTML 标签中的图片，之后再执行脚本，这样页面可以更快完成加载。 如果脚本改变了要下载的图片，用户代理可能会开启两个独立的下载，会导致更坏的页面加载性能。
如果作者避免在 HTML 标签中指定图片，而是从脚本中发起一个下载来避免上述两个下载的问题， 那么对于禁用脚本的用户就不会下载任何脚本，预先下载图片的优化也会失效。
考虑到这些，本规范引入了一些功能，以声明的方式解决上述问题。
在 img 元素上可以使用
src 和 srcset 属性，
使用 x 描述符来提供多个只有大小不同的图片（小图是大图的缩略版）。
当图片的渲染大小依赖于 viewport 宽度时
（基于视口的选择），
使用 x 描述符不合适，
但该描述符可以与 美术设计 一起使用。
<h2>From today's featured article</h2>
<img src="/uploads/100-marie-lloyd.jpg"
srcset="/uploads/150-marie-lloyd.jpg 1.5x, /uploads/200-marie-lloyd.jpg 2x"
alt="" width="100" height="150">
<p><b><a href="/wiki/Marie_Lloyd">Marie Lloyd</a></b> (1870–1922)
was an English <a href="/wiki/Music_hall">music hall</a> singer, ...
用户代理可以根据用户屏幕像素密度、缩放级别，以及可能的其他因素比如用户的网络条件选择任何一个给出的资源。
为了向后兼容其他老的不知道 srcset 属性的用户代理 ，
其中一个 URL 在 img 元素的 src 属性上也有指定。
这样即使在老的用户代理中也能显示一些有用的（虽然可能比用户预期的分辨率低）东西。
对于新的用户代理，src 属性也会参与资源选择，就像它被写在
srcset 中并有一个 1x 描述符一样。
可以使用 srcset 和 sizes 属性，使用 w 描述符来提供多个只有大小不同的图片
（小图是大图的缩略版）。
在这个例子中，有一个横幅（使用适当的 CSS）横跨整个 视口 宽度。
<h1><img sizes="100vw" srcset="wolf-400.jpg 400w, wolf-800.jpg 800w, wolf-1600.jpg 1600w"
src="wolf-400.jpg" alt="The rad wolf"></h1>
用户代理将会根据指定的 w 描述符和 sizes
属性指定的渲染大小为每个图片计算等效像素密度。
然后它可以根据用户屏幕的像素密度、缩放级别和任何可能的其他因素（比如用户网络状态）选择任何一个资源。
如果用户屏幕是 320 CSS 像素 宽，这等价于指定
wolf-400.jpg 1.25x, wolf-800.jpg 2.5x, wolf-1600.jpg 5x。
另一方面，如果用户屏幕是 1200 CSS 像素 宽，这等价于指定
wolf-400.jpg 0.33x, wolf-800.jpg 0.67x, wolf-1600.jpg 1.33x.。
通过使用 w 描述符和 sizes 属性，
不论用户设备有多大用户代理都可以选择下载正确的图片资源。
为了向后兼容，其中一个 URL 也指定在 img 元素的
src 属性上。在新的用户代理中，当
srcset 属性使用 w 描述符时，
src 属性会被忽略。
在这个例子中，网页基于 视口 宽度有三个布局。
窄的布局有一列图片（每个图片宽度大约是 100%），
中等布局有两列图片（每个图片宽度大约是 50%），
最宽的布局有三列图片和一些页面边距（每个图片大约是 33%）。
在 视口 宽度为
30em 和 50em 时会切换布局。
<img sizes="(max-width: 30em) 100vw, (max-width: 50em) 50vw, calc(33vw - 100px)"
srcset="swing-200.jpg 200w, swing-400.jpg 400w, swing-800.jpg 800w, swing-1600.jpg 1600w"
src="swing-400.jpg" alt="Kettlebell Swing">
sizes 属性设置了布局的切换点为
30em 和 50em，也声明了在这些切换点之间图片大小为
100vw, 50vw, 或 calc(33vw - 100px)。
这些大小不一定要精确匹配 CSS 中指定的真正的图片宽度。
用户代理会从 sizes 属性中选一个宽度，
使用第一个 <media-condition> （括号中的部分）结果为真的大小，
如果所有结果都为假就使用最后一项（calc(33vw - 100px)）。
例如，如果 视口 宽度为 29em，那么
(max-width: 30em) 计算为真，就会使用 100vw，
所以资源选择后的图片大小就是 29em。如果
视口 宽度是 32em，那么
(max-width: 30em) 计算为假，但 (max-width: 50em)
计算为真，就会使用 50vw，所以资源选择后图片大小就是
16em（半个 视口 宽度）。
注意更宽一点的 视口 因为布局不同而导致图片更小。
用户代理可以计算等效像素密度并选择合适的资源，类似上一个例子那样。
可以使用 picture 元素和 source 元素和
media 属性提供多张内容不同的图片
（例如小的图片是大图的裁剪版）。
<picture> <source media="(min-width: 45em)" srcset="large.jpg"> <source media="(min-width: 32em)" srcset="med.jpg"> <img src="small.jpg" alt="The wolf runs through the snow."> </picture>
用户代理会选择 media 属性中的媒体查询匹配的第一个
source 元素，然后从它的 srcset
属性中选择一个合适的 URL。
图片的渲染大小取决于选择了哪张图片。可以使用 CSS 来指定用户代理下载图片之前使用的宽高。
img { width: 300px; height: 300px }
@media (min-width: 32em) { img { width: 500px; height:300px } }
@media (min-width: 45em) { img { width: 700px; height:400px } }
可以使用 source 元素上的 type
属性来提供多种格式的图片。
<h2>From today's featured article</h2> <picture> <source srcset="/uploads/100-marie-lloyd.webp" type="image/webp"> <source srcset="/uploads/100-marie-lloyd.jxr" type="image/vnd.ms-photo"> <img src="/uploads/100-marie-lloyd.jpg" alt="" width="100" height="150"> </picture> <p><b><a href="/wiki/Marie_Lloyd">Marie Lloyd</a></b> (1870–1922) was an English <a href="/wiki/Music_hall">music hall</a> singer, ...
在这个例子中，用户代理会选择第一个
type 属性为支持的 MIME 类型的源。
如果用户代理支持 WebP 就会选择第一个 source 元素。
如果不支持 WebP，但支持 JPEG XR 就会选择第二个 source 元素。
如果这两种都不支持，就会选择 img 元素。
This section is non-normative.
可以使用 CSS 和媒体查询来构造动态适应用户环境的页面布局，
特别是对不同的 视口 宽高和像素密度。
但 CSS 无法操作内容；这时可以使用 img 元素的
srcset 属性和 picture 元素。
本节将介绍一个示例案例，演示如何使用这些功能。
考虑一个场景，在宽屏（宽于 600 CSS 像素）上要使用一个
300×150 的图片 a-rectangle.png，
但在小一点的屏幕上（不大于 600 CSS 像素）要使用一个
100×100 的图片 a-square.png。这样的标记可能像这样：
<figure> <picture> <source srcset="a-square.png" media="(max-width: 600px)"> <img src="a-rectangle.png" alt="Barney Frank wears a suit and glasses."> </picture> <figcaption>Barney Frank, 2011</figcaption> </figure>
在 alt 属性中放什么内容，参考
图片替代文本的要求 一节。
问题在于用户代理在图片加载时不知道使用什么样的宽高。 为了避免页面加载中布局被重排很多次，可以使用 CSS 和 CSS 媒体查询来提供宽高：
<style>
#a { width: 300px; height: 150px; }
@media (max-width: 600px) { #a { width: 100px; height: 100px; } }
</style>
<figure>
<picture>
<source srcset="a-square.png" media="(max-width: 600px)">
<img src="a-rectangle.png" alt="Barney Frank wears a suit and glasses." id="a">
</picture>
<figcaption>Barney Frank, 2011</figcaption>
</figure>
也可以使用 width 和 height 使用来为提供旧的用户代理提供宽高，
CSS 只用于支持 picture 的用户代理：
<style media="(max-width: 600px)">
#a { width: 100px; height: 100px; }
</style>
<figure>
<picture>
<source srcset="a-square.png" media="(max-width: 600px)">
<img src="a-rectangle.png" width="300" height="150"
alt="Barney Frank wears a suit and glasses." id="a">
</picture>
<figcaption>Barney Frank, 2011</figcaption>
</figure>
img 元素指定了 src 属性，
它为不支持 picture 的旧的用户代理给出了图片的 URL。
这就要讨论 src 属性应该提供哪个图片的问题了。
如果作者在旧的用户代理中想要最大的图片，标记可以这样写：
<picture> <source srcset="pear-mobile.jpeg" media="(max-width: 720px)"> <source srcset="pear-tablet.jpeg" media="(max-width: 1280px)"> <img src="pear-desktop.jpeg" alt="The pear is juicy."> </picture>
但如果旧的用户代理更重要，就可以把所有三个图片都列在
source 元素中，完全重写掉 src 属性。
<picture> <source srcset="pear-mobile.jpeg" media="(max-width: 720px)"> <source srcset="pear-tablet.jpeg" media="(max-width: 1280px)"> <source srcset="pear-desktop.jpeg"> <img src="pear-mobile.jpeg" alt="The pear is juicy."> </picture>
因为在这里 src 属性在支持 picture
的用户代理下会被完全忽略，src 属性可以默认写成任何图片，
包括既不是最大的也不是最小的那个：
<picture> <source srcset="pear-mobile.jpeg" media="(max-width: 720px)"> <source srcset="pear-tablet.jpeg" media="(max-width: 1280px)"> <source srcset="pear-desktop.jpeg"> <img src="pear-tablet.jpeg" alt="The pear is juicy."> </picture>
上面用到了 max-width 媒体特性，给出了该图片适用的最大
（视口）大小。也可以使用 min-width。
<picture> <source srcset="pear-desktop.jpeg" media="(min-width: 1281px)"> <source srcset="pear-tablet.jpeg" media="(min-width: 721px)"> <img src="pear-mobile.jpeg" alt="The pear is juicy."> </picture>
source,
img, and link elementsA srcset attribute is an attribute with requirements defined in this section.
If present, its value must consist of one or more image candidate strings, each separated from the next by a U+002C COMMA character (,). If an image candidate string contains no descriptors and no ASCII whitespace after the URL, the following image candidate string, if there is one, must begin with one or more ASCII whitespace.
An image candidate string consists of the following components, in order, with the further restrictions described below this list:
Zero or more ASCII whitespace.
A valid non-empty URL that does not start or end with a U+002C COMMA character (,), referencing a non-interactive, optionally animated, image resource that is neither paged nor scripted.
Zero or more ASCII whitespace.
Zero or one of the following:
A width descriptor, consisting of: ASCII whitespace, a valid non-negative integer giving a number greater than zero representing the width descriptor value, and a U+0077 LATIN SMALL LETTER W character.
A pixel density descriptor, consisting of: ASCII whitespace, a valid floating-point number giving a number greater than zero representing the pixel density descriptor value, and a U+0078 LATIN SMALL LETTER X character.
Zero or more ASCII whitespace.
There must not be an image candidate string for an element that has the same width descriptor value as another image candidate string's width descriptor value for the same element.
There must not be an image candidate string for an element that has the same
pixel density descriptor value as another image candidate string's
pixel density descriptor value for the same element. For the purpose of this
requirement, an image candidate string with no descriptors is equivalent to an
image candidate string with a 1x descriptor.
If an image candidate string for an element has the width descriptor specified, all other image candidate strings for that element must also have the width descriptor specified.
The specified width in an image candidate string's width descriptor must match the intrinsic width in the resource given by the image candidate string's URL, if it has an intrinsic width.
If an element has a sizes attribute present, all image candidate strings for that element must have the width descriptor specified.
A sizes attribute is an attribute with requirements defined in this section.
If present, the value must be a valid source size list.
A valid source size list is a string that matches the following grammar: [CSSVALUES] [MQ]
<source-size-list> = [ <source-size># , ]? <source-size-value>
<source-size> = <media-condition> <source-size-value>
<source-size-value> = <length>A <source-size-value> must not be negative, and must not use CSS functions other than the math functions.
The <source-size-value> gives the intended layout width of the image. The author can specify different widths for different environments with <media-condition>s.
Percentages are not allowed in a <source-size-value>, to avoid confusion about what it would be relative to. The 'vw' unit can be used for sizes relative to the viewport width.
An img element has a current request and a pending request.
The current request is initially set to a new image request.
The pending request is initially set to null.
An image request has a state, current URL, and image data.
An image request's state is one of the following:
An image request's current URL is initially the empty string.
An image request's image data is the decoded image data.
When an image request's state is either partially available or completely available, the image request is said to be available.
When an img element's current request's state is completely available and the
user agent can decode the media data without errors, then the img element is said to
be fully decodable.
An image request's state is initially unavailable.
When an img element's current request is available, the img element provides a paint
source whose width is the image's density-corrected intrinsic width (if any), whose height is the image's density-corrected intrinsic height
(if any), and whose appearance is the intrinsic appearance of the image.
An img element is said to use srcset or
picture if it has a srcset attribute
specified or if it has a parent that is a picture element.
Each img element has a last selected source, which must initially be
null.
Each image request has a current pixel density, which must initially be undefined.
When an img element has a current pixel density that is not 1.0, the
element's image data must be treated as if its resolution, in device pixels per CSS pixels, was the current pixel density. The image's
density-corrected intrinsic width and height are the intrinsic width and height after taking into account the current pixel
density.
For example, if the current pixel density is 3.125, that means that there are 300 device pixels per CSS inch, and thus if the image data is 300x600, it has intrinsic dimensions of 96 CSS pixels by 192 CSS pixels.
All img and link elements are associated with a source set.
A source set is an ordered set of zero or more image sources and a source size.
An image source is a URL, and optionally either a pixel density descriptor, or a width descriptor.
A source size is a <source-size-value>.
When a source size has a unit relative to the viewport,
it must be interpreted relative to the img element's node document's
viewport.
Other units must be interpreted the same as in Media Queries. [MQ]
A parse error for algorithms in this section indicates a non-fatal mismatch between input and requirements. User agents are encouraged to expose parse errors somehow.
Whether the image is fetched successfully or not (e.g. whether the response status was an ok status) must be ignored when determining the image's type and whether it is a valid image.
This allows servers to return images with error responses, and have them displayed.
The user agent should apply the image sniffing rules to determine the type of the image, with the image's associated Content-Type headers giving the official type. If these rules are not applied, then the type of the image must be the type given by the image's associated Content-Type headers.
User agents must not support non-image resources with the img element (e.g. XML
files whose document element is an HTML element). User agents must not run executable
code (e.g. scripts) embedded in the image resource. User agents must only display the first page
of a multipage resource (e.g. a PDF file). User agents must not allow the resource to act in an
interactive fashion, but should honour any animation in the resource.
This specification does not specify which image types are to be supported.
By default, images are obtained immediately. User agents may provide users with the option to instead obtain them on-demand. (The on-demand option might be used by bandwidth-constrained users, for example.)
When obtaining images immediately, the user agent must synchronously update the image
data of the img element, with the restart animation flag set if so
stated, whenever that element is created or has experienced relevant mutations.
When obtaining images on demand, the user agent must update the image data of an
img element whenever it needs the image data (i.e., on demand), but only if the
img element's current request's state is unavailable. When an
img element has experienced relevant mutations, if the user agent only
obtains images on demand, the img element's current request's state must return to unavailable.
img 元素 相关的变化 包括：
该元素的 src 属性设置为与之前的值一样的值。
这时必须为 更新图片数据 算法设置
restart animation 标志。
该元素的 crossorigin 属性状态变化。
该元素的父元素是 picture 元素且
是前驱节点的 source 元素的 srcset, sizes, media 或 type 属性被设置、改变和移除。
执行了该元素的 adopting steps。
Each Document object must have a list of available images. Each image
in this list is identified by a tuple consisting of an absolute URL, a CORS
settings attribute mode, and, if the mode is not No
CORS, an origin.
Each image furthermore has an ignore higher-layer caching flag.
User agents may copy entries from one Document
object's list of available images to another at any time (e.g. when the
Document is created, user agents can add to it all the images that are loaded in
other Documents), but must not change the keys of entries copied in this way when
doing so, and must unset the ignore higher-layer caching flag for the copied entry.
User agents may also remove images from such lists at any time (e.g. to save
memory).
User agents must remove entries in the list of available images as appropriate
given higher-layer caching semantics for the resource (e.g. the HTTP `Cache-Control` response header) when the ignore
higher-layer caching flag is unset.
The list of available images is intended to enable synchronous
switching when changing the src attribute to a URL that has
previously been loaded, and to avoid re-downloading images in the same document even when they
don't allow caching per HTTP. It is not used to avoid re-downloading the same image while the
previous image is still loading.
The user agent can also store the image data separately from the list of available images.
For example, if a resource has the HTTP response header
`Cache-Control: must-revalidate`, and its ignore higher-layer
caching flag is unset, the user agent would remove it from the list of available
images but could keep the image data separately, and use that if the server responds with a
304 Not Modified status.
Image data is usually encoded in order to reduce file size. This means that in order for the user agent to present the image to the screen, the data needs to be decoded. Decoding is the process which converts an image's media data into a bitmap form, suitable for presentation to the screen. Note that this process can be slow relative to other processes involved in presenting content. Thus, the user agent can choose when to perform decoding, in order to create the best user experience.
Image decoding is said to be synchronous if it prevents presentation of other content until it is finished. Typically, this has an effect of atomically presenting the image and any other content at the same time. However, this presentation is delayed by the amount of time it takes to perform the decode.
Image decoding is said to be asynchronous if it does not prevent presentation of other content. This has an effect of presenting non-image content faster. However, the image content is missing on screen until the decode finishes. Once the decode is finished, the screen is updated with the image.
In both synchronous and asynchronous decoding modes, the final content is presented to screen after the same amount of time has elapsed. The main difference is whether the user agent presents non-image content ahead of presenting the final content.
In order to aid the user agent in deciding whether to perform synchronous or asynchronous
decode, the decoding attribute can be set on
img elements. The possible values of the decoding attribute are the following image decoding
hint keywords:
| Keyword | State | Description |
|---|---|---|
sync
| Sync | Indicates a preference to decode this image synchronously for atomic presentation with other content. |
async
| Async | Indicates a preference to decode this image asynchronously to avoid delaying presentation of other content. |
auto
| Auto | Indicates no preference in decoding mode (the default). |
When decoding an image, the user agent should
respect the preference indicated by the decoding
attribute's state. If the state indicated is auto, then the user agent is free to choose any
decoding behavior.
It is also possible to control the decoding behavior using the decode() method. Since the decode() method performs decoding independently from the process responsible for
presenting content to screen, it is unaffected by the decoding attribute.
This algorithm cannot be called from steps running in parallel. If a user agent needs to call this algorithm from steps running in parallel, it needs to queue a task to do so.
When the user agent is to update the image data of an img element,
optionally with the restart animations flag set,
it must run the following steps:
If the element's node document is not the active document, then:
Continue running this algorithm in parallel.
Wait until the element's node document is the active document.
If another instance of this algorithm for this img element was started after this instance
(even if it aborted and is no longer running), then return.
Queue a microtask to continue this algorithm.
If the user agent cannot support images, or its support for images has been disabled, then abort the image request for the current request and the pending request, set current request's state to unavailable, set pending request to null, and return.
Let selected source be null and selected pixel density be undefined.
If the element does not use srcset or picture and
it has a src attribute specified whose value is not the empty
string, then set selected source to the value of the element's src attribute and set selected pixel density to
1.0.
Set the element's last selected source to selected source.
If selected source is not null, then:
Parse selected source, relative to the element's node document. If that is not successful, then abort this inner set of steps. Otherwise, let urlString be the resulting URL string.
Let key be a tuple consisting of urlString, the img
element's crossorigin attribute's mode, and, if that
mode is not No CORS, the node
document's origin.
If the list of available images contains an entry for key, then:
Set the ignore higher-layer caching flag for that entry.
Abort the image request for the current request and the pending request.
Set pending request to null.
Let current request be a new image request whose image data is that of the entry and whose state is completely available.
Update the presentation of the image appropriately.
Set current request's current pixel density to selected pixel density.
Queue an element task on the DOM manipulation task source
given the img element and following steps:
If restart animation is set, then restart the animation.
Set current request's current URL to urlString.
Fire an event named load at the img element.
Abort the update the image data algorithm.
Queue a microtask to perform the rest of this algorithm, allowing the task that invoked this algorithm to continue.
If another instance of this algorithm for this img element was started after
this instance (even if it aborted and is no longer running), then return.
Only the last instance takes effect, to avoid multiple requests when, for
example, the src, srcset,
and crossorigin attributes are all set in
succession.
Let selected source and selected pixel density be the URL and pixel density that results from selecting an image source, respectively.
If selected source is null, then:
Set the current request's state to broken, abort the image request for the current request and the pending request, and set pending request to null.
Queue an element task on the DOM manipulation task source given
the img element and the following steps:
Change the current request's current URL to the empty string.
If the element has a src attribute or it uses srcset or picture, fire an event named error at the img element.
Return.
Parse selected source, relative to the element's node document, and let urlString be the resulting URL string. If that is not successful, then:
Abort the image request for the current request and the pending request.
Set the current request's state to broken.
Set pending request to null.
Queue an element task on the DOM manipulation task
source given the img element and the following steps:
Change the current request's current URL to selected source.
Fire an event named error at the img element.
Return.
If the pending request is not null and urlString is the same as the pending request's current URL, then return.
If urlString is the same as the current request's current URL and current request's state is partially available, then
abort the image request for the pending request, queue an element
task on the DOM manipulation task source given the img element
to restart the animation if restart animation is set, and return.
If the pending request is not null, then abort the image request for the pending request.
Set image request to a new image request whose current URL is urlString.
If current request's state is unavailable or broken, then set the current request to image request. Otherwise, set the pending request to image request.
Let request be the result of creating a potential-CORS request given urlString, "image", and the current state of the element's crossorigin content attribute.
Set request's client to the element's node document's relevant settings object.
If the element uses srcset or
picture, set request's initiator to "imageset".
Set request's referrer
policy to the current state of the element's referrerpolicy attribute.
Let delay load event be true if the img's lazy loading
attribute is in the Eager state, or if
scripting is disabled for the img, and
false otherwise.
If the will lazy load element steps given the img return true,
then:
Set the img's lazy load resumption steps to the rest of this
algorithm starting with the step labeled fetch the image.
Start intersection-observing a lazy loading element for the
img element.
Return.
Fetch the image: Fetch request. Let this instance of the fetching algorithm be associated with image request.
The resource obtained in this fashion, if any, is image request's image data. It can be either CORS-same-origin or
CORS-cross-origin; this affects the origin of the image itself (e.g.
when used on a canvas).
When delay load event is true, fetching the image must delay the load event of the element's node document until the task that is queued by the networking task source once the resource has been fetched (defined below) has been run.
This, unfortunately, can be used to perform a rudimentary port scan of the user's local network (especially in conjunction with scripting, though scripting isn't actually necessary to carry out such an attack). User agents may implement cross-origin access control policies that are stricter than those described above to mitigate this attack, but unfortunately such policies are typically not compatible with existing web content.
Continue the remaining steps in parallel, but without missing any data from fetching.
As soon as possible, jump to the first applicable entry from the following list:
multipart/x-mixed-replaceThe next task that is queued by the networking task source while the image is being fetched must run the following steps:
If image request is the pending request and at least one body part has been completely decoded, abort the image request for the current request, upgrade the pending request to the current request.
Otherwise, if image request is the pending request and the user agent is able to determine that image request's image is corrupted in some fatal way such that the image dimensions cannot be obtained, abort the image request for the current request, upgrade the pending request to the current request, and set the current request's state to broken.
Otherwise, if image request is the current request, its state is unavailable, and the user agent is able to determine image request's image's width and height, set the current request's state to partially available.
Otherwise, if image request is the current request, its state is unavailable, and the user agent is able to determine that image request's image is corrupted in some fatal way such that the image dimensions cannot be obtained, set the current request's state to broken.
Each task that is queued by the networking task source while the image is being
fetched must update the presentation of the image, but as each new body part comes in, it must
replace the previous image. Once one body part has been completely decoded, the user agent
must set the img element's current request's state to completely available and
queue an element task on the DOM manipulation task source given the
img element to fire an event named load at the img element.
The next task that is queued by the networking task source while the image is being fetched must run the following steps:
If the user agent is able to determine image request's image's width and height, and image request is pending request, set image request's state to partially available.
Otherwise, if the user agent is able to determine image request's image's
width and height, and image request is current request, update the
img element's presentation appropriately and set image request's
state to partially
available.
Otherwise, if the user agent is able to determine that image request's
image is corrupted in some fatal way such that the image dimensions cannot be obtained, and
image request is pending request, abort the image
request for the current request and the pending request,
upgrade the pending request to the current request, set current
request's state to broken, and fire an event
named error at the img element.
Otherwise, if the user agent is able to determine that image request's
image is corrupted in some fatal way such that the image dimensions cannot be obtained, and
image request is current request, abort the image
request for image request and fire an
event named error at the img
element.
That task, and each subsequent task, that is queued by the networking task source while the image is being fetched, if image request is the current request, must update the presentation of the image appropriately (e.g., if the image is a progressive JPEG, each packet can improve the resolution of the image).
Furthermore, the last task that is queued by the networking task source once the resource has been fetched must additionally run these steps:
If image request is the pending request,
abort the image request for the current request,
upgrade the pending request to the current request and
update the img element's presentation appropriately.
Set image request to the completely available state.
Add the image to the list of available images using the key key, with the ignore higher-layer caching flag set.
Fire an event named load at the img element.
The image data is not in a supported file format; the user agent must set image
request's state to broken, abort the image request for the current
request and the pending request, upgrade the pending request to the
current request if image request is the pending request, and
then queue an element task on the DOM manipulation task source given
the img element to fire an event named
error at the img element.
While a user agent is running the above algorithm for an element x, there must be a strong reference from the element's node document to the element x, even if that element is not connected.
To abort the image request for an image request image request means to run the following steps:
Forget image request's image data, if any.
Abort any instance of the fetching algorithm for image request, discarding any pending tasks generated by that algorithm.
To upgrade the pending request to the current request for an img element means to run the following steps:
Let the img element's current request be the pending request.
Let the img element's pending request be null.
When asked to select an image source for a given img or
link element el, user agents must do the following:
Update the source set for el.
If el's source set is empty, return null as the URL and undefined as the pixel density.
Otherwise, take el's source set and let it be source set.
If an entry b in source set has the same associated pixel density descriptor as an earlier entry a in source set, then remove entry b. Repeat this step until none of the entries in source set have the same associated pixel density descriptor as an earlier entry.
In a user agent-specific manner, choose one image source from source set. Let this be selected source.
Return selected source and its associated pixel density.
When asked to update the source set for a given img or
link element el, user agents must do the following:
Set el's source set to an empty source set.
Let elements be « el ».
If el is an img element whose parent node is a
picture element, then replace the contents of
elements with el's parent node's child elements, retaining relative
order.
For each child in elements:
If child is el:
Let source set be an empty source set.
If child has a srcset or imagesrcset attribute, parse child's srcset attribute and set
source set to the returned source set.
Parse child's sizes attribute, and let source set's source size be the returned value.
If child has a src or href attribute whose value is not the empty string and
source set does not contain an image source with a pixel
density descriptor value of 1, and no image source with a width
descriptor, append child's src or href attribute value to source set.
Normalize the source densities of source set.
Let el's source set be source set.
Return.
If el is a link element, then elements
contains only el, so this step will be reached immediately and the rest of the
algorithm will not run.
Assert: child is not a link element.
If child is not a source element, continue to the next child.
Otherwise, child is a source element.
If child does not have a srcset
attribute, continue to the next child.
Parse child's srcset attribute and let the returned source set be source set.
If source set has zero image sources, continue to the next child.
If child has a media attribute, and
its value does not match the environment,
continue to the next child.
Parse child's sizes attribute with the fallback width width, and let source set's source size be the returned value.
If child has a type attribute, and its
value is an unknown or unsupported MIME type, continue to the next child.
Normalize the source densities of source set.
Let el's source set be source set.
Return.
Each img element independently considers its previous sibling
source elements plus the img element itself for selecting an image
source, ignoring any other (invalid) elements, including other img elements in
the same picture element, or source elements that are following siblings
of the relevant img element.
When asked to parse a srcset attribute from an element, parse the value of the element's srcset attribute as follows:
Let input be the value passed to this algorithm.
Let position be a pointer into input, initially pointing at the start of the string.
Let candidates be an initially empty source set.
Splitting loop: Collect a sequence of code points that are ASCII whitespace or U+002C COMMA characters from input given position. If any U+002C COMMA characters were collected, that is a parse error.
If position is past the end of input, return candidates.
Collect a sequence of code points that are not ASCII whitespace from input given position, and let that be url.
Let descriptors be a new empty list.
If url ends with U+002C (,), then:
Remove all trailing U+002C COMMA characters from url. If this removed more than one character, that is a parse error.
Otherwise:
Descriptor tokenizer: Skip ASCII whitespace within input given position.
Let current descriptor be the empty string.
Let state be in descriptor.
Let c be the character at position. Do the following depending on the value of state. For the purpose of this step, "EOF" is a special character representing that position is past the end of input.
Do the following, depending on the value of c:
If current descriptor is not empty, append current descriptor to descriptors and let current descriptor be the empty string. Set state to after descriptor.
Advance position to the next character in input. If current descriptor is not empty, append current descriptor to descriptors. Jump to the step labeled descriptor parser.
Append c to current descriptor. Set state to in parens.
If current descriptor is not empty, append current descriptor to descriptors. Jump to the step labeled descriptor parser.
Append c to current descriptor.
Do the following, depending on the value of c:
Append c to current descriptor. Set state to in descriptor.
Append current descriptor to descriptors. Jump to the step labeled descriptor parser.
Append c to current descriptor.
Do the following, depending on the value of c:
Stay in this state.
Jump to the step labeled descriptor parser.
Set state to in descriptor. Set position to the previous character in input.
Advance position to the next character in input. Repeat this step.
In order to be compatible with future additions, this algorithm supports multiple descriptors and descriptors with parens.
Descriptor parser: Let error be no.
Let width be absent.
Let density be absent.
Let future-compat-h be absent.
For each descriptor in descriptors, run the appropriate set of steps from the following list:
If the user agent does not support the sizes attribute,
let error be yes.
A conforming user agent will support the sizes attribute.
However, user agents typically implement and ship features in an incremental manner in practice.
If width and density are not both absent, then let error be yes.
Apply the rules for parsing non-negative integers to the descriptor. If the result is zero, let error be yes. Otherwise, let width be the result.
If width, density and future-compat-h are not all absent, then let error be yes.
Apply the rules for parsing floating-point number values to the descriptor. If the result is less than zero, let error be yes. Otherwise, let density be the result.
If density is zero, the intrinsic dimensions will be infinite. User agents are expected to have limits in how big images can be rendered, which is allowed by the hardware limitations clause.
This is a parse error.
If future-compat-h and density are not both absent, then let error be yes.
Apply the rules for parsing non-negative integers to the descriptor. If the result is zero, let error be yes. Otherwise, let future-compat-h be the result.
Let error be yes.
If future-compat-h is not absent and width is absent, let error be yes.
If error is still no, then append a new image source to candidates whose URL is url, associated with a width width if not absent and a pixel density density if not absent. Otherwise, there is a parse error.
Return to the step labeled splitting loop.
When asked to parse a sizes attribute from an element, parse a comma-separated list of component values from the value of the element's sizes attribute (or the empty string, if the attribute is absent), and let unparsed sizes list be the result. [CSSSYNTAX]
For each unparsed size in unparsed sizes list:
Remove all consecutive <whitespace-token>s from the end of unparsed size. If unparsed size is now empty, that is a parse error; continue to the next iteration of this algorithm.
If the last component value in unparsed size is a valid non-negative <source-size-value>, let size be its value and remove the component value from unparsed size. Any CSS function other than the math functions is invalid. Otherwise, there is a parse error; continue to the next iteration of this algorithm.
Remove all consecutive <whitespace-token>s from the end of unparsed size. If unparsed size is now empty, return size and exit this algorithm. If this was not the last item in unparsed sizes list, that is a parse error.
Parse the remaining component values in unparsed size as a <media-condition>. If it does not parse correctly, or it does parse correctly but the <media-condition> evaluates to false, continue to the next iteration of this algorithm. [MQ]
Return size and exit this algorithm.
If the above algorithm exhausts unparsed sizes list without returning a
size value, then return 100vw.
While a valid source size list only contains a bare <source-size-value> (without an accompanying <media-condition>) as the last entry in the <source-size-list>, the parsing algorithm technically allows such at any point in the list, and will accept it immediately as the size if the preceding entries in the list weren't used. This is to enable future extensions, and protect against simple author errors such as a final trailing comma.
image source 除 URL 之外可以有一个 像素密度描述符, 一个 宽度描述符 或者什么描述符都没有。 source set 规范化后会给每个 image source 一个 像素密度描述符。
当要求 规范化 source set source set 的源像素密度时， 用户代理必须执行以下操作：
令 source size 为 source set 的 source size。
对 source set 中的每个 image source：
如果 image source 有一个 像素密度描述符， 继续下一个 image source。
否则，如果 image source 有一个 宽度描述符，
把 宽度描述符 替换为
像素密度描述符，其
值 为
宽度描述符的值 除以
源大小，单位是 x。
否则，给出 image source 的 像素密度描述符
为 1x。
The user agent may at any time run the following algorithm to update an img
element's image in order to react to changes in the environment. (User agents are not
required to ever run this algorithm; for example, if the user is not looking at the page any
more, the user agent might want to wait until the user has returned to the page before determining
which image to use, in case the environment changes again in the meantime.)
User agents are encouraged to run this algorithm in particular when the user changes
the viewport's size (e.g. by resizing the window or changing the page zoom), and when
an img element is inserted into a
document, so that the density-corrected intrinsic width and height match the
new viewport, and so that the correct image is chosen when art direction
is involved.
Await a stable state. The synchronous section consists of all the remaining steps of this algorithm until the algorithm says the synchronous section has ended. (Steps in synchronous sections are marked with ⌛.)
⌛ If the img element does not use srcset or
picture, its node document is not the active
document, has image data whose resource type is multipart/x-mixed-replace, or
the pending request is not null, then return.
⌛ Let selected source and selected pixel density be the URL and pixel density that results from selecting an image source, respectively.
⌛ If selected source is null, then return.
⌛ If selected source and selected pixel density are the same as the element's last selected source and current pixel density, then return.
⌛ Parse selected source, relative to the element's node document, and let urlString be the resulting URL string. If that is not successful, then return.
⌛ Let corsAttributeState be the state of the element's crossorigin content attribute.
⌛ Let origin be the origin of the img element's node document.
⌛ Let client be the img element's node
document's relevant settings object.
⌛ Let key be a tuple consisting of urlString, corsAttributeState, and, if corsAttributeState is not No CORS, origin.
⌛ Let image request be a new image request whose current URL is urlString
⌛ Let the element's pending request be image request.
End the synchronous section, continuing the remaining steps in parallel.
If the list of available images contains an entry for key, then set image request's image data to that of the entry. Continue to the next step.
Otherwise:
Let request be the result of creating a potential-CORS request given
urlString, "image", and
corsAttributeState.
Set request's client to
client, initiator to "imageset", and set request's synchronous
flag.
Set request's
referrer policy to the current state of
the element's referrerpolicy attribute.
Let response be the result of fetching request.
If response's unsafe response is a network error or
if the image format is unsupported (as determined by applying the image sniffing rules, again as mentioned earlier),
or if the user agent is able to determine that image request's image is corrupted in
some fatal way such that the image dimensions cannot be obtained, or if the resource type is
multipart/x-mixed-replace, then let pending request be null and abort
these steps.
Otherwise, response's unsafe response is image
request's image data. It can be either
CORS-same-origin or CORS-cross-origin; this affects the
origin of the image itself (e.g., when used on a canvas).
Queue an element task on the DOM manipulation task source given
the img element and the following steps:
If the img element has experienced relevant mutations
since this algorithm started, then let pending request be null and abort these
steps.
Let the img element's last selected source be selected source
and the img element's current pixel density be selected pixel density.
Set the image request's state to completely available.
Add the image to the list of available images using the key key, with the ignore higher-layer caching flag set.
Update the img element's presentation appropriately.
Fire an event named load at the img element.
除非另有说明，必须指定 alt
属性且它的值不能为空，且必须适合替代图片。
对 alt 属性具体的要求取决于它要表示什么图片，在下面给出描述。
编写替代文本时要考虑的最一般规则如下：
其目的是用其 alt 属性的文本替换每个图像不会改变页面的含义。
所以，通常在编写替代文本时，考虑如果不能包含一个图片时会怎样写。
基于此的一个推论是 alt 属性的值不应该包含
图片的 caption, title 或 legend 文本。
应该包含的替代文本是用于替代图片的；而不是图片的补充。
用 title 属性可以提供补充信息。
另一个推论是 alt 属性的值不应该重复已经由后面的文字提供的信息。
考虑替代文字的一种方式是，假设你在跟某人打电话， 这时如何直接读出这个包含图片的页面，而不用提这里有一张图片。 无论你说什么来替代图像，这通常都是编写替代文本的良好开端。
当创建 超链接 的 a 元素或 button 元素
没有文本内容只包含一个或更多图片时，alt
属性必须包含一些文字来传达链接或按钮的用途。
在这个例子中，要求用户从三个颜色中选择一个喜欢的颜色。 每个颜色由图片给出，但对于用户代理配置为不显示图片的用户会用颜色名来替代：
<h1>Pick your color</h1> <ul> <li><a href="green.html"><img src="green.jpeg" alt="Green"></a></li> <li><a href="blue.html"><img src="blue.jpeg" alt="Blue"></a></li> <li><a href="red.html"><img src="red.jpeg" alt="Red"></a></li> </ul>
在这个例子中，每个按钮有很多图片来表示用户想要输出的颜色类型。 每种情况都用第一个图片提供替代文本。
<button name="rgb"><img src="red" alt="RGB"><img src="green" alt=""><img src="blue" alt=""></button> <button name="cmyk"><img src="cyan" alt="CMYK"><img src="magenta" alt=""><img src="yellow" alt=""><img src="black" alt=""></button>
由于每个图片表示文字的一部分，也可以写成这样：
<button name="rgb"><img src="red" alt="R"><img src="green" alt="G"><img src="blue" alt="B"></button> <button name="cmyk"><img src="cyan" alt="C"><img src="magenta" alt="M"><img src="yellow" alt="Y"><img src="black" alt="K"></button>
但是对于其他的替代文本，可能不能这样写，把所有替代文本放到每种情况的一张图片中可能更合理：
<button name="rgb"><img src="red" alt="sRGB profile"><img src="green" alt=""><img src="blue" alt=""></button> <button name="cmyk"><img src="cyan" alt="CMYK profile"><img src="magenta" alt=""><img src="yellow" alt=""><img src="black" alt=""></button>
Sometimes something can be more clearly stated in graphical form, for example as a flowchart, a
diagram, a graph, or a simple map showing directions. In such cases, an image can be given using
the img element, but the lesser textual version must still be given, so that users
who are unable to view the image (e.g. because they have a very slow connection, or because they
are using a text-only browser, or because they are listening to the page being read out by a
hands-free automobile voice web browser, or simply because they are blind) are still able to
understand the message being conveyed.
The text must be given in the alt attribute, and must convey
the same message as the image specified in the src
attribute.
It is important to realize that the alternative text is a replacement for the image, not a description of the image.
In the following example we have a flowchart
in image form, with text in the alt attribute rephrasing the
flowchart in prose form:
<p>In the common case, the data handled by the tokenization stage
comes from the network, but it can also come from script.</p>
<p><img src="images/parsing-model-overview.svg" alt="The Network
passes data to the Input Stream Preprocessor, which passes it to the
Tokenizer, which passes it to the Tree Construction stage. From there,
data goes to both the DOM and to Script Execution. Script Execution is
linked to the DOM, and, using document.write(), passes data to the
Tokenizer."></p>Here's another example, showing a good solution and a bad solution to the problem of including an image in a description.
First, here's the good solution. This sample shows how the alternative text should just be what you would have put in the prose if the image had never existed.
<!-- This is the correct way to do things. -->
<p>
You are standing in an open field west of a house.
<img src="house.jpeg" alt="The house is white, with a boarded front door.">
There is a small mailbox here.
</p>Second, here's the bad solution. In this incorrect way of doing things, the alternative text is simply a description of the image, instead of a textual replacement for the image. It's bad because when the image isn't shown, the text doesn't flow as well as in the first example.
<!-- This is the wrong way to do things. -->
<p>
You are standing in an open field west of a house.
<img src="house.jpeg" alt="A white house, with a boarded front door.">
There is a small mailbox here.
</p>Text such as "Photo of white house with boarded door" would be equally bad alternative text
(though it could be suitable for the title attribute or in the
figcaption element of a figure with this image).
一个文档可以包含图标信息，图标是为了帮助视觉浏览器的用户一目了然地识别功能。
有些情况下，图标是文本标签的补充，它们传达同样的含义。
那些情况下，alt 属性必须存在且为空。
下面的图标在文本旁边表达同样的含义，所以它们的 alt 属性为空：
<nav> <p><a href="/help/"><img src="/icons/help.png" alt=""> Help</a></p> <p><a href="/configure/"><img src="/icons/configuration.png" alt=""> Configuration Tools</a></p> </nav>
另外一些情况下，图标旁边没有文字来描述它的含义；这样的图标就应该解释自己的含义。
那些情况下，同等的文本标签必须通过alt 属性给出。
下面是一个新闻站点，每个文章用一个图标来表示对应的话题。
<body> <article> <header> <h1>Ratatouille wins <i>Best Movie of the Year</i> award</h1> <p><img src="movies.png" alt="Movies"></p> </header> <p>Pixar has won yet another <i>Best Movie of the Year</i> award, making this its 8th win in the last 12 years.</p> </article> <article> <header> <h1>Latest TWiT episode is online</h1> <p><img src="podcasts.png" alt="Podcasts"></p> </header> <p>The latest TWiT episode has been posted, in which we hear several tech news stories as well as learning much more about the iPhone. This week, the panelists compare how reflective their iPhones' Apple logos are.</p> </article> </body>
许多页面包含标志（Logo），徽章，旗帜或徽记，代表某个特定实体，如公司，组织，项目，乐队，软件包，国家或某些特定实体。
如果标志（Logo）被用于表示实体，例如作为页面标题。
alt 属性必须包含由徽标表示的实体的名称。
alt 属性 不得 包含类似 “logo” 的文字，因为它传达的不是标志，而是实体本身。
如果标志（Logo）用在它表示的实体的名字旁边，那这个标志就是补充性的，
其 alt 属性必须为空。
如果标志（Logo）仅用作装饰材料（例如品牌标识，或者例如，作为提及徽标所属实体的文章中的配图），那么它适用下文的纯粹装饰性图片的条目。 如果实际上讨论的就是标志，那么它就用作了短语或段落（徽标的描述）并且有替代的图形表示（徽标本身），它适用上面的第一个条目。
在下面的代码片段中，上面四种情况都存在。首先这个标志用于表示一个公司：
<h1><img src="XYZ.gif" alt="The XYZ company"></h1>
然后下面的段落中，在公司名旁边使用标志，所以没有替代文本：
<article> <h2>News</h2> <p>We have recently been looking at buying the <img src="alpha.gif" alt=""> ΑΒΓ company, a small Greek company specializing in our type of product.</p>
在这第三个代码片段中，在边栏中有一个标志，作为一个大的讨论并购的文章的一部分：
<aside><p><img src="alpha-large.gif" alt=""></p></aside> <p>The ΑΒΓ company has had a good quarter, and our pie chart studies of their accounts suggest a much bigger blue slice than its green and orange slices, which is always a good sign.</p> </article>
最后，我们有一个意见部分谈论一个标志，因此在替代文本中细致地描述了这个标志。
<p>Consider for a moment their logo:</p> <p><img src="/images/logo" alt="It consists of a green circle with a green question mark centered inside it."></p> <p>How unoriginal can you get? I mean, oooooh, a question mark, how <em>revolutionary</em>, how utterly <em>ground-breaking</em>, I'm sure everyone will rush to adopt those specifications now! They could at least have tried for some sort of, I don't know, sequence of rounded squares with varying shades of green and bold white outlines, at least that would look good on the cover of a blue book.</p>
图片 不可用 时就会使用文本。 这个例子展示了替代文本应该怎样写，使得这些文本可以无缝地嵌入周围的文本， 就像那里从来没有图片一样。
Sometimes, an image just consists of text, and the purpose of the image is not to highlight the actual typographic effects used to render the text, but just to convey the text itself.
In such cases, the alt attribute must be present but must
consist of the same text as written in the image itself.
Consider a graphic containing the text "Earth Day", but with the letters all decorated with flowers and plants. If the text is merely being used as a heading, to spice up the page for graphical users, then the correct alternative text is just the same text "Earth Day", and no mention need be made of the decorations:
<h1><img src="earthdayheading.png" alt="Earth Day"></h1>An illuminated manuscript might use graphics for some of its images. The alternative text in such a situation is just the character that the image represents.
<p><img src="initials/o.svg" alt="O">nce upon a time and a long long time ago, late at
night, when it was dark, over the hills, through the woods, across a great ocean, in a land far
away, in a small house, on a hill, under a full moon...When an image is used to represent a character that cannot otherwise be represented in Unicode, for example gaiji, itaiji, or new characters such as novel currency symbols, the alternative text should be a more conventional way of writing the same thing, e.g. using the phonetic hiragana or katakana to give the character's pronunciation.
In this example from 1997, a new-fangled currency symbol that looks like a curly E with two bars in the middle instead of one is represented using an image. The alternative text gives the character's pronunciation.
<p>Only <img src="euro.png" alt="euro ">5.99!An image should not be used if characters would serve an identical purpose. Only when the text cannot be directly represented using text, e.g., because of decorations or because there is no appropriate character (as in the case of gaiji), would an image be appropriate.
If an author is tempted to use an image because their default system font does not support a given character, then web Fonts are a better solution than images.
In many cases, the image is actually just supplementary, and its presence merely reinforces the
surrounding text. In these cases, the alt attribute must be
present but its value must be the empty string.
In general, an image falls into this category if removing the image doesn't make the page any less useful, but including the image makes it a lot easier for users of visual browsers to understand the concept.
A flowchart that repeats the previous paragraph in graphical form:
<p>The Network passes data to the Input Stream Preprocessor, which
passes it to the Tokenizer, which passes it to the Tree Construction
stage. From there, data goes to both the DOM and to Script Execution.
Script Execution is linked to the DOM, and, using document.write(),
passes data to the Tokenizer.</p>
<p><img src="images/parsing-model-overview.svg" alt=""></p>In these cases, it would be wrong to include alternative text that consists of just a caption.
If a caption is to be included, then either the title attribute
can be used, or the figure and figcaption elements can be used. In the
latter case, the image would in fact be a phrase or paragraph with an alternative graphical
representation, and would thus require alternative text.
<!-- Using the title="" attribute -->
<p>The Network passes data to the Input Stream Preprocessor, which
passes it to the Tokenizer, which passes it to the Tree Construction
stage. From there, data goes to both the DOM and to Script Execution.
Script Execution is linked to the DOM, and, using document.write(),
passes data to the Tokenizer.</p>
<p><img src="images/parsing-model-overview.svg" alt=""
title="Flowchart representation of the parsing model."></p><!-- Using <figure> and <figcaption> -->
<p>The Network passes data to the Input Stream Preprocessor, which
passes it to the Tokenizer, which passes it to the Tree Construction
stage. From there, data goes to both the DOM and to Script Execution.
Script Execution is linked to the DOM, and, using document.write(),
passes data to the Tokenizer.</p>
<figure>
<img src="images/parsing-model-overview.svg" alt="The Network leads to
the Input Stream Preprocessor, which leads to the Tokenizer, which
leads to the Tree Construction stage. The Tree Construction stage
leads to two items. The first is Script Execution, which leads via
document.write() back to the Tokenizer. The second item from which
Tree Construction leads is the DOM. The DOM is related to the Script
Execution.">
<figcaption>Flowchart representation of the parsing model.</figcaption>
</figure><!-- This is WRONG. Do not do this. Instead, do what the above examples do. -->
<p>The Network passes data to the Input Stream Preprocessor, which
passes it to the Tokenizer, which passes it to the Tree Construction
stage. From there, data goes to both the DOM and to Script Execution.
Script Execution is linked to the DOM, and, using document.write(),
passes data to the Tokenizer.</p>
<p><img src="images/parsing-model-overview.svg"
alt="Flowchart representation of the parsing model."></p>
<!-- Never put the image's caption in the alt="" attribute! -->A graph that repeats the previous paragraph in graphical form:
<p>According to a study covering several billion pages,
about 62% of documents on the web in 2007 triggered the Quirks
rendering mode of web browsers, about 30% triggered the Almost
Standards mode, and about 9% triggered the Standards mode.</p>
<p><img src="rendering-mode-pie-chart.png" alt=""></p>有些图片对内容不那么重要，可能是纯粹装饰性图片，也可能与文本完全冗余。
这些情况下，alt 属性也必须存在，它的值应该是空字符串，
或者是一个图片传达的信息的文本表示。如果图片的标题部分给出了图片标题，那么
alt 属性值不得为空（因为那可能会让非视觉媒体的读者感到困惑）。
考虑一篇关于政治人物的新文章，其中的图片显示了人物的面部，通过样式表浮动到右侧。 这张图片就不是纯装饰性的，因为它与故事相关。这张图片也不完全与故事冗余，因为它显示了政客的长相。 是否提供替代文本取决于作者，也部分取决于这张图片是否对文章起到润色的作用。
下面第一个例子中，图片没有上下文也没有替代文本：
<p><img src="alexsalmond.jpeg" alt=""> Ahead of today's referendum, the First Minister of Scotland, Alex Salmond, wrote an open letter to all registered voters. In it, he admitted that all countries make mistakes.</p>
如果图片只是一张脸，可能没有必要描述它。 读者完全不关心他的头发是红色还是棕色，白皮肤或黑皮肤，有一个眼睛还是两个眼睛。
但是如果图片更加动态，比如显示了政客很生气，或者非常开心，或者很颓废， 可能就应该加一些替代文本来给文章一个基调。如果没有替代文本可能这个基调就会被读者遗漏。
<p><img src="alexsalmond.jpeg" alt="Alex Salmond is sad."> Ahead of today's referendum, the First Minister of Scotland, Alex Salmond, wrote an open letter to all registered voters. In it, he admitted that all countries make mistakes.</p>
<p><img src="alexsalmond.jpeg" alt="Alex Salmond is ecstatic!"> Ahead of today's referendum, the First Minister of Scotland, Alex Salmond, wrote an open letter to all registered voters. In it, he admitted that all countries make mistakes.</p>
任务是 "伤心" 还是 "高兴" 会影响后续段落的解读： 他可能是说由于即将举行的公民投票会对他不利而向民众辞职， 也可能是说这次选举出了一些问题，但是接下来的投票率可能使它变得无关紧要。 解读因图片内容而异。
如果该图片有标题部分，那么包含一段替代文本可以避免让非视觉媒体的用户困惑标题指代的内容。
<p>Ahead of today's referendum, the First Minister of Scotland, Alex Salmond,
wrote an open letter to all registered voters. In it, he admitted that all
countries make mistakes.</p>
<figure>
<img src="alexsalmond.jpeg"
alt="A high forehead, cheerful disposition, and dark hair round out Alex Salmond's face.">
<figcaption> Alex Salmond, SNP. Photo © 2014 PolitiPhoto. </figcaption>
</figure>
如果图片是装饰性的而且不是页面特定的 — 比如这个图片作为站点设计的一部分。 — 那么这个图片应该通过站点 CSS 来指定，而不是写在文档标记中。
但是如果一张图片没有在周围的文字中加以讨论，但仍然有些关联，也可以使用
img 元素把它包含在页面中。这样的图片的 alt
属性必须存在而且值必须是空字符串。
有些关联的纯装饰性图片的例子比如，一篇介绍在 Burning Man 发生的事件的博文会引入一张 Black Rock City 景色的照片，或者在显示诗歌的页面上引入一张受这首诗启发的绘画作品。 下面的代码片段是第二种情况的例子（代码中只引入了诗歌的第一小节）：
<h1>The Lady of Shalott</h1> <p><img src="shalott.jpeg" alt=""></p> <p>On either side the river lie<br> Long fields of barley and of rye,<br> That clothe the wold and meet the sky;<br> And through the field the road run by<br> To many-tower'd Camelot;<br> And up and down the people go,<br> Gazing where the lilies blow<br> Round an island there below,<br> The island of Shalott.</p>
当一张图片被切分为若干个小图片文件，用来拼装成原来的大图时，这些图片中要有一个的
alt 属性按照相关规则设置为大图的替代文本。
然后其他所有剩余图片必须把 alt 属性设为空字符串。
下面的例子中，表示 XYZ Corp 公司 Logo 的图片被分为两部分， 第一个包含字母 "XYZ" 第二个包含单词 "Corp"。 都包含在了第一个图片的替代文本（"XYZ Corp"）中。
<h1><img src="logo1.png" alt="XYZ Corp"><img src="logo2.png" alt=""></h1>
下面的例子中，评分显示为三颗星星和两颗空星。 虽然替代文本可以是 "★★★☆☆"，但作者决定提供更有帮助的评分格式比如 "3 / 5"。它设置为第一个图片的替代文本。其他图片的替代文本为空。
<p>Rating: <meter max=5 value=3><img src="1" alt="3 out of 5" ><img src="1" alt=""><img src="1" alt=""><img src="0" alt="" ><img src="0" alt=""></meter></p>
通常带连接的图片应该使用 图片映射 而不是切分图片。
但是，如果图片确实已经被切分了，而且切分后的部分是链接的唯一内容，
那么每个链接必须有一个图片必须把它的 alt
属性设置为表示链接用途的替代文本。
下面的例子是一张表示飞行面条怪物徽章的图片，左右两侧的面条状物是单独的两张图片， 这样用户就可以在探险中选择左右两侧的图片了。
<h1>The Church</h1> <p>You come across a flying spaghetti monster. Which side of His Noodliness do you wish to reach out for?</p> <p><a href="?go=left" ><img src="fsm-left.png" alt="Left side. "></a ><img src="fsm-middle.png" alt="" ><a href="?go=right"><img src="fsm-right.png" alt="Right side."></a></p>
有些情况下，图片是内容的关键部分。比如照片库页面中的图片就是这样的情况。 这时图片就是包含它的页面的重点。
如何为内容关键的图片提供替代文本取决于图片的出处。
如果可以提供详细的替代文本，如果图像是杂志评论中的一系列截图的一部分，或者是连环漫画的一部分，
或者是关于该照片的博客文章中的那个照片，则必须通过 alt
属性给出替代图片的文字。
新操作系统的屏幕截图，其中包含一些替代文字：
<figure>
<img src="KDE%20Light%20desktop.png"
alt="The desktop is blue, with icons along the left hand side in
two columns, reading System, Home, K-Mail, etc. A window is
open showing that menus wrap to a second line if they
cannot fit in the window. The window has a list of icons
along the top, with an address bar below it, a list of
icons for tabs along the left edge, a status bar on the
bottom, and two panes in the middle. The desktop has a bar
at the bottom of the screen with a few buttons, a pager, a
list of open applications, and a clock.">
<figcaption>Screenshot of a KDE desktop.</figcaption>
</figure>
财务报告中的图表：
<img src="sales.gif"
title="Sales graph"
alt="From 1998 to 2005, sales increased by the following percentages
with each year: 624%, 75%, 138%, 40%, 35%, 9%, 21%">
请注意，“销售图”对于销售图而言是不充分的替代文本。 做 标题 很好的文本通常不适合做替代文本。
有些情况下图片的特性决定了提供完整的替代文本不现实。 例如，图像可能模糊不清，或者可能是复杂的分形，或者可能是详细的地形图。
这些情况下 alt 属性也必须包含一些适当的替代文本，
但可以比较简短。
有时候根本无法用文字对图片做出正确的描述。例如对罗夏墨迹测验就无法提供有用的描述。 但是，即使是一个简单的描述也比什么都没有要好：
<figure> <img src="/commons/a/a7/Rorschach1.jpg" alt="A shape with left-right symmetry with indistinct edges, with a small gap in the center, two larger gaps offset slightly from the center, with two similar gaps under them. The outline is wider in the top half than the bottom half, with the sides extending upwards higher than the center, and the center extending below the sides."> <figcaption>A black outline of the first of the ten cards in the Rorschach inkblot test.</figcaption> </figure>
注意下面是一个替代文本的非常糟糕的使用方式：
<!-- This example is wrong. Do not copy it. --> <figure> <img src="/commons/a/a7/Rorschach1.jpg" alt="A black outline of the first of the ten cards in the Rorschach inkblot test."> <figcaption>A black outline of the first of the ten cards in the Rorschach inkblot test.</figcaption> </figure>
这样把标题放到替代文字中根本没用，因为对于没有图片的用户它只是重复了标题， 相比于只读到或听到一次标题，再来重复一次根本不会有任何帮助。
无法描述的图片的另一个例子是分型，根据定义它的细节是无限的。
下面的例子展示了为曼德博集合提供替代文字的一种可能的方式。
<img src="ms1.jpeg" alt="The Mandelbrot set appears as a cardioid with its cusp on the real axis in the positive direction, with a smaller bulb aligned along the same center line, touching it in the negative direction, and with these two shapes being surrounded by smaller bulbs of various sizes.">
类似地，在生物学中人脸的照片也是内容的关键部分，也很难完全用文字替代：
<section class="bio"> <h1>A Biography of Isaac Asimov</h1> <p>Born <b>Isaak Yudovich Ozimov</b> in 1920, Isaac was a prolific author.</p> <p><img src="headpics/asimov.jpeg" alt="Isaac Asimov had dark hair, a tall forehead, and wore glasses. Later in life, he wore long white sideburns."> <p>Asimov was born in Russia, and moved to the US when he was three years old.</p> <p>... </section>
在这样的情况下，在替代文本中引述图片本身是不必要的（并且实际上不鼓励）， 因为这样的文本与浏览器自身的图片报告是冗余的。 例如，如果替代文本是“Isaac Asimov的照片”，那么标准的用户代理可能将其读作 “（Image）Isaac Asimov 的照片” 而不是（更有用） “（Image）Isaac Asimov 是黑发 ，额头很高，戴着眼镜......”。
有些情况比较不幸，根本没有替代文字。可能是因为图片是动态获取的且没有关联的替代文本 （比如 Web 摄像头），也可能因为页面是由用户提供的图片通过脚本生成的， 而用户并没有提供合适的可用替代文本（例如照片分享站点）， 也可能是因为作者自己不知道图片表示着什么（比如一个盲人摄影师在博客上分享了一张图片）。
这种情况下，alt 属性可以忽略，但必须符合以下条件之一：
img 元素在
包含 figcaption 的 figure 元素中，
且这个 figcaption 元素包含除
元素间空白 的内容，
而且忽略 figcaption 元素及其后代节点后， figure 元素没有其他
流式内容 后代，
元素间空白 和 img 元素除外。
title 属性存在且值非空。
目前不推荐依赖 title 属性，
因为很多用户代理不会按照标准把这个属性暴露给用户
（例如用户代理会要求比如鼠标这样的指点设备才能显示提示内容，
这排除了纯键盘用户和触摸设备用户，比如任何拥有现代手机和平板的用户。）
这种情况应该保持在最低限度。只要作者有能力提供真正的替代文本，
那么忽略 alt 属性就是不可接受的。
如果一个照片分享网站收到图片时除了标题没有其他任何元数据， 这个照片可以这样标记：
<figure> <img src="1100670787_6a7c664aef.jpg"> <figcaption>Bubbles traveled everywhere with us.</figcaption> </figure>
当然如果能详细描述图片的重要部分，并通过替代文本提供出来会更好。
一个盲人的博客中，显示了一张用户拍的照片。刚开始用户可能不知道他们拍的照片上有什么：
<article> <h1>I took a photo</h1> <p>I went out today and took a photo!</p> <figure> <img src="photo2.jpeg"> <figcaption>A photograph taken blindly from my front porch.</figcaption> </figure> </article>
最后如果用户从他们的朋友那里得到了图片的描述，他们就可以包含一些替代文本：
<article> <h1>I took a photo</h1> <p>I went out today and took a photo!</p> <figure> <img src="photo2.jpeg" alt="The photograph shows my squirrel feeder hanging from the edge of my roof. It is half full, but there are no squirrels around. In the background, out-of-focus trees fill the shot. The feeder is made of wood with a metal grate, and it contains peanuts. The edge of the roof is wooden too, and is painted white with light blue streaks."> <figcaption>A photograph taken blindly from my front porch.</figcaption> </figure> </article>
有时图片的整个意义就是没有文本描述，要用户为它提供描述。
例如，CAPTCHA 图片的目的就是看用户能否从用文字描述图形。
下面是标记 CAPTCHA 的一种方式（注意 title 属性）：
<p><label>What does this image say? <img src="captcha.cgi?id=8934" title="CAPTCHA"> <input type=text name=captcha></label> (If you cannot see the image, you can use an <a href="?audio">audio</a> test instead.)</p>
另一个例子是显示图片并询问替代文本的软件，这些文本之后会被写入到页面中。 这样的页面可能会有一个图片表格，像这样：
<table> <thead> <tr> <th> Image <th> Description <tbody> <tr> <td> <img src="2421.png" title="Image 640 by 100, filename 'banner.gif'"> <td> <input name="alt2421"> <tr> <td> <img src="2422.png" title="Image 200 by 480, filename 'ad3.gif'"> <td> <input name="alt2422"> </table>
注意即使在这个例子中，在 title
属性中还是提供了尽可能多的有用信息。
因为有些用户根本不能使用照片
（比如因为他们的网络很慢，或因为他们在使用文本浏览器，或者因为他们正在听汽车的语音 Web
浏览器阅读页面内容，或者只是因为他们是盲人），
只在没有可用的替代文本而且没人可以提供时，才允许忽略 alt 属性。
作者没有付出努力不是一个可以接受的理由。
通常作者应该避免使用不用来显示的 img 元素。
如果一个 img 元素不是用于显示的，比如用于计算页面浏览量的服务。
那么 alt 属性必须为空字符串。
在这些场景下，width 和 height 属性必须设为 0。
This section does not apply to documents that are publicly accessible, or whose target audience is not necessarily personally known to the author, such as documents on a web site, emails sent to public mailing lists, or software documentation.
When an image is included in a private communication (such as an HTML email) aimed at a
specific person who is known to be able to view images, the alt
attribute may be omitted. However, even in such cases authors are strongly urged to include
alternative text (as appropriate according to the kind of image involved, as described in the
above entries), so that the email is still usable should the user use a mail client that does not
support images, or should the document be forwarded on to other users whose abilities might not
include easily seeing images.
Markup generators (such as WYSIWYG authoring tools) should, wherever possible, obtain alternative text from their users. However, it is recognized that in many cases, this will not be possible.
For images that are the sole contents of links, markup generators should examine the link target to determine the title of the target, or the URL of the target, and use information obtained in this manner as the alternative text.
For images that have captions, markup generators should use the figure and
figcaption elements, or the title attribute, to
provide the image's caption.
As a last resort, implementers should either set the alt
attribute to the empty string, under the assumption that the image is a purely decorative image
that doesn't add any information but is still specific to the surrounding content, or omit the
alt attribute altogether, under the assumption that the image is
a key part of the content.
Markup generators may specify a generator-unable-to-provide-required-alt
attribute on img elements for which they have been unable to obtain alternative text
and for which they have therefore omitted the alt attribute. The
value of this attribute must be the empty string. Documents containing such attributes are not
conforming, but conformance checkers will silently
ignore this error.
This is intended to avoid markup generators from being pressured into replacing
the error of omitting the alt attribute with the even more
egregious error of providing phony alternative text, because state-of-the-art automated
conformance checkers cannot distinguish phony alternative text from correct alternative text.
Markup generators should generally avoid using the image's own file name as the alternative text. Similarly, markup generators should avoid generating alternative text from any content that will be equally available to presentation user agents (e.g., web browsers).
This is because once a page is generated, it will typically not be updated, whereas the browsers that later read the page can be updated by the user, therefore the browser is likely to have more up-to-date and finely-tuned heuristics than the markup generator did when generating the page.
规范检查工具必须把缺少 alt 属性报告为错误，
符合以下条件之一的情形除外：
规范检查工具的配置中声明了，该文档是一封邮件，或该文档的受众是特定的肯定可以查看图片的人。
img 元素有一个 (不合规范的)
generator-unable-to-provide-required-alt
属性其值为非空字符串。规范检查工具如果不把缺失
alt 属性报告为错误，也必须不把空的
generator-unable-to-provide-required-alt
属性报告为错误。
（该情况并不代表文档符合规范，只是说生成器不能够确定合适的替代文本 —
所以不要求验证器在这种情况下显示错误。因为这会鼓励标签生成器为了禁用检查工具报错而引入假的替代文本。
正常情况下，规范检查工具 可以 把缺失 alt 属性报告为错误，
即使指定了 generator-unable-to-provide-required-alt
属性；例如可能有一个用户选项是报告 所有 规范错误，
即使这些错误可能是使用标记生成器或多或少不可避免的。）
iframe elementSupport in all current engines.
Support in all current engines.
src — Address of the resourcesrcdoc — A document to render in the iframename — Name of nested browsing contextsandbox — Security rules for nested contentallow — Permissions policy to be applied to the iframe's contentsallowfullscreen — Whether to allow the iframe's contents to use requestFullscreen()width — Horizontal dimensionheight — Vertical dimensionreferrerpolicy — Referrer policy for fetches initiated by the elementloading — Used when determining loading deferral[Exposed=Window]
interface HTMLIFrameElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute USVString src;
[CEReactions] attribute DOMString srcdoc;
[CEReactions] attribute DOMString name;
[SameObject, PutForwards=value] readonly attribute DOMTokenList sandbox;
[CEReactions] attribute DOMString allow;
[CEReactions] attribute boolean allowFullscreen;
[CEReactions] attribute DOMString width;
[CEReactions] attribute DOMString height;
[CEReactions] attribute DOMString referrerPolicy;
[CEReactions] attribute DOMString loading;
readonly attribute Document? contentDocument;
readonly attribute WindowProxy? contentWindow;
Document? getSVGDocument();
// also has obsolete members
};The iframe element represents its nested browsing
context.
The src attribute
gives the URL of a page that the element's nested browsing context is to
contain. The attribute, if present, must be a valid non-empty URL potentially surrounded by
spaces. If the itemprop attribute is specified on an
iframe element, then the src attribute must
also be specified.
Support in all current engines.
The srcdoc
attribute gives the content of the page that the element's nested browsing context is
to contain. The value of the attribute is the source of an iframe srcdoc document.
The srcdoc attribute, if present, must have a value
using the HTML syntax that consists of the following syntactic components, in the
given order:
html element.The above requirements apply in XML documents as well.
Here a blog uses the srcdoc attribute in conjunction
with the sandbox attribute described below to provide
users of user agents that support this feature with an extra layer of protection from script
injection in the blog post comments:
<article>
<h1>I got my own magazine!</h1>
<p>After much effort, I've finally found a publisher, and so now I
have my own magazine! Isn't that awesome?! The first issue will come
out in September, and we have articles about getting food, and about
getting in boxes, it's going to be great!</p>
<footer>
<p>Written by <a href="/users/cap">cap</a>, 1 hour ago.
</footer>
<article>
<footer> Thirteen minutes ago, <a href="/users/ch">ch</a> wrote: </footer>
<iframe sandbox srcdoc="<p>did you get a cover picture yet?"></iframe>
</article>
<article>
<footer> Nine minutes ago, <a href="/users/cap">cap</a> wrote: </footer>
<iframe sandbox srcdoc="<p>Yeah, you can see it <a href="/gallery?mode=cover&amp;page=1">in my gallery</a>."></iframe>
</article>
<article>
<footer> Five minutes ago, <a href="/users/ch">ch</a> wrote: </footer>
<iframe sandbox srcdoc="<p>hey that's earl's table.
<p>you should get earl&amp;me on the next cover."></iframe>
</article>Notice the way that quotes have to be escaped (otherwise the srcdoc attribute would end prematurely), and the way raw
ampersands (e.g. in URLs or in prose) mentioned in the sandboxed content have to be
doubly escaped — once so that the ampersand is preserved when originally parsing
the srcdoc attribute, and once more to prevent the
ampersand from being misinterpreted when parsing the sandboxed content.
Furthermore, notice that since the DOCTYPE is optional in
iframe srcdoc documents, and the html,
head, and body elements have optional
start and end tags, and the title element is also optional in iframe srcdoc
documents, the markup in a srcdoc attribute can be
relatively succinct despite representing an entire document, since only the contents of the
body element need appear literally in the syntax. The other elements are still
present, but only by implication.
In the HTML syntax, authors need only remember to use U+0022
QUOTATION MARK characters (") to wrap the attribute contents and then to escape all U+0026
AMPERSAND (&) and U+0022 QUOTATION MARK (") characters, and to specify the sandbox attribute, to ensure safe embedding of content. (And
remember to escape ampersands before quotation marks, to ensure quotation marks become "
and not &quot;.)
In XML the U+003C LESS-THAN SIGN character (<) needs to be escaped as well. In order to prevent attribute-value normalization, some of XML's whitespace characters — specifically U+0009 CHARACTER TABULATION (tab), U+000A LINE FEED (LF), and U+000D CARRIAGE RETURN (CR) — also need to be escaped. [XML]
If the src attribute and the srcdoc attribute are both specified together, the srcdoc attribute takes priority. This allows authors to provide
a fallback URL for legacy user agents that do not support the srcdoc attribute.
When an iframe element element is inserted into a document whose browsing
context is non-null, the user agent must run these steps:
Create a new nested browsing context for element.
Process the iframe attributes for element, with initialInsertion set to true.
When an iframe element is removed
from a document, the user agent must discard the element's nested browsing context, if it is not null,
and then set the element's nested browsing context to null.
This happens without any unload events firing
(the element's nested browsing context and its Document are discarded, not unloaded).
Whenever an iframe element with a non-null nested browsing context
has its srcdoc attribute set, changed, or removed, the
user agent must process the iframe attributes.
Similarly, whenever an iframe element with a non-null nested browsing
context but with no srcdoc attribute specified has
its src attribute set, changed, or removed, the user agent
must process the iframe attributes.
To process the iframe attributes for an element element,
with an optional boolean initialInsertion (default false):
If element's srcdoc attribute is
specified, then:
Set element's current navigation was lazy loaded boolean to false.
If the will lazy load element steps given element return true, then:
Set element's lazy load resumption steps to the rest of this algorithm starting with the step labeled navigate to the srcdoc resource.
Set element's current navigation was lazy loaded boolean to true.
Start intersection-observing a lazy loading element for element.
Return.
Navigate to the srcdoc resource: navigate an iframe or
frame given element and a new response whose url
list consists of about:srcdoc, header list consists of `Content-Type`/`text/html`, body is the value of element's srcdoc attribute, and CSP list is a clone
of element's node document's CSP list.
The resulting Document must be considered an iframe srcdoc document.
Otherwise, if initialInsertion is false, run the otherwise steps for
iframe or frame elements given element.
The otherwise steps for iframe or frame elements, given an
element element, are:
If element has no src attribute specified,
or its value is the empty string, let url be the URL
"about:blank".
Otherwise, parse the value of element's src attribute, relative to element's node
document.
If that is not successful, then let url be the URL
"about:blank". Otherwise, let url be the resulting URL
record.
If there exists an ancestor browsing context of element's nested browsing context whose active document's URL, ignoring fragments, is equal to url, then return.
Let resource be a new request whose url is url and whose referrer policy is the current state of
element's referrerpolicy content
attribute.
If element is an iframe element, then set element's
current navigation was lazy loaded boolean to false.
If element is an iframe element, and the will lazy load element
steps given element return true, then:
Set element's lazy load resumption steps to the rest of this algorithm starting with the step labeled navigate to the resource.
Set element's current navigation was lazy loaded boolean to true.
Start intersection-observing a lazy loading element for element.
Return.
Navigate to the resource: navigate an iframe or
frame given element and resource.
To navigate an iframe or frame given an element
element and a resource resource:
Let historyHandling be "default".
If element's nested browsing context's session
history contains only one Document, and that was the about:blank
Document created when element's nested browsing context was
created, then set historyHandling to "replace".
If element's nested browsing context's active
document is not completely loaded, then set historyHandling to
"replace".
Navigate element's nested browsing context to resource, with historyHandling set to historyHandling and the source browsing context set to element's node document's browsing context.
A load event is also fired at the
iframe element when it is created if no other data is loaded in it.
Each Document has an iframe load in progress flag and a mute
iframe load flag. When a Document is created, these flags must be unset for
that Document.
To run the iframe load event steps, given an iframe element
element:
Assert: element's nested browsing context is not null.
Let childDocument be the active document of element's nested browsing context.
If childDocument has its mute iframe load flag set, then return.
Set childDocument's iframe load in progress flag.
Fire an event named load at element.
Unset childDocument's iframe load in progress flag.
This, in conjunction with scripting, can be used to probe the URL space of the local network's HTTP servers. User agents may implement cross-origin access control policies that are stricter than those described above to mitigate this attack, but unfortunately such policies are typically not compatible with existing web content.
If an element type potentially delays the load event, then for each element element of that type, the user agent must delay the load event of element's node document if element's nested browsing context is non-null and any of the following are true:
element's nested browsing context's active document is not ready for post-load tasks.
Anything is delaying the load event of element's nested browsing context's active document.
element's nested browsing context is in the delaying load events mode.
If, during the handling of the load event,
element's nested browsing context is again navigated, that will further delay the load event.
Each iframe element has an associated current navigation was lazy
loaded boolean, initially false. It is set and unset in the process the
iframe attributes algorithm.
An iframe element whose current navigation was lazy loaded boolean is
false potentially delays the load event.
If, when the element is created, the srcdoc attribute is not set, and the src attribute is either also not set or set but its value cannot be
parsed, the browsing context will remain at the initial
about:blank page.
If the user navigates away from this page, the
iframe's nested browsing context's WindowProxy object will
proxy new Window objects for new Document objects, but the src attribute will not change.
The name attribute,
if present, must be a valid browsing context name. The given value is used to name
the element's nested browsing context if present when that is created.
Support in all current engines.
The sandbox
attribute, when specified, enables a set of extra restrictions on any content hosted by the
iframe. Its value must be an unordered set of unique space-separated
tokens that are ASCII case-insensitive. The allowed values are allow-forms, allow-modals, allow-orientation-lock, allow-pointer-lock, allow-popups, allow-popups-to-escape-sandbox,
allow-presentation, allow-same-origin, allow-scripts, allow-top-navigation, allow-top-navigation-by-user-activation,
and allow-downloads.
When the attribute is set, the content is treated as being from a unique origin,
forms, scripts, and various potentially annoying APIs are disabled, links are prevented from
targeting other browsing contexts, and plugins are secured.
The allow-same-origin keyword causes
the content to be treated as being from its real origin instead of forcing it into a unique
origin; the allow-top-navigation
keyword allows the content to navigate its top-level browsing context;
the allow-top-navigation-by-user-activation
keyword behaves similarly but allows such navigation only when the
browsing context's active window has transient activation; and the allow-forms, allow-modals, allow-orientation-lock, allow-pointer-lock, allow-popups, allow-presentation, allow-scripts, and allow-popups-to-escape-sandbox
keywords re-enable forms, modal dialogs, screen orientation lock, the pointer lock API, popups,
the presentation API, scripts, and the creation of unsandboxed auxiliary browsing contexts respectively. [POINTERLOCK]
[SCREENORIENTATION] [PRESENTATION]
The allow-top-navigation and allow-top-navigation-by-user-activation
keywords must not both be specified, as doing so is redundant; only allow-top-navigation will have an effect
in such non-conformant markup.
Setting both the allow-scripts and allow-same-origin keywords together when the
embedded page has the same origin as the page containing the iframe
allows the embedded page to simply remove the sandbox
attribute and then reload itself, effectively breaking out of the sandbox altogether.
These flags only take effect when the nested browsing context of
the iframe element is navigated. Removing them, or
removing the entire sandbox attribute, has no effect on
an already-loaded page.
Potentially hostile files should not be served from the same server as the file
containing the iframe element. Sandboxing hostile content is of minimal help if an
attacker can convince the user to just visit the hostile content directly, rather than in the
iframe. To limit the damage that can be caused by hostile HTML content, it should be
served from a separate dedicated domain. Using a different domain ensures that scripts in the
files are unable to attack the site, even if the user is tricked into visiting those pages
directly, without the protection of the sandbox
attribute.
When an iframe element with a sandbox
attribute has its nested browsing context created (before the initial
about:blank Document is created), and when an iframe
element's sandbox attribute is set or changed while it
has a nested browsing context, the user agent must parse the sandboxing directive using the attribute's value as the
input and the iframe element's iframe sandboxing flag
set as the output.
When an iframe element's sandbox
attribute is removed while it has a non-null nested browsing context, the user agent
must empty the iframe element's iframe sandboxing flag
set.
In this example, some completely-unknown, potentially hostile, user-provided HTML content is embedded in a page. Because it is served from a separate domain, it is affected by all the normal cross-site restrictions. In addition, the embedded page has scripting disabled, plugins disabled, forms disabled, and it cannot navigate any frames or windows other than itself (or any frames or windows it itself embeds).
<p>We're not scared of you! Here is your content, unedited:</p>
<iframe sandbox src="https://usercontent.example.net/getusercontent.cgi?id=12193"></iframe>It is important to use a separate domain so that if the attacker convinces the user to visit that page directly, the page doesn't run in the context of the site's origin, which would make the user vulnerable to any attack found in the page.
In this example, a gadget from another site is embedded. The gadget has scripting and forms enabled, and the origin sandbox restrictions are lifted, allowing the gadget to communicate with its originating server. The sandbox is still useful, however, as it disables plugins and popups, thus reducing the risk of the user being exposed to malware and other annoyances.
<iframe sandbox="allow-same-origin allow-forms allow-scripts"
src="https://maps.example.com/embedded.html"></iframe>Suppose a file A contained the following fragment:
<iframe sandbox="allow-same-origin allow-forms" src=B></iframe>Suppose that file B contained an iframe also:
<iframe sandbox="allow-scripts" src=C></iframe>Further, suppose that file C contained a link:
<a href=D>Link</a>For this example, suppose all the files were served as text/html.
Page C in this scenario has all the sandboxing flags set. Scripts are disabled, because the
iframe in A has scripts disabled, and this overrides the allow-scripts keyword set on the
iframe in B. Forms are also disabled, because the inner iframe (in B)
does not have the allow-forms keyword
set.
Suppose now that a script in A removes all the sandbox attributes in A and B.
This would change nothing immediately. If the user clicked the link in C, loading page D into the
iframe in B, page D would now act as if the iframe in B had the allow-same-origin and allow-forms keywords set, because that was the
state of the nested browsing context in the iframe in A when page B was
loaded.
Generally speaking, dynamically removing or changing the sandbox attribute is ill-advised, because it can make it quite
hard to reason about what will be allowed and what will not.
The allow
attribute, when specified, determines the container
policy that will be used when the permissions policy for a Document
in the iframe's nested browsing context is initialized. Its value must
be a serialized permissions policy.
[PERMISSIONSPOLICY]
In this example, an iframe is used to embed a map from an online navigation
service. The allow attribute is used to enable the
Geolocation API within the nested context.
<iframe src="https://maps.example.com/" allow="geolocation"></iframe>The allowfullscreen attribute is a boolean
attribute. When specified, it indicates that Document objects in the
iframe element's nested browsing context will be initialized with a
permissions policy which allows the
"fullscreen" feature to be used from any origin. This is
enforced by the process permissions policy
attributes algorithm. [PERMISSIONSPOLICY]
Here, an iframe is used to embed a player from a video site. The allowfullscreen attribute is needed to enable the
player to show its video fullscreen.
<article>
<header>
<p><img src="/usericons/1627591962735"> <b>Fred Flintstone</b></p>
<p><a href="/posts/3095182851" rel=bookmark>12:44</a> — <a href="#acl-3095182851">Private Post</a></p>
</header>
<p>Check out my new ride!</p>
<iframe src="https://video.example.com/embed?id=92469812" allowfullscreen></iframe>
</article>Neither allow nor
allowfullscreen can grant access to a feature
in an iframe element's nested browsing context if the element's
node document is not already allowed to use that feature.
To determine whether a Document object document
is allowed to use the policy-controlled-feature feature, run these
steps:
If document's browsing context is null, then return false.
If document's browsing context's active document is not document, then return false.
If the result of running is feature enabled in document
for origin on feature, document, and document's
origin is "Enabled", then return true.
Return false.
Because they only influence the permissions policy of the nested browsing
context's active document, the allow
and allowfullscreen attributes only take effect
when the nested browsing context of the iframe is navigated. Adding or removing them has no effect on an already-loaded
document.
The iframe element supports dimension attributes for cases where the
embedded content has specific dimensions (e.g. ad units have well-defined dimensions).
An iframe element never has fallback content, as it will always
create a new nested browsing context, regardless of whether the specified initial
contents are successfully used.
The referrerpolicy attribute is a
referrer policy attribute. Its purpose is to set the referrer policy
used when processing the iframe
attributes. [REFERRERPOLICY]
The loading attribute is a lazy
loading attribute. Its purpose is to indicate the policy for loading iframe
elements that are outside the viewport.
When the loading attribute's state is changed to the
Eager state, the user agent must run these
steps:
Let resumptionSteps be the iframe element's lazy load
resumption steps.
If resumptionSteps is null, then return.
Set the iframe's lazy load resumption steps to null.
Invoke resumptionSteps.
Descendants of iframe elements represent nothing. (In legacy user agents that do
not support iframe elements, the contents would be parsed as markup that could act as
fallback content.)
The HTML parser treats markup inside iframe elements as
text.
Support in all current engines.
Support in all current engines.
The IDL attributes src, srcdoc, name, sandbox, and allow must reflect the respective content
attributes of the same name.
The supported tokens for sandbox's DOMTokenList are the allowed
values defined in the sandbox attribute and supported by
the user agent.
The allowFullscreen IDL attribute
must reflect the allowfullscreen
content attribute.
HTMLIFrameElement/referrerPolicy
Support in all current engines.
The referrerPolicy IDL attribute
must reflect the referrerpolicy
content attribute, limited to only known values.
The loading IDL attribute must
reflect the loading content attribute,
limited to only known values.
HTMLIFrameElement/contentDocument
Support in all current engines.
The contentDocument IDL attribute,
on getting, must return the iframe element's content document.
HTMLIFrameElement/contentWindow
Support in all current engines.
The contentWindow IDL attribute must
return the WindowProxy object of the iframe element's nested
browsing context, if its nested browsing context is non-null, or null
otherwise.
Here is an example of a page using an iframe to include advertising from an
advertising broker:
<iframe src="https://ads.example.com/?customerid=923513721&format=banner"
width="468" height="60"></iframe>embed elementSupport in all current engines.
Support in all current engines.
src — Address of the resourcetype — Type of embedded resourcewidth — Horizontal dimensionheight — Vertical dimension[Exposed=Window]
interface HTMLEmbedElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute USVString src;
[CEReactions] attribute DOMString type;
[CEReactions] attribute DOMString width;
[CEReactions] attribute DOMString height;
Document? getSVGDocument();
// also has obsolete members
};Depending on the type of content instantiated by the
embed element, the node may also support other
interfaces.
The embed element provides an integration point for an external (typically
non-HTML) application or interactive content.
The src attribute
gives the URL of the resource being embedded. The attribute, if present, must contain
a valid non-empty URL potentially surrounded by spaces.
Authors should avoid referencing untrusted resources, as such a resource can be used to instantiate plugins or run scripts, even if the author has used features such as the Flash "allowScriptAccess" parameter.
If the itemprop attribute is specified on an
embed element, then the src attribute must also
be specified.
The type attribute,
if present, gives the MIME type by which the plugin to instantiate is selected. The
value must be a valid MIME type string. If both the type attribute and the src
attribute are present, then the type attribute must specify
the same type as the explicit Content-Type metadata of the
resource given by the src attribute.
While any of the following conditions are occurring, any plugin instantiated for
the element must be removed, and the embed element represents
nothing:
The element has neither a src attribute nor a type attribute.
The element has a media element ancestor.
The element has an ancestor object element that is not showing its
fallback content.
An embed element is said to be potentially
active when the following conditions are all met simultaneously:
src attribute set or a type attribute set (or both).src attribute is either absent or its value
is not the empty string.object element that is not showing its
fallback content.Whenever an embed element that was not potentially active becomes potentially active, and whenever a potentially active embed element that is
remaining potentially active and has its src attribute set, changed, or removed or its type attribute set, changed, or removed, the user agent must
queue an element task on the embed task source given the element
to run the embed element setup steps for that element.
The embed element setup steps for a given embed element
element are as follows:
If another task has since been queued to run the
embed element setup steps for element, then return.
If the Should plugin element be blocked a priori by Content
Security Policy? algorithm returns "Blocked" when executed on
element, then return. [CSP]
If element has a src attribute set, then:
Let url be the result of parsing the value
of element's src attribute, relative to
element's node document.
If url is failure, then return.
Let request be a new request whose
url is url, client is element's node
document's relevant settings object, destination is "embed",
credentials mode is "include", mode is "navigate", and whose use-URL-credentials flag is set.
Fetch request.
Fetching the resource must delay the load event of element's node document.
To process response for the response response:
If another task has since been queued to run
the embed element setup steps for element, then
return.
If response is a network error, then fire an event named load at element, and return.
Let type be the result of determining the type of content given element and response.
Switch on type:
Display no plugin for element.
image/svg+xmlIf element's nested browsing context is null, then create a new nested browsing context for element.
Navigate element's nested browsing context to
response, with historyHandling set to
"replace" and the source browsing context
set to element's node document's browsing context.
element's src attribute
does not get updated if the browsing context gets further navigated to other
locations.
element now represents its nested browsing context.
Display a plugin for element, given type and response.
Otherwise:
Let type be the value of element's type attribute.
If type is a type that a plugin supports, then display a plugin for element given type.
Otherwise, display no plugin for element.
To determine the type of the content given an
embed element element and a response response, run the following steps:
If element has a type attribute, and that
attribute's value is a type that a plugin supports, then return the value of the
type attribute.
If the path component of response's url matches a pattern that a plugin supports, then return the type that that plugin can handle.
For example, a plugin might say that it can handle URLs with path components that end with the four character string
".swf".
If response has explicit Content-Type metadata, and that value is a type that a plugin supports, then return that value.
Return null.
It is intentional that the above algorithm allows response to have a non-ok status. This allows servers to return data for plugins even with error responses (e.g., HTTP 500 Internal Server Error codes can still contain plugin data).
To display a plugin for an embed element element, given a
string type and optionally a response
response:
If element's nested browsing context is not null, then:
Discard element's nested browsing context.
Set element's nested browsing context to null.
Find and instantiate an appropriate plugin based on type, replacing any previously-instantiated plugin for element. If response was given, forward it to the plugin.
element now represents this plugin instance.
Once the plugin, and response if given, are completely loaded, queue an
element task on the DOM manipulation task source give element to
fire an event named load at element.
To display no plugin for an embed element element:
If element's nested browsing context is not null, then:
Discard element's nested browsing context.
Set element's nested browsing context to null.
Display an indication that no plugin could be found for element, replacing any previously-instantiated plugin for element.
element now represents nothing.
The embed element has no fallback content; its
descendants are ignored.
Whenever an embed element that was potentially
active stops being potentially active, any
plugin that had been instantiated for that element must be unloaded.
When a plugin is to be instantiated but it cannot be secured and the sandboxed plugins browsing context
flag is set on the embed element's node document's active
sandboxing flag set, then the user agent must not instantiate the plugin, and
must instead render the embed element in a manner that conveys that the
plugin was disabled. The user agent may offer the user the option to override the
sandbox and instantiate the plugin anyway; if the user invokes such an option, the
user agent must act as if the conditions above did not apply for the purposes of this element.
Plugins that cannot be secured are disabled in sandboxed browsing contexts because they might not honor the restrictions imposed by the sandbox (e.g. they might allow scripting even when scripting in the sandbox is disabled). User agents should convey the danger of overriding the sandbox to the user if an option to do so is provided.
The embed element potentially delays the load event.
Any namespace-less attribute other than name, align, hspace, and vspace may be
specified on the embed element, so long as its name is XML-compatible
and contains no ASCII upper alphas. These attributes are
then passed as parameters to the plugin.
All attributes in HTML documents get lowercased automatically, so the restriction on uppercase letters doesn't affect such documents.
The four exceptions are to exclude legacy attributes that have side-effects beyond just sending parameters to the plugin.
The user agent should pass the names and values of all the attributes of the embed
element that have no namespace to the plugin used, when one is instantiated.
The HTMLEmbedElement object representing the element must expose the scriptable
interface of the plugin instantiated for the embed element, if any.
The embed element supports dimension attributes.
The IDL attributes src and type each must reflect the respective content
attributes of the same name.
Here's a way to embed a resource that requires a proprietary plugin, like Flash:
<embed src="catgame.swf">If the user does not have the plugin (for example if the plugin vendor doesn't support the user's platform), then the user will be unable to use the resource.
To pass the plugin a parameter "quality" with the value "high", an attribute can be specified:
<embed src="catgame.swf" quality="high">This would be equivalent to the following, when using an object element
instead:
<object data="catgame.swf">
<param name="quality" value="high">
</object>object elementSupport in all current engines.
Support in all current engines.
usemap attribute: Interactive content.param elements, then, transparent.data — Address of the resourcetype — Type of embedded resourcename — Name of nested browsing contextusemap — Name of image map to useform — Associates the element with a form elementwidth — Horizontal dimensionheight — Vertical dimension[Exposed=Window]
interface HTMLObjectElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute USVString data;
[CEReactions] attribute DOMString type;
[CEReactions] attribute DOMString name;
[CEReactions] attribute DOMString useMap;
readonly attribute HTMLFormElement? form;
[CEReactions] attribute DOMString width;
[CEReactions] attribute DOMString height;
readonly attribute Document? contentDocument;
readonly attribute WindowProxy? contentWindow;
Document? getSVGDocument();
readonly attribute boolean willValidate;
readonly attribute ValidityState validity;
readonly attribute DOMString validationMessage;
boolean checkValidity();
boolean reportValidity();
undefined setCustomValidity(DOMString error);
// also has obsolete members
};Depending on the type of content instantiated by the
object element, the node also supports other
interfaces.
The object element can represent an external resource, which, depending on the
type of the resource, will either be treated as an image, as a child browsing
context, or as an external resource to be processed by a plugin.
The data attribute,
if present, specifies the URL of the resource. If present, the attribute must be a
valid non-empty URL potentially surrounded by spaces.
Authors should avoid referencing untrusted resources, as such a resource can be used to instantiate plugins or run scripts, even if the author has used features such as the Flash "allowScriptAccess" parameter.
The type attribute,
if present, specifies the type of the resource. If present, the attribute must be a valid
MIME type string.
At least one of either the data attribute or the type attribute must be present.
If the itemprop attribute is specified on an object
element, then the data attribute must also be specified.
The name attribute,
if present, must be a valid browsing context name. The given value is used to name
the element's nested browsing context, if applicable, and if present when the
element's nested browsing context is created.
Whenever one of the following conditions occur:
object elements changes to or from showing its
fallback content,
classid attribute is set, changed, or
removed,
classid attribute is not present, and
its data attribute is set, changed, or removed,
classid attribute nor its
data attribute are present, and its type attribute is set, changed, or removed,
...the user agent must queue an element task on the DOM manipulation task
source given the object element to run the following steps to (re)determine
what the object element represents. This task
being queued or actively running must delay the load
event of the element's node document.
If the user has indicated a preference that this object element's fallback
content be shown instead of the element's usual behavior, then jump to the step below
labeled fallback.
For example, a user could ask for the element's fallback content to be shown because that content uses a format that the user finds more accessible.
If the element has an ancestor media element, or has an ancestor
object element that is not showing its fallback content, or
if the element is not in a document whose browsing context is non-null, or if the element's node
document is not fully active, or if the element is still in the stack
of open elements of an HTML parser or XML parser, or if the
element is not being rendered, or if the Should plugin element be blocked
a priori by Content Security Policy? algorithm returns "Blocked" when executed on the element, then jump to the step below labeled
fallback. [CSP]
If the classid attribute is present, and has a
value that isn't the empty string, then: if the user agent can find a plugin
suitable according to the value of the classid
attribute, and either plugins aren't being sandboxed or that
plugin can be secured, then that
plugin should be used, and the value of the data attribute, if any, should be passed to the
plugin. If no suitable plugin can be found, or if the
plugin reports an error, jump to the step below labeled fallback.
If the data attribute is present and its value is
not the empty string, then:
If the type attribute is present and its value is
not a type that the user agent supports, and is not a type that the user agent can find a
plugin for, then the user agent may jump to the step below labeled fallback
without fetching the content to examine its real type.
Parse the URL specified by the data attribute, relative to the element's node
document.
If that failed, fire an event named error at the element, then jump to the step below labeled
fallback.
Let request be a new request whose
url is the resulting URL record,
client is the element's node
document's relevant settings object, destination is "object",
credentials mode is "include", mode is "navigate", and whose use-URL-credentials flag is set.
Fetch request.
Fetching the resource must delay the load event of the element's node document until the task that is queued by the networking task source once the resource has been fetched (defined next) has been run.
For the purposes of the application cache networking model, this fetch operation is not for a child browsing context (though it might end up being used for one after all, as defined below).
If the resource is not yet available (e.g. because the resource was not available in the cache, so that loading the resource required making a request over the network), then jump to the step below labeled fallback. The task that is queued by the networking task source once the resource is available must restart this algorithm from this step. Resources can load incrementally; user agents may opt to consider a resource "available" whenever enough data has been obtained to begin processing the resource.
If the load failed (e.g. there was an HTTP 404 error, there was a DNS error), fire an event named error
at the element, then jump to the step below labeled fallback.
Determine the resource type, as follows:
Let the resource type be unknown.
If the user agent is configured to strictly obey Content-Type headers for this resource, and the resource has associated Content-Type metadata, then let the resource type be the type specified in the resource's Content-Type metadata, and jump to the step below labeled handler.
This can introduce a vulnerability, wherein a site is trying to embed a resource that uses a particular plugin, but the remote site overrides that and instead furnishes the user agent with a resource that triggers a different plugin with different security characteristics.
If there is a type attribute present on the
object element, and that attribute's value is not a type that the user agent
supports, but it is a type that a plugin supports, then let the resource type be the type specified in that type attribute, and jump to the step below labeled
handler.
Run the appropriate set of steps from the following list:
Let binary be false.
If the type specified in the resource's Content-Type
metadata is "text/plain", and the result of applying the rules for distinguishing if a resource is
text or binary to the resource is that the resource is not
text/plain, then set binary to true.
If the type specified in the resource's Content-Type
metadata is "application/octet-stream", then set binary to true.
If binary is false, then let the resource type be the type specified in the resource's Content-Type metadata, and jump to the step below labeled handler.
If there is a type attribute present on the
object element, and its value is not application/octet-stream,
then run the following steps:
If the attribute's value is a type that a plugin supports, or the
attribute's value is a type that starts with "image/" that is
not also an XML MIME type, then let the resource type be the
type specified in that type attribute.
Jump to the step below labeled handler.
If there is a type attribute present on the
object element, then let the tentative type be the type
specified in that type attribute.
Otherwise, let tentative type be the computed type of the resource.
If tentative type is not
application/octet-stream, then let resource type be
tentative type and jump to the step below labeled
handler.
If applying the URL parser algorithm to the URL of the specified resource (after any redirects) results in a URL record whose path component matches a pattern that a plugin supports, then let resource type be the type that that plugin can handle.
For example, a plugin might say that it can handle resources with path components that end with the four character string
".swf".
It is possible for this step to finish, or for one of the substeps above to jump straight to the next step, with resource type still being unknown. In both cases, the next step will trigger fallback.
Handler: Handle the content as given by the first of the following cases that matches:
If the object element's nested browsing context is non-null,
then it must be discarded and then set
to null.
If plugins are being sandboxed and the plugin that supports resource type cannot be secured, jump to the step below labeled fallback.
Otherwise, the user agent should use the plugin that supports resource type and pass the content of the resource to that plugin. If the plugin reports an error, then jump to the step below labeled fallback.
image/"If the object element's nested browsing context is null, then
create a new nested browsing context for the element.
If the URL of the given resource is not about:blank, then
navigate the element's nested browsing
context to that resource, with historyHandling set
to "replace" and the source browsing context
set to the object element's node document's browsing context. (The data attribute of the object element doesn't
get updated if the browsing context gets further navigated to other locations.)
The object element represents its nested browsing
context.
In certain situations, e.g., if the resource was fetched from an
application cache but it is an HTML file with a manifest attribute that points to a different application cache manifest, the navigation of the browsing context will be restarted
so as to load the resource afresh from the network or a different application
cache. Even if the resource is then found to have a different type, it is still used
as part of a browsing context: only the navigate algorithm is
restarted, not this object algorithm.
image/", and support
for images has not been disabledIf the object element's nested browsing context is non-null,
then it must be discarded and then set
to null.
Apply the image sniffing rules to determine the type of the image.
The object element represents the specified image.
If the image cannot be rendered, e.g. because it is malformed or in an unsupported format, jump to the step below labeled fallback.
The given resource type is not supported. Jump to the step below labeled fallback.
If the previous step ended with the resource type being unknown, this is the case that is triggered.
The element's contents are not part of what the object element
represents.
If the object element does not represent its nested browsing
context, then once the resource is completely loaded, queue an element
task on the DOM manipulation task source given the object
element to fire an event named load at the element.
If the element does represent its nested browsing
context, then an analogous task will be queued when the created Document
is completely finished loading.
Return.
If the data attribute is absent but the type attribute is present, and the user agent can find a
plugin suitable according to the value of the type attribute, and either plugins aren't being sandboxed or the plugin can be
secured, then that plugin should be used. If these conditions cannot be met, or if the
plugin reports an error, jump to the step below labeled fallback. Otherwise
return; once the plugin is completely loaded, queue an element task on the
DOM manipulation task source given the object element to fire an event named load at
the element.
Fallback: The object element represents the element's
children, ignoring any leading param element children. This is the element's
fallback content. If the element has an instantiated plugin, then
unload it. If the element's nested browsing context is non-null, then it must be
discarded and then set to null.
When the algorithm above instantiates a plugin, the user agent
should pass to the plugin used the names and values of all the attributes on the
element, in the order they were added to the element, with the attributes added by the parser
being ordered in source order, followed by a parameter named "PARAM" whose value is null, followed
by all the names and values of parameters given by
param elements that are children of the object element, in tree
order. If the plugin supports a scriptable interface, the
HTMLObjectElement object representing the element should expose that interface. The
object element represents the plugin. The
plugin is not a nested browsing context.
Plugins are considered sandboxed for the purpose of an
object element if the sandboxed plugins browsing context flag is set on
the object element's node document's active sandboxing flag
set.
Due to the algorithm above, the contents of object elements act as fallback
content, used only when referenced resources can't be shown (e.g. because it returned a 404
error). This allows multiple object elements to be nested inside each other,
targeting multiple user agents with different capabilities, with the user agent picking the first
one it supports.
The object element potentially delays the load event.
The usemap attribute, if present while the
object element represents an image, can indicate that the object has an associated
image map. The attribute must be ignored if the
object element doesn't represent an image.
The form attribute is used to explicitly associate the
object element with its form owner.
Constraint validation: object elements are always barred
from constraint validation.
The object element supports dimension attributes.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
The IDL attributes data, type
and name each must reflect the
respective content attributes of the same name. The useMap IDL attribute must reflect the usemap content attribute.
HTMLObjectElement/contentDocument
Support in all current engines.
The contentDocument IDL attribute,
on getting, must return the object element's content document.
HTMLObjectElement/contentWindow
Support in all current engines.
The contentWindow IDL attribute must
return the WindowProxy object of the object element's nested
browsing context, if its nested browsing context is non-null; otherwise, it
must return null.
The willValidate, validity, and validationMessage attributes, and the checkValidity(), reportValidity(), and setCustomValidity() methods, are part of the
constraint validation API. The form IDL attribute
is part of the element's forms API.
In this example, an HTML page is embedded in another using the object
element.
<figure>
<object data="clock.html"></object>
<figcaption>My HTML Clock</figcaption>
</figure>The following example shows how a plugin can be used in HTML (in this case the Flash plugin,
to show a video file). Fallback is provided for users who do not have Flash enabled, in this case
using the video element to show the video for those using user agents that support
video, and finally providing a link to the video for those who have neither Flash
nor a video-capable browser.
<p>Look at my video:
<object type="application/x-shockwave-flash">
<param name=movie value="https://video.example.com/library/watch.swf">
<param name=allowfullscreen value=true>
<param name=flashvars value="https://video.example.com/vids/315981">
<video controls src="https://video.example.com/vids/315981">
<a href="https://video.example.com/vids/315981">View video</a>.
</video>
</object>
</p>param elementobject element, before any flow content.name — Name of parametervalue — Value of parameter[Exposed=Window]
interface HTMLParamElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString name;
[CEReactions] attribute DOMString value;
// also has obsolete members
};The param element defines parameters for plugins invoked by object
elements. It does not represent anything on its own.
The name attribute
gives the name of the parameter.
The value attribute
gives the value of the parameter.
Both attributes must be present. They may have any value.
If both attributes are present, and if the parent element of the param is an
object element, then the element defines a parameter with the given name-value pair.
If either the name or value of a parameter defined
by a param element that is the child of an object element that
represents an instantiated plugin changes, and if that
plugin is communicating with the user agent using an API that features the ability to
update the plugin when the name or value of a parameter so changes, then the user agent must
appropriately exercise that ability to notify the plugin of the change.
The IDL attributes name and value must both reflect the respective content
attributes of the same name.
The following example shows how the param element can be used to pass a parameter
to a plugin, in this case the O3D plugin.
<!DOCTYPE HTML>
<html lang="en">
<head>
<title>O3D Utah Teapot</title>
</head>
<body>
<p>
<object type="application/vnd.o3d.auto">
<param name="o3d_features" value="FloatingPointTextures">
<img src="o3d-teapot.png"
title="3D Utah Teapot illustration rendered using O3D."
alt="When O3D renders the Utah Teapot, it appears as a squat
teapot with a shiny metallic finish on which the
surroundings are reflected, with a faint shadow caused by
the lighting.">
<p>To see the teapot actually rendered by O3D on your
computer, please download and install the <a
href="http://code.google.com/apis/o3d/docs/gettingstarted.html#install">O3D plugin</a>.</p>
</object>
<script src="o3d-teapot.js"></script>
</p>
</body>
</html>video 元素Support in all current engines.
Support in all current engines.
controls 属性: Interactive content.src 属性：
后代可以有零个或多个 track 元素，均为透明的，并且没有媒体元素。src 属性： 后代可以有零个或多个 source 元素，零个或多个 track 元素，后代元素均是透明的，并且没有媒体元素。src — Address of the resourcecrossorigin — How the element handles crossorigin requestsposter — Poster frame to show prior to video playbackpreload — Hints how much buffering the media resource will likely needautoplay — Hint that the media resource can be started automatically when the page is loadedplaysinline — Encourage the user agent to display video content within the element's playback arealoop — Whether to loop the media resourcemuted — Whether to mute the media resource by defaultcontrols — Show user agent controlswidth — Horizontal dimensionheight — Vertical dimension[Exposed=Window]
interface HTMLVideoElement : HTMLMediaElement {
[HTMLConstructor] constructor();
[CEReactions] attribute unsigned long width;
[CEReactions] attribute unsigned long height;
readonly attribute unsigned long videoWidth;
readonly attribute unsigned long videoHeight;
[CEReactions] attribute USVString poster;
[CEReactions] attribute boolean playsInline;
};video 元素用于播放视频或电影，以及带有字幕的音频文件。
可以在 video 元素内提供一些文本内容。用户代理不应向用户展示这些内容；这些内容是用来提供给那些不支持 video元素的老旧浏览器，这样他们可以尝试替换为以前的视频插件，或者向用户展示这些文字，以便告知用户如何访问视频内容。
需要注意的是，video 标签内部的文本并不是为了解决可访问性问题。有许多其他特性能帮助弱视、盲人、听力障碍者、聋人以及有其他身体或认知障碍的人获取视频内容。要提供字幕，可以将其直接嵌入到视频中，也可以通过 track 元素外引文件的方式。可以将手语作为额外的视频轨道嵌入到视频流中。音频描述可以直接嵌入到视频流中或者通过 WebVTT 文件以文本的形式提供，通过 track 元素引用 WebVTT 文件时，浏览器会自动将文本合成为语音。WebVTT 同样可以用来提供章节标题。对于那些完全不想看视频的用户，可以在视频附近为他们提供一些链接用于跳转到一些文本内容。 [WEBVTT]
video 元素表面上只有视频数据，但其中也可能包含一些音频数据。
src，crossorigin，
preload， autoplay，
loop， muted，以及 controls 属性是媒体元素的共有属性。
poster 属性提供图片文件的 URL，用于在浏览器没有视频数据可用时显示。该属性的值必需是一个可能由空格包裹的合法的非空 URL。
如果要使用指定的资源，那么在该元素创建时，或者 poster 属性被设置，更改或移除时，浏览器必须执行以下步骤以确定该元素的 poster frame（无论该元素的封面展示标志位的值如何）：
如果 video 元素已经有一个此类型的算法实例在运行，在不改变当前 poster
frame 的情况下终止该算法实例。
如果 poster 属性的值是空字符串或者没有设置该属性，那么便没有 poster frame；返回。
以相对于当前节点文档的路径解析 poster 属性的值。如果解析失败，那么便没有 poster frame；返回。
新建一个请求，其
url 是 resulting URL record，客户端是元素的节点文档 的 相关设置对象，请求目的是"image"，凭据模式是 "include"，并且设置其 use-URL-credentials 标志位。
获取请求。这个请求应当延迟文档节点的加载事件。
如果成功加载图片，则 poster frame 就是该图片。否则没有 poster frame。
poster 属性提供的图片（poster frame），旨在为用户提供一个该视频的大体印象。
playsinline 属性是一个布尔属性。如果展现该属性，则表明希望浏览器默认以“内联”的形式播放视频，”内联“是指限制在视频元素所在的区域播放，而不是直接全屏播放或者位于一个独立的可随意调整大小的窗口中。
没有显式地声明 playsinline 属性并不意味着视频会默认全屏播放，这完全由浏览器决定。实际上大多数浏览器更倾向于默认以“内联”的形式播放视频，因此在那些浏览器中，加不加 playsinline 属性都是一样的。
A video element represents what is given for the first matching condition in the
list below:
readyState attribute is either HAVE_NOTHING, or HAVE_METADATA but no video data has yet been obtained at
all, or the element's readyState attribute is any
subsequent value but the media resource does not have a video channel)video element represents its poster frame, if any,
or else transparent black with no intrinsic dimensions.video element is paused, the current playback position is the first frame of video,
and the element's show poster flag is setvideo element represents its poster frame, if any,
or else the first frame of the video.video element is paused, and the
frame of video corresponding to the current playback
position is not available (e.g. because the video is seeking or buffering)video element is neither potentially playing nor paused (e.g. when seeking or stalled)video element represents the last frame of the video to have
been rendered.video element is pausedvideo element represents the frame of video corresponding to
the current playback position.video element has a video channel and is potentially
playing)video element represents the frame of video at the continuously
increasing "current" position. When the
current playback position changes such that the last frame rendered is no longer the
frame corresponding to the current playback position in the video, the new frame
must be rendered.Frames of video must be obtained from the video track that was selected when the event loop last reached step 1.
视频中的每一帧对应哪个播放位置由视频流的格式决定。
The video element also represents any text track cues whose text track cue active flag is set and whose
text track is in the showing mode, and any
audio from the media resource, at the current playback position.
Any audio associated with the media resource must, if played, be played synchronized with the current playback position, at the element's effective media volume. The user agent must play the audio from audio tracks that were enabled when the event loop last reached step 1.
除了上述提到的之外，用户代理可以以文字、图标或其他方式在播放区域为用户提供一些信息（比如缓冲中，未发现视频资源，视频加载出错等）。
无法渲染 video 标签的用户代理可以展示一个链接用于链接至其他的视频播放程序或者视频资源本身。
当一个 video 元素的媒体资源具有视频通道时, 该元素提供一个绘制源(paint source)，绘制源的宽度是媒体资源的固有宽度，高度是媒体资源的固有高度，外观是当前播放位置对应的视频帧，如果当前帧不可用（比如在加载中时），则显示之前帧，如果没有之前的帧可用（比如视频第一帧都没加载完成），则显示黑色背景。
videoWidthSupport in all current engines.
videoHeightSupport in all current engines.
这些属性返回视频的固有尺寸，如果尺寸未知，则返回零。
媒体资源的固有宽度 和 固有高度以 CSS 像素为单位，这些值是在考虑过资源的尺寸、高宽比、净光圈、分辨率以及视频格式后得出的。如果一个有所变化的格式没有定义如何将宽高比作用于视频数据以获得“正确”的视频尺寸，那么浏览器必须通过增加增加某一维度同时使得另一维度保持不变来应用该比例。
videoWidth IDL 属性必须返回以 CSS 像素为单位的视频固有宽度。 videoHeight IDL 属性必须返回以 CSS 像素为单位的视频固有高度。如果一个元素的 readyState 属性是 HAVE_NOTHING，那么两个属性都必须返回 0。
无论何时，当视频的固有宽度或者固有高度改变时，（比如因为 selected video track 改变），如果此时元素的 readyState 属性不是 HAVE_NOTHING，
那么浏览器必须 入队一个媒体元素任务（指定元素为 媒体元素）来触发媒体元素上的 resize 事件。
video 元素支持 dimension attributes.
在没有其他的样式规则时，视频内容应当在播放区域中展现完整，并且高宽比保持不变，居中且尽可能的占据空间。这样一来，如果播放区域的高宽比与视频的高宽比并不一致，视频就会以 letterboxed 或 pillarboxed 的形式展现。不被视频内容占据的播放区域就什么都不显示（显示黑色）。
用户代理实现 CSS 样式时，上述要求可以使用渲染 video 时建议的样式来实现。
The intrinsic width of a video element's playback area is the
intrinsic width of the poster frame, if that is available and the
element currently represents its poster frame; otherwise, it is the intrinsic width of the video resource, if that is
available; otherwise the intrinsic width is missing.
The intrinsic height of a video element's playback area is the
intrinsic height of the poster frame, if that is available and the
element currently represents its poster frame; otherwise it is the intrinsic height of the video resource, if that is
available; otherwise the intrinsic height is missing.
video 元素的默认尺寸大小是宽 300 像素，高 150 像素。 [CSSIMAGES]
浏览器应当提供控制字幕的显隐，音频以及其他视频附加数据的功能，并且这些功能应该不会影响页面的正常渲染。
用户代理可以通过一些方式使得用户更加舒服地观看视频，比如全屏或者以一个独立的可改变大小的窗口播放视频。用户代理甚至可以在播放视频时默认触发这种模式，不过当 playsinline 属性被指定时，他们就不应该这么做了。跟其他的用户界面功能一样，除非显式地展现了切换不同模式时的功能按钮，否则进行模式切换时不应当影响页面的正常渲染。在这种独立的视口模式下，就算 controls 属性被省略，用户代理也可以显示完整的控制组件。
在播放视频时，浏览器可以调整某些系统功能来提升用户体验，比如禁用屏幕保护程序。
The poster IDL attribute must
reflect the poster content attribute.
The playsInline IDL attribute must
reflect the playsinline content
attribute.
以下示例说明当视频播放失败时如何检测：
<script>
function failed(e) {
// video playback failed - show a message saying why
switch (e.target.error.code) {
case e.target.error.MEDIA_ERR_ABORTED:
alert('You aborted the video playback.');
break;
case e.target.error.MEDIA_ERR_NETWORK:
alert('A network error caused the video download to fail part-way.');
break;
case e.target.error.MEDIA_ERR_DECODE:
alert('The video playback was aborted due to a corruption problem or because the video used features your browser did not support.');
break;
case e.target.error.MEDIA_ERR_SRC_NOT_SUPPORTED:
alert('The video could not be loaded, either because the server or network failed or because the format is not supported.');
break;
default:
alert('An unknown error occurred.');
break;
}
}
</script>
<p><video src="tgif.vid" autoplay controls onerror="failed(event)"></video></p>
<p><a href="tgif.vid">Download the video file</a>.</p>audio elementSupport in all current engines.
Support in all current engines.
controls attribute: Interactive content.controls attribute: Palpable content.src attribute:
zero or more track elements, then
transparent, but with no media element descendants.src attribute: zero or more source elements, then
zero or more track elements, then
transparent, but with no media element descendants.src — Address of the resourcecrossorigin — How the element handles crossorigin requestspreload — Hints how much buffering the media resource will likely needautoplay — Hint that the media resource can be started automatically when the page is loadedloop — Whether to loop the media resourcemuted — Whether to mute the media resource by defaultcontrols — Show user agent controls[Exposed=Window,
LegacyFactoryFunction=Audio(optional DOMString src)]
interface HTMLAudioElement : HTMLMediaElement {
[HTMLConstructor] constructor();
};An audio element represents a sound or audio stream.
Content may be provided inside the audio element. User agents
should not show this content to the user; it is intended for older web browsers which do
not support audio, so that legacy audio plugins can be tried, or to show text to the
users of these older browsers informing them of how to access the audio contents.
In particular, this content is not intended to address accessibility concerns. To
make audio content accessible to the deaf or to those with other physical or cognitive
disabilities, a variety of features are available. If captions or a sign language video are
available, the video element can be used instead of the audio element to
play the audio, allowing users to enable the visual alternatives. Chapter titles can be provided
to aid navigation, using the track element and a WebVTT file. And,
naturally, transcripts or other textual alternatives can be provided by simply linking to them in
the prose near the audio element. [WEBVTT]
The audio element is a media element whose media data is
ostensibly audio data.
The src, crossorigin,
preload, autoplay,
loop, muted, and controls attributes are the attributes common to all media elements.
Audio( [ url ] )Support in all current engines.
Returns a new audio element, with the src
attribute set to the value passed in the argument, if applicable.
A legacy factory function is provided for creating HTMLAudioElement objects (in
addition to the factory methods from DOM such as createElement()):
Audio(src). When invoked,
the legacy factory function must perform the following steps:
Let document be the current global object's associated Document.
Let audio be the result of creating an
element given document, audio, and the HTML
namespace.
Set an attribute value for
audio using "preload" and "auto".
If src is given, then set
an attribute value for audio using "src"
and src. (This will cause the user
agent to invoke the object's resource selection
algorithm before returning.)
Return audio.
track elementSupport in all current engines.
Support in all current engines.
kind — The type of text tracksrc — Address of the resourcesrclang — Language of the text tracklabel — User-visible labeldefault — Enable the track if no other text track is more suitable[Exposed=Window]
interface HTMLTrackElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString kind;
[CEReactions] attribute USVString src;
[CEReactions] attribute DOMString srclang;
[CEReactions] attribute DOMString label;
[CEReactions] attribute boolean default;
const unsigned short NONE = 0;
const unsigned short LOADING = 1;
const unsigned short LOADED = 2;
const unsigned short ERROR = 3;
readonly attribute unsigned short readyState;
readonly attribute TextTrack track;
};The track element allows authors to specify explicit external timed text tracks for media elements. It
does not represent anything on its own.
The kind attribute is
an enumerated attribute. The following table lists the keywords defined for this
attribute. The keyword given in the first cell of each row maps to the state given in the second
cell.
| Keyword | State | Brief description |
|---|---|---|
subtitles
| Subtitles | Transcription or translation of the dialogue, suitable for when the sound is available but not understood (e.g. because the user does not understand the language of the media resource's audio track). Overlaid on the video. |
captions
| Captions | Transcription or translation of the dialogue, sound effects, relevant musical cues, and other relevant audio information, suitable for when sound is unavailable or not clearly audible (e.g. because it is muted, drowned-out by ambient noise, or because the user is deaf). Overlaid on the video; labeled as appropriate for the hard-of-hearing. |
descriptions
| Descriptions | Textual descriptions of the video component of the media resource, intended for audio synthesis when the visual component is obscured, unavailable, or not usable (e.g. because the user is interacting with the application without a screen while driving, or because the user is blind). Synthesized as audio. |
chapters
| Chapters metadata | Tracks intended for use from script. Not displayed by the user agent. |
metadata
| Metadata |
The attribute may be omitted. The missing value default is the subtitles state. The invalid value default is the metadata state.
The src attribute
gives the URL of the text track data. The value must be a valid non-empty URL
potentially surrounded by spaces. This attribute must be present.
If the element has a src attribute whose value is not the
empty string and whose value, when the attribute was set, could be successfully parsed relative to the element's node document, then the
element's track URL is the resulting URL string. Otherwise, the element's
track URL is the empty string.
If the element's track URL identifies a WebVTT resource, and the element's kind attribute is not in the chapters metadata or metadata state, then the WebVTT file must be a
WebVTT file using cue text. [WEBVTT]
The srclang
attribute gives the language of the text track data. The value must be a valid BCP 47 language
tag. This attribute must be present if the element's kind
attribute is in the subtitles state. [BCP47]
If the element has a srclang attribute whose value is
not the empty string, then the element's track language is the value of the attribute.
Otherwise, the element has no track language.
The label attribute
gives a user-readable title for the track. This title is used by user agents when listing subtitle, caption, and audio description tracks in their user interface.
The value of the label attribute, if the attribute is
present, must not be the empty string. Furthermore, there must not be two track
element children of the same media element whose kind attributes are in the same state, whose srclang attributes are both missing or have values that
represent the same language, and whose label attributes are
again both missing or both have the same value.
If the element has a label attribute whose value is not
the empty string, then the element's track label is the value of the attribute.
Otherwise, the element's track label is an empty string.
The default
attribute is a boolean attribute, which, if specified, indicates that the track is to
be enabled if the user's preferences do not indicate that another track would be more
appropriate.
Each media element must have no more than one track element child
whose kind attribute is in the subtitles or captions state and whose default attribute is specified.
Each media element must have no more than one track element child
whose kind attribute is in the description state and whose default attribute is specified.
Each media element must have no more than one track element child
whose kind attribute is in the chapters metadata state and whose default attribute is specified.
There is no limit on the number of track elements whose kind attribute is in the metadata state and whose default attribute is specified.
readyStateReturns the text track readiness state, represented by a number from the following list:
NONE (0)The text track not loaded state.
LOADING (1)The text track loading state.
LOADED (2)The text track loaded state.
ERROR (3)The text track failed to load state.
trackReturns the TextTrack object corresponding to the text track of the track element.
The readyState attribute must return the
numeric value corresponding to the text track readiness state of the
track element's text track, as defined by the following list:
NONE (numeric value 0)LOADING (numeric value 1)LOADED (numeric value 2)ERROR (numeric value 3)The track IDL
attribute must, on getting, return the track element's text track's
corresponding TextTrack object.
Support in all current engines.
The src, srclang, label, and default IDL
attributes must reflect the respective content attributes of the same name. The kind IDL attribute
must reflect the content attribute of the same name, limited to only known
values.
This video has subtitles in several languages:
<video src="brave.webm">
<track kind=subtitles src=brave.en.vtt srclang=en label="English">
<track kind=captions src=brave.en.hoh.vtt srclang=en label="English for the Hard of Hearing">
<track kind=subtitles src=brave.fr.vtt srclang=fr lang=fr label="Français">
<track kind=subtitles src=brave.de.vtt srclang=de lang=de label="Deutsch">
</video>(The lang attributes on the last two describe the language of
the label attribute, not the language of the subtitles
themselves. The language of the subtitles is given by the srclang attribute.)
HTMLMediaElement objects (audio and video, in this
specification) are simply known as media elements.
Support in all current engines.
enum CanPlayTypeResult { "" /* empty string */, "maybe", "probably" };
typedef (MediaStream or MediaSource or Blob) MediaProvider;
[Exposed=Window]
interface HTMLMediaElement : HTMLElement {
// error state
readonly attribute MediaError? error;
// network state
[CEReactions] attribute USVString src;
attribute MediaProvider? srcObject;
readonly attribute USVString currentSrc;
[CEReactions] attribute DOMString? crossOrigin;
const unsigned short NETWORK_EMPTY = 0;
const unsigned short NETWORK_IDLE = 1;
const unsigned short NETWORK_LOADING = 2;
const unsigned short NETWORK_NO_SOURCE = 3;
readonly attribute unsigned short networkState;
[CEReactions] attribute DOMString preload;
readonly attribute TimeRanges buffered;
undefined load();
CanPlayTypeResult canPlayType(DOMString type);
// ready state
const unsigned short HAVE_NOTHING = 0;
const unsigned short HAVE_METADATA = 1;
const unsigned short HAVE_CURRENT_DATA = 2;
const unsigned short HAVE_FUTURE_DATA = 3;
const unsigned short HAVE_ENOUGH_DATA = 4;
readonly attribute unsigned short readyState;
readonly attribute boolean seeking;
// playback state
attribute double currentTime;
undefined fastSeek(double time);
readonly attribute unrestricted double duration;
object getStartDate();
readonly attribute boolean paused;
attribute double defaultPlaybackRate;
attribute double playbackRate;
attribute boolean preservesPitch;
readonly attribute TimeRanges played;
readonly attribute TimeRanges seekable;
readonly attribute boolean ended;
[CEReactions] attribute boolean autoplay;
[CEReactions] attribute boolean loop;
Promise<undefined> play();
undefined pause();
// controls
[CEReactions] attribute boolean controls;
attribute double volume;
attribute boolean muted;
[CEReactions] attribute boolean defaultMuted;
// tracks
[SameObject] readonly attribute AudioTrackList audioTracks;
[SameObject] readonly attribute VideoTrackList videoTracks;
[SameObject] readonly attribute TextTrackList textTracks;
TextTrack addTextTrack(TextTrackKind kind, optional DOMString label = "", optional DOMString language = "");
};The media element attributes, src, crossorigin, preload, autoplay,
loop, muted, and
controls, apply to all media elements. They are defined in this section.
Media elements are used to present audio data, or video and audio data, to the user. This is referred to as media data in this section, since this section applies equally to media elements for audio or for video. The term media resource is used to refer to the complete set of media data, e.g. the complete video file, or complete audio file.
A media resource can have multiple audio and video tracks. For the purposes of a
media element, the video data of the media resource is only that of the
currently selected track (if any) as given by the element's videoTracks attribute when the event loop last
reached step 1, and the audio data of the media resource is the result of mixing all
the currently enabled tracks (if any) given by the element's audioTracks attribute when the event loop last
reached step 1.
Both audio and video elements can be used for both audio
and video. The main difference between the two is simply that the audio element has
no playback area for visual content (such as video or captions), whereas the video
element does.
Each media element has a unique media element event task source.
To queue a media element task with a media element element and a series of steps steps, queue an element task on the media element's media element event task source given element and steps.
Support in all current engines.
errorSupport in all current engines.
Returns a MediaError object representing the current error state of the
element.
Returns null if there is no error.
All media elements have an associated error status, which
records the last error the element encountered since its resource selection algorithm was last invoked. The
error attribute,
on getting, must return the MediaError object created for this last error, or null if
there has not been an error.
[Exposed=Window]
interface MediaError {
const unsigned short MEDIA_ERR_ABORTED = 1;
const unsigned short MEDIA_ERR_NETWORK = 2;
const unsigned short MEDIA_ERR_DECODE = 3;
const unsigned short MEDIA_ERR_SRC_NOT_SUPPORTED = 4;
readonly attribute unsigned short code;
readonly attribute DOMString message;
};error . codeSupport in all current engines.
Returns the current error's error code, from the list below.
error . messageSupport in all current engines.
Returns a specific informative diagnostic message about the error condition encountered. The message and message format are not generally uniform across different user agents. If no such message is available, then the empty string is returned.
Every MediaError object has a message, which is a string, and a code, which is one of the following:
MEDIA_ERR_ABORTED (numeric value 1)MEDIA_ERR_NETWORK (numeric value 2)MEDIA_ERR_DECODE (numeric value 3)MEDIA_ERR_SRC_NOT_SUPPORTED (numeric value 4)src
attribute or assigned media provider object was not suitable.To create a MediaError, given an
error code which is one of the above values, return a new MediaError object whose
code is the given error code and whose message is a string containing any details the user
agent is able to supply about the cause of the error condition, or the empty string if the user
agent is unable to supply such details. This message string must not contain only the information
already available via the supplied error code; for example, it must not simply be a translation of
the code into a string format. If no additional information is available beyond that provided by
the error code, the message must be set to the
empty string.
The code attribute of a
MediaError object must return this MediaError object's code.
The message attribute of a
MediaError object must return this MediaError object's message.
The src content
attribute on media elements gives the URL of the
media resource (video, audio) to show. The attribute, if present, must contain a valid
non-empty URL potentially surrounded by spaces.
If the itemprop attribute is specified on the media
element, then the src attribute must also be
specified.
The crossorigin content attribute on media elements is a CORS settings attribute.
If a media element is created with a
src attribute, the user agent must immediately invoke the
media element's resource selection
algorithm.
If a src attribute of a media element is set
or changed, the user agent must invoke the media element's media element load
algorithm. (Removing the src attribute does
not do this, even if there are source elements present.)
Support in all current engines.
The src IDL
attribute on media elements must reflect the
content attribute of the same name.
Support in all current engines.
The crossOrigin IDL attribute must
reflect the crossorigin content
attribute, limited to only known values.
A media provider object is an object that can represent a media
resource, separate from a URL. MediaStream objects,
MediaSource objects, and Blob objects are all media provider objects.
Each media element can have an assigned media provider object, which is a media provider object. When a media element is created, it has no assigned media provider object.
srcObject [ = source ]Support in one engine only.
Allows the media element to be assigned a media provider object.
currentSrcSupport in all current engines.
Returns the URL of the current media resource, if any.
Returns the empty string when there is no media resource, or it doesn't have a URL.
The currentSrc IDL attribute must initially be set to
the empty string. Its value is changed by the resource
selection algorithm defined below.
The srcObject IDL attribute, on getting,
must return the element's assigned media provider object, if any, or null otherwise.
On setting, it must set the element's assigned media provider object to the new
value, and then invoke the element's media element load algorithm.
There are three ways to specify a media resource: the srcObject IDL attribute, the src content attribute, and source elements. The IDL
attribute takes priority, followed by the content attribute, followed by the elements.
A media resource can be described in terms of its type, specifically a
MIME type, in some cases with a codecs parameter. (Whether the
codecs parameter is allowed or not depends on the MIME type.) [RFC6381]
Types are usually somewhat incomplete descriptions; for example "video/mpeg" doesn't say anything except what the container type is, and even a
type like "video/mp4; codecs="avc1.42E01E, mp4a.40.2"" doesn't
include information like the actual bitrate (only the maximum bitrate). Thus, given a type, a user
agent can often only know whether it might be able to play media of that type (with
varying levels of confidence), or whether it definitely cannot play media of that
type.
A type that the user agent knows it cannot render is one that describes a resource that the user agent definitely does not support, for example because it doesn't recognize the container type, or it doesn't support the listed codecs.
The MIME type "application/octet-stream" with no parameters is never
a type that the user agent knows it cannot render. User agents must treat that type
as equivalent to the lack of any explicit Content-Type metadata
when it is used to label a potential media resource.
Only the MIME type "application/octet-stream" with no
parameters is special-cased here; if any parameter appears with it, it will be treated just like
any other MIME type. This is a deviation from the rule that unknown MIME type parameters should be
ignored.
canPlayType(type)Support in all current engines.
Returns the empty string (a negative response), "maybe", or "probably" based on how confident the user agent is that it can play media resources of the given type.
The canPlayType(type) method must return the
empty string if type is a type that the user agent knows it cannot
render or is the type "application/octet-stream"; it must return "probably" if the user agent is confident
that the type represents a media resource that it can render if used in with this
audio or video element; and it must return "maybe" otherwise. Implementors are encouraged
to return "maybe" unless the type can be
confidently established as being supported or not. Generally, a user agent should never return
"probably" for a type that allows the codecs parameter if that parameter is not present.
This script tests to see if the user agent supports a (fictional) new format to dynamically
decide whether to use a video element or a plugin:
<section id="video">
<p><a href="playing-cats.nfv">Download video</a></p>
</section>
<script>
var videoSection = document.getElementById('video');
var videoElement = document.createElement('video');
var support = videoElement.canPlayType('video/x-new-fictional-format;codecs="kittens,bunnies"');
if (support != "probably" && "New Fictional Video Plugin" in navigator.plugins) {
// not confident of browser support
// but we have a plugin
// so use plugin instead
videoElement = document.createElement("embed");
} else if (support == "") {
// no support from browser and no plugin
// do nothing
videoElement = null;
}
if (videoElement) {
while (videoSection.hasChildNodes())
videoSection.removeChild(videoSection.firstChild);
videoElement.setAttribute("src", "playing-cats.nfv");
videoSection.appendChild(videoElement);
}
</script>The type attribute of the
source element allows the user agent to avoid downloading resources that use formats
it cannot render.
networkStateSupport in all current engines.
从下面的代码列表中，返回该元素网络活动的当前状态。
媒体元素 与网络交互时，它们当前的网络活动由
networkState 属性表示。
读取时，它必须返回该元素的当前网络状态，它的值必须是下面一种：
NETWORK_EMPTY (数字值 0)NETWORK_IDLE (数字值 1)NETWORK_LOADING (数字值 2)NETWORK_NO_SOURCE (数字值 3)下面定义的 资源选择算法 确切地描述了
何时 networkState 属性的值会改变，
以及触发什么样的事件来表示这个状态改变。
load()Support in all current engines.
Causes the element to reset and start selecting and loading a new media resource from scratch.
All media elements have a can autoplay flag, which must begin in the true state, and a delaying-the-load-event flag, which must begin in the false state. While the delaying-the-load-event flag is true, the element must delay the load event of its document.
When the load()
method on a media element is invoked, the user agent must run the media element
load algorithm.
The media element load algorithm consists of the following steps.
Abort any already-running instance of the resource selection algorithm for this element.
Let pending tasks be a list of all tasks from the media element's media element event task source in one of the task queues.
For each task in pending tasks that would resolve pending play promises or reject pending play promises, immediately resolve or reject those promises in the order the corresponding tasks were queued.
Remove each task in pending tasks from its task queue
Basically, pending events and callbacks are discarded and promises in-flight to be resolved/rejected are resolved/rejected immediately when the media element starts loading a new resource.
If the media element's networkState is set to NETWORK_LOADING or NETWORK_IDLE, queue a media element task
given the media element to fire an event
named abort at the media element.
If the media element's networkState
is not set to NETWORK_EMPTY, then:
Queue a media element task given the media element to fire an event named emptied at the media element.
If a fetching process is in progress for the media element, the user agent should stop it.
If the media element's assigned media provider object is a
MediaSource object, then detach it.
If readyState is not set to HAVE_NOTHING, then set it to that state.
If the paused attribute is false, then:
Set the paused attribute to true.
Take pending play promises and reject pending play promises
with the result and an "AbortError"
DOMException.
If seeking is true, set it to false.
Set the current playback position to 0.
Set the official playback position to 0.
If this changed the official playback position, then queue a media
element task given the media element to fire an event named timeupdate at the media element.
Set the timeline offset to Not-a-Number (NaN).
Update the duration attribute to Not-a-Number
(NaN).
The user agent will not fire a durationchange event for this particular change of
the duration.
Set the playbackRate attribute to the value of
the defaultPlaybackRate attribute.
Set the error attribute to null and the
can autoplay flag to true.
Invoke the media element's resource selection algorithm.
Playback of any previously playing media resource for this element stops.
The resource selection algorithm for a media element is as follows. This algorithm is always invoked as part of a task, but one of the first steps in the algorithm is to return and continue running the remaining steps in parallel. In addition, this algorithm interacts closely with the event loop mechanism; in particular, it has synchronous sections (which are triggered as part of the event loop algorithm). Steps in such sections are marked with ⌛.
Set the element's networkState attribute to
the NETWORK_NO_SOURCE value.
Set the element's show poster flag to true.
Set the media element's delaying-the-load-event flag to true (this delays the load event).
Await a stable state, allowing the task that invoked this algorithm to continue. The synchronous section consists of all the remaining steps of this algorithm until the algorithm says the synchronous section has ended. (Steps in synchronous sections are marked with ⌛.)
⌛ If the media element's blocked-on-parser flag is false, then populate the list of pending text tracks.
⌛ If the media element has an assigned media provider object, then let mode be object.
⌛ Otherwise, if the media element has no assigned media provider
object but has a src
attribute, then let mode be attribute.
⌛ Otherwise, if the media element does not have an assigned media provider
object and does not have a src attribute, but does have a source element child, then
let mode be children and let candidate
be the first such source element child in tree order.
⌛ Otherwise the media element has no assigned media provider
object and has neither a src attribute nor a source element child:
⌛ Set the networkState to NETWORK_EMPTY.
⌛ Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
End the synchronous section and return.
⌛ Set the media element's networkState to NETWORK_LOADING.
⌛ Queue a media element task given the media element to
fire an event named loadstart at the media element.
Run the appropriate steps from the following list:
⌛ Set the currentSrc attribute to
the empty string.
End the synchronous section, continuing the remaining steps in parallel.
Run the resource fetch algorithm with the assigned media provider object. If that algorithm returns without aborting this one, then the load failed.
Failed with media provider: Reaching this step indicates that the media resource failed to load. Take pending play promises and queue a media element task given the media element to run the dedicated media source failure steps with the result.
Wait for the task queued by the previous step to have executed.
Return. The element won't attempt to load another resource until this algorithm is triggered again.
⌛ If the src
attribute's value is the empty string, then end the synchronous section, and jump
down to the failed with attribute step below.
⌛ Let urlString and urlRecord be the resulting URL
string and the resulting URL record, respectively, that would have
resulted from parsing the URL specified by the
src attribute's value relative to the media
element's node document when the src
attribute was last changed.
⌛ If urlString was obtained successfully, set the currentSrc attribute to urlString.
End the synchronous section, continuing the remaining steps in parallel.
If urlRecord was obtained successfully, run the resource fetch algorithm with urlRecord. If that algorithm returns without aborting this one, then the load failed.
Failed with attribute: Reaching this step indicates that the media resource failed to load or that the given URL could not be parsed. Take pending play promises and queue a media element task given the media element to run the dedicated media source failure steps with the result.
Wait for the task queued by the previous step to have executed.
Return. The element won't attempt to load another resource until this algorithm is triggered again.
⌛ Let pointer be a position defined by two adjacent nodes in the media element's child list, treating the start of the list (before the first child in the list, if any) and end of the list (after the last child in the list, if any) as nodes in their own right. One node is the node before pointer, and the other node is the node after pointer. Initially, let pointer be the position between the candidate node and the next node, if there are any, or the end of the list, if it is the last node.
As nodes are inserted and removed into the media element, pointer must be updated as follows:
Other changes don't affect pointer.
⌛ Process candidate: If candidate does not have a
src attribute, or if its src attribute's value is the empty string, then end the
synchronous section, and jump down to the failed with elements step
below.
⌛ Let urlString and urlRecord be the resulting URL
string and the resulting URL record, respectively, that would have
resulted from parsing the URL specified by
candidate's src attribute's value relative
to the candidate's node document when the src attribute was last changed.
⌛ If urlString was not obtained successfully, then end the synchronous section, and jump down to the failed with elements step below.
⌛ If candidate has a type attribute whose value, when parsed as a MIME
type (including any codecs described by the codecs parameter, for
types that define that parameter), represents a type that the user agent knows it cannot
render, then end the synchronous section, and jump down to the failed with elements step below.
⌛ Set the currentSrc attribute to
urlString.
End the synchronous section, continuing the remaining steps in parallel.
Run the resource fetch algorithm with urlRecord. If that algorithm returns without aborting this one, then the load failed.
Failed with elements: Queue a media element task given the
media element to fire an event named
error at candidate.
Await a stable state. The synchronous section consists of all the remaining steps of this algorithm until the algorithm says the synchronous section has ended. (Steps in synchronous sections are marked with ⌛.)
⌛ Forget the media element's media-resource-specific tracks.
⌛ Find next candidate: Let candidate be null.
⌛ Search loop: If the node after pointer is the end of the list, then jump to the waiting step below.
⌛ If the node after pointer is a source element,
let candidate be that element.
⌛ Advance pointer so that the node before pointer is now the node that was after pointer, and the node after pointer is the node after the node that used to be after pointer, if any.
⌛ If candidate is null, jump back to the search loop step. Otherwise, jump back to the process candidate step.
⌛ Waiting: Set the element's networkState attribute to the NETWORK_NO_SOURCE value.
⌛ Set the element's show poster flag to true.
⌛ Queue a media element task given the media element given the element to set the element's delaying-the-load-event flag to false. This stops delaying the load event.
End the synchronous section, continuing the remaining steps in parallel.
Wait until the node after pointer is a node other than the end of the list. (This step might wait forever.)
Await a stable state. The synchronous section consists of all the remaining steps of this algorithm until the algorithm says the synchronous section has ended. (Steps in synchronous sections are marked with ⌛.)
⌛ Set the element's delaying-the-load-event flag back to true (this delays the load event again, in case it hasn't been fired yet).
⌛ Set the networkState back to NETWORK_LOADING.
⌛ Jump back to the find next candidate step above.
The dedicated media source failure steps with a list of promises promises are the following steps:
Set the error attribute to the result of
creating a MediaError with MEDIA_ERR_SRC_NOT_SUPPORTED.
Set the element's networkState attribute to
the NETWORK_NO_SOURCE value.
Set the element's show poster flag to true.
Fire an event named error at the media element.
Reject pending play promises with promises and a
"NotSupportedError" DOMException.
Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
The resource fetch algorithm for a media element and a given URL record or media provider object is as follows:
If the algorithm was invoked with media provider object or a URL record whose object is a media provider object, then let mode be local. Otherwise let mode be remote.
If mode is remote, then let the current media resource be the resource given by the URL record passed to this algorithm; otherwise, let the current media resource be the resource given by the media provider object. Either way, the current media resource is now the element's media resource.
Remove all media-resource-specific text tracks from the media element's list of pending text tracks, if any.
Run the appropriate steps from the following list:
Optionally, run the following substeps. This is the expected behavior if the user agent
intends to not attempt to fetch the resource until the user requests it explicitly (e.g. as
a way to implement the preload attribute's none keyword).
Set the networkState to NETWORK_IDLE.
Queue a media element task given the media element to
fire an event named suspend at the element.
Queue a media element task given the media element to set the element's delaying-the-load-event flag to false. This stops delaying the load event.
Wait for the task to be run.
Wait for an implementation-defined event (e.g., the user requesting that the media element begin playback).
Set the element's delaying-the-load-event flag back to true (this delays the load event again, in case it hasn't been fired yet).
Set the networkState to NETWORK_LOADING.
Let destination be "audio" if the media
element is an audio element and to "video"
otherwise.
Let request be the result of creating a potential-CORS request given
current media resource's URL record, destination, and the
media element's crossorigin
content attribute value.
Set request's client to the media element's node document's relevant settings object.
Fetch request.
The response's unsafe response obtained in this fashion, if any,
contains the media data. It can be CORS-same-origin or
CORS-cross-origin; this affects whether subtitles referenced in the media
data are exposed in the API and, for video elements, whether a
canvas gets tainted when the video is drawn on it.
The stall timeout is an implementation-defined length of time,
which should be about three seconds. When a media element that is actively
attempting to obtain media data has failed to receive any data for a duration
equal to the stall timeout, the user agent must queue a media element
task given the media element to fire
an event named stalled at the element.
User agents may allow users to selectively block or slow media data downloads. When a media element's download has been blocked altogether, the user agent must act as if it was stalled (as opposed to acting as if the connection was closed). The rate of the download may also be throttled automatically by the user agent, e.g. to balance the download with other connections sharing the same bandwidth.
User agents may decide to not download more content at any time,
e.g. after buffering five minutes of a one hour media resource, while waiting for the user
to decide whether to play the resource or not, while waiting for user input in an
interactive resource, or when the user navigates away from the page. When a media
element's download has been suspended, the user agent must queue a media
element task given the media element to set the networkState to NETWORK_IDLE and fire an event named suspend at the element. If and when downloading of the
resource resumes, the user agent must queue a media element task given the
media element to set the networkState to NETWORK_LOADING. Between the queuing of these
tasks, the load is suspended (so progress events
don't fire, as described above).
The preload attribute provides a hint
regarding how much buffering the author thinks is advisable, even in the absence of the autoplay attribute.
When a user agent decides to completely suspend a download, e.g., if it is waiting until the user starts playback before downloading any further content, the user agent must queue a media element task given the media element to set the element's delaying-the-load-event flag to false. This stops delaying the load event.
The user agent may use whatever means necessary to fetch the resource (within the constraints put forward by this and other specifications); for example, reconnecting to the server in the face of network errors, using HTTP range retrieval requests, or switching to a streaming protocol. The user agent must consider a resource erroneous only if it has given up trying to fetch it.
To determine the format of the media resource, the user agent must use the rules for sniffing audio and video specifically.
While the load is not suspended (see below), every 350ms (±200ms) or for every byte
received, whichever is least frequent, queue a media element task
given the media element to fire an
event named progress at the element.
The networking task source tasks to process the data as it is being fetched must each immediately queue a media element task given the media element to run the first appropriate steps from the media data processing steps list below. (A new task is used for this so that the work described below occurs relative to the appropriate media element event task source rather than using the networking task source.)
When the networking task source has queued the last task as part of fetching the media resource (i.e. once the download has completed), if the fetching process completes without errors, including decoding the media data, and if all of the data is available to the user agent without network access, then, the user agent must move on to the final step below. This might never happen, e.g. when streaming an infinite resource such as web radio, or if the resource is longer than the user agent's ability to cache data.
While the user agent might still need network access to obtain parts of the media resource, the user agent must remain on this step.
For example, if the user agent has discarded the first half of a video, the
user agent will remain at this step even once the playback has
ended, because there is always the chance the user will seek back to the start. In fact,
in this situation, once playback has ended, the user agent
will end up firing a suspend event, as described
earlier.
The resource described by the current media resource, if any, contains the media data. It is CORS-same-origin.
If the current media resource is a raw data stream (e.g. from a
File object), then to determine the format of the media resource,
the user agent must use the rules for sniffing
audio and video specifically. Otherwise, if the data stream is pre-decoded, then the
format is the format given by the relevant specification.
Whenever new data for the current media resource becomes available, queue a media element task given the media element to run the first appropriate steps from the media data processing steps list below.
When the current media resource is permanently exhausted (e.g. all the bytes of
a Blob have been processed), if there were no decoding errors, then the user
agent must move on to the final step below. This might never happen, e.g. if the
current media resource is a MediaStream.
The media data processing steps list is as follows:
DNS errors, HTTP 4xx and 5xx errors (and equivalents in other protocols), and other fatal network errors that occur before the user agent has established whether the current media resource is usable, as well as the file using an unsupported container format, or using unsupported codecs for all the data, must cause the user agent to execute the following steps:
The user agent should cancel the fetching process.
Abort this subalgorithm, returning to the resource selection algorithm.
Create an AudioTrack object to represent the audio track.
Update the media element's audioTracks attribute's AudioTrackList
object with the new AudioTrack object.
Let enable be unknown.
If either the media resource or the URL of the current media resource indicate a particular set of audio tracks to enable, or if the user agent has information that would facilitate the selection of specific audio tracks to improve the user's experience, then: if this audio track is one of the ones to enable, then set enable to true, otherwise, set enable to false.
This could be triggered by media fragment syntax, but it could also be triggered e.g. by the user agent selecting a 5.1 surround sound audio track over a stereo audio track.
If enable is still unknown, then, if the media element does not yet have an enabled audio track, then set enable to true, otherwise, set enable to false.
If enable is true, then enable this audio track, otherwise, do not enable this audio track.
Fire an event named addtrack at this AudioTrackList object,
using TrackEvent, with the track
attribute initialized to the new AudioTrack object.
Create a VideoTrack object to represent the video track.
Update the media element's videoTracks attribute's VideoTrackList
object with the new VideoTrack object.
Let enable be unknown.
If either the media resource or the URL of the current media resource indicate a particular set of video tracks to enable, or if the user agent has information that would facilitate the selection of specific video tracks to improve the user's experience, then: if this video track is the first such video track, then set enable to true, otherwise, set enable to false.
This could again be triggered by media fragment syntax.
If enable is still unknown, then, if the media element does not yet have a selected video track, then set enable to true, otherwise, set enable to false.
If enable is true, then select this track and unselect any
previously selected video tracks, otherwise, do not select this video track. If other tracks
are unselected, then a change event will be fired.
Fire an event named addtrack at this VideoTrackList object,
using TrackEvent, with the track
attribute initialized to the new VideoTrack object.
This indicates that the resource is usable. The user agent must follow these substeps:
Establish the media timeline for the purposes of the current playback position and the earliest possible position, based on the media data.
Update the timeline offset to the date and time that corresponds to the zero time in the media timeline established in the previous step, if any. If no explicit time and date is given by the media resource, the timeline offset must be set to Not-a-Number (NaN).
Set the current playback position and the official playback position to the earliest possible position.
Update the duration attribute with the time of
the last frame of the resource, if known, on the media timeline established
above. If it is not known (e.g. a stream that is in principle infinite), update the duration attribute to the value positive Infinity.
The user agent will queue a media
element task given the media element to fire an event named durationchange at the element at this point.
For video elements, set the videoWidth and videoHeight attributes, and queue a media
element task given the media element to fire an event named resize at the media element.
Further resize events will be fired
if the dimensions subsequently change.
Set the readyState attribute to HAVE_METADATA.
A loadedmetadata DOM event
will be fired as part of setting the readyState attribute to a new value.
Let jumped be false.
If the media element's default playback start position is greater than zero, then seek to that time, and let jumped be true.
Let the media element's default playback start position be zero.
Let the initial playback position be zero.
If either the media resource or the URL of the current media resource indicate a particular start time, then set the initial playback position to that time and, if jumped is still false, seek to that time.
For example, with media formats that support media fragment syntax, the fragment can be used to indicate a start position.
If there is no enabled audio track, then
enable an audio track. This will cause a change event to be fired.
If there is no selected video track,
then select a video track. This will cause a change event to be fired.
Once the readyState attribute reaches HAVE_CURRENT_DATA, after
the loadeddata event has been fired, set the
element's delaying-the-load-event flag to false. This stops delaying the load event.
A user agent that is attempting to reduce network usage while still fetching
the metadata for each media resource would also stop buffering at this point,
following the rules described previously, which involve the
networkState attribute switching to the NETWORK_IDLE value and a suspend event firing.
The user agent is required to determine the duration of the media resource and go through this step before playing.
Fire an event named progress at the media element.
Set the networkState to NETWORK_IDLE and fire an event named
suspend at the media element.
If the user agent ever discards any media data and then needs to resume the
network activity to obtain it again, then it must queue a media element task
given the media element to set the networkState to NETWORK_LOADING.
If the user agent can keep the media resource loaded, then the algorithm will continue to its final step below, which aborts the algorithm.
Fatal network errors that occur after the user agent has established whether the current media resource is usable (i.e. once the media element's
readyState attribute is no longer HAVE_NOTHING) must cause the user agent to execute the
following steps:
The user agent should cancel the fetching process.
Set the error attribute to the result of
creating a MediaError with MEDIA_ERR_NETWORK.
Set the element's networkState attribute
to the NETWORK_IDLE value.
Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
Fire an event named error at the media element.
Abort the overall resource selection algorithm.
Fatal errors in decoding the media data that occur after the user agent has
established whether the current media resource is usable (i.e. once the media element's
readyState attribute is no longer HAVE_NOTHING) must cause the
user agent to execute the following steps:
The user agent should cancel the fetching process.
Set the error attribute to the result of
creating a MediaError with MEDIA_ERR_DECODE.
Set the element's networkState attribute
to the NETWORK_IDLE value.
Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
Fire an event named error at the media element.
Abort the overall resource selection algorithm.
The fetching process is aborted by the user, e.g. because the user
pressed a "stop" button, the user agent must execute the following steps. These steps are not
followed if the load() method itself is invoked while
these steps are running, as the steps above handle that particular kind of abort.
The user agent should cancel the fetching process.
Set the error attribute to the result of
creating a MediaError with MEDIA_ERR_ABORTED.
Fire an event named abort at the media element.
If the media element's readyState
attribute has a value equal to HAVE_NOTHING, set
the element's networkState attribute to the
NETWORK_EMPTY value, set the element's
show poster flag to true, and fire an
event named emptied at the element.
Otherwise, set the element's networkState
attribute to the NETWORK_IDLE value.
Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
Abort the overall resource selection algorithm.
The server returning data that is partially usable but cannot be optimally rendered must cause the user agent to render just the bits it can handle, and ignore the rest.
If the media data is CORS-same-origin, run the steps to expose a media-resource-specific text track with the relevant data.
Cross-origin videos do not expose their subtitles, since that would allow attacks such as hostile sites reading subtitles from confidential videos on a user's intranet.
Final step: If the user agent ever reaches this step (which can only happen if the entire resource gets loaded and kept available): abort the overall resource selection algorithm.
When a media element is to forget the media element's media-resource-specific
tracks, the user agent must remove from the media element's list of text
tracks all the media-resource-specific
text tracks, then empty the media element's audioTracks attribute's AudioTrackList object,
then empty the media element's videoTracks
attribute's VideoTrackList object. No events (in particular, no removetrack events) are fired as part of this; the error and emptied
events, fired by the algorithms that invoke this one, can be used instead.
The preload
attribute is an enumerated attribute. The following table lists the keywords and
states for the attribute — the keywords in the left column map to the states in the cell in
the second column on the same row as the keyword. The attribute can be changed even once the
media resource is being buffered or played; the descriptions in the table below are
to be interpreted with that in mind.
| Keyword | State | Brief description |
|---|---|---|
none
| None | Hints to the user agent that either the author does not expect the user to need the media resource, or that the server wants to minimize unnecessary traffic. This state does not provide a hint regarding how aggressively to actually download the media resource if buffering starts anyway (e.g. once the user hits "play"). |
metadata
| Metadata | Hints to the user agent that the author does not expect the user to need the media resource, but that fetching the resource metadata (dimensions, track list, duration, etc), and maybe even the first few frames, is reasonable. If the user agent precisely fetches no more than the metadata, then the media element will end up with its readyState attribute set to HAVE_METADATA; typically though, some frames will be obtained as well and it will probably be HAVE_CURRENT_DATA or HAVE_FUTURE_DATA.
When the media resource is playing, hints to the user agent that bandwidth is to be considered scarce, e.g. suggesting throttling the download so that the media data is obtained at the slowest possible rate that still maintains consistent playback.
|
auto
| Automatic | Hints to the user agent that the user agent can put the user's needs first without risk to the server, up to and including optimistically downloading the entire resource. |
The empty string is also a valid keyword, and maps to the Automatic state. The attribute's missing value default and invalid value default are implementation-defined, though the Metadata state is suggested as a compromise between reducing server load and providing an optimal user experience.
Authors might switch the attribute from "none" or "metadata" to "auto" dynamically once the user begins playback. For
example, on a page with many videos this might be used to indicate that the many videos are not to
be downloaded unless requested, but that once one is requested it is to be downloaded
aggressively.
The preload attribute is intended to provide a hint to
the user agent about what the author thinks will lead to the best user experience. The attribute
may be ignored altogether, for example based on explicit user preferences or based on the
available connectivity.
The preload
IDL attribute must reflect the content attribute of the same name, limited to
only known values.
The autoplay attribute can override the
preload attribute (since if the media plays, it naturally
has to buffer first, regardless of the hint given by the preload attribute). Including both is not an error, however.
bufferedSupport in all current engines.
Returns a TimeRanges object that represents the ranges of the media
resource that the user agent has buffered.
The buffered attribute must return a new static
normalized TimeRanges object that represents the ranges of the
media resource, if any, that the user agent has buffered, at the time the attribute
is evaluated. Users agents must accurately determine the ranges available, even for media streams
where this can only be determined by tedious inspection.
Typically this will be a single range anchored at the zero point, but if, e.g. the user agent uses HTTP range requests in response to seeking, then there could be multiple ranges.
User agents may discard previously buffered data.
Thus, a time position included within a range of the objects return by the buffered attribute at one time can end up being not included in
the range(s) of objects returned by the same attribute at later times.
Returning a new object each time is a bad pattern for attribute getters and is only enshrined here as it would be costly to change it. It is not to be copied to new APIs.
durationSupport in all current engines.
Returns the length of the media resource, in seconds, assuming that the start of the media resource is at time zero.
Returns NaN if the duration isn't available.
Returns Infinity for unbounded streams.
currentTime [ = value ]Support in all current engines.
Returns the official playback position, in seconds.
Can be set, to seek to the given time.
A media resource has a media timeline that maps times (in seconds) to positions in the media resource. The origin of a timeline is its earliest defined position. The duration of a timeline is its last defined position.
Establishing the media
timeline: if the media resource somehow specifies an explicit timeline whose
origin is not negative (i.e. gives each frame a specific time offset and gives the first frame a
zero or positive offset), then the media timeline should be that timeline. (Whether
the media resource can specify a timeline or not depends on the media resource's format.) If the media resource specifies an
explicit start time and date, then that time and date should be considered the zero point
in the media timeline; the timeline offset will be the time and date,
exposed using the getStartDate() method.
If the media resource has a discontinuous timeline, the user agent must extend the timeline used at the start of the resource across the entire resource, so that the media timeline of the media resource increases linearly starting from the earliest possible position (as defined below), even if the underlying media data has out-of-order or even overlapping time codes.
For example, if two clips have been concatenated into one video file, but the video format exposes the original times for the two clips, the video data might expose a timeline that goes, say, 00:15..00:29 and then 00:05..00:38. However, the user agent would not expose those times; it would instead expose the times as 00:15..00:29 and 00:29..01:02, as a single video.
In the rare case of a media resource that does not have an explicit timeline, the
zero time on the media timeline should correspond to the first frame of the
media resource. In the even rarer case of a media resource with no
explicit timings of any kind, not even frame durations, the user agent must itself determine the
time for each frame in an implementation-defined manner.
An example of a file format with no explicit timeline but with explicit frame
durations is the Animated GIF format. An example of a file format with no explicit timings at all
is the JPEG-push format (multipart/x-mixed-replace with JPEG frames, often
used as the format for MJPEG streams).
If, in the case of a resource with no timing information, the user agent will nonetheless be able to seek to an earlier point than the first frame originally provided by the server, then the zero time should correspond to the earliest seekable time of the media resource; otherwise, it should correspond to the first frame received from the server (the point in the media resource at which the user agent began receiving the stream).
At the time of writing, there is no known format that lacks explicit frame time offsets yet still supports seeking to a frame before the first frame sent by the server.
Consider a stream from a TV broadcaster, which begins streaming on a sunny Friday afternoon in
October, and always sends connecting user agents the media data on the same media timeline, with
its zero time set to the start of this stream. Months later, user agents connecting to this
stream will find that the first frame they receive has a time with millions of seconds. The getStartDate() method would always return the date that the
broadcast started; this would allow controllers to display real times in their scrubber (e.g.
"2:30pm") rather than a time relative to when the broadcast began ("8 months, 4 hours, 12
minutes, and 23 seconds").
Consider a stream that carries a video with several concatenated fragments, broadcast by a
server that does not allow user agents to request specific times but instead just streams the
video data in a predetermined order, with the first frame delivered always being identified as
the frame with time zero. If a user agent connects to this stream and receives fragments defined
as covering timestamps 2010-03-20 23:15:00 UTC to 2010-03-21 00:05:00 UTC and 2010-02-12 14:25:00
UTC to 2010-02-12 14:35:00 UTC, it would expose this with a media timeline starting
at 0s and extending to 3,600s (one hour). Assuming the streaming server disconnected at the end
of the second clip, the duration attribute would then
return 3,600. The getStartDate() method would return a
Date object with a time corresponding to 2010-03-20 23:15:00 UTC. However, if a
different user agent connected five minutes later, it would (presumably) receive
fragments covering timestamps 2010-03-20 23:20:00 UTC to 2010-03-21 00:05:00 UTC and 2010-02-12
14:25:00 UTC to 2010-02-12 14:35:00 UTC, and would expose this with a media timeline
starting at 0s and extending to 3,300s (fifty five minutes). In this case, the getStartDate() method would return a Date object
with a time corresponding to 2010-03-20 23:20:00 UTC.
In both of these examples, the seekable attribute
would give the ranges that the controller would want to actually display in its UI; typically, if
the servers don't support seeking to arbitrary times, this would be the range of time from the
moment the user agent connected to the stream up to the latest frame that the user agent has
obtained; however, if the user agent starts discarding earlier information, the actual range
might be shorter.
In any case, the user agent must ensure that the earliest possible position (as defined below) using the established media timeline, is greater than or equal to zero.
The media timeline also has an associated clock. Which clock is used is user-agent defined, and may be media resource-dependent, but it should approximate the user's wall clock.
Media elements have a current playback position, which must initially (i.e. in the absence of media data) be zero seconds. The current playback position is a time on the media timeline.
Media elements also have an official playback position, which must initially be set to zero seconds. The official playback position is an approximation of the current playback position that is kept stable while scripts are running.
Media elements also have a default playback start position, which must initially be set to zero seconds. This time is used to allow the element to be seeked even before the media is loaded.
Each media element has a show poster flag. When a media
element is created, this flag must be set to true. This flag is used to control when the
user agent is to show a poster frame for a video element instead of showing the video
contents.
The currentTime attribute must, on
getting, return the media element's default playback start position,
unless that is zero, in which case it must return the element's official playback
position. The returned value must be expressed in seconds. On setting, if the media
element's readyState is HAVE_NOTHING, then it must set the media
element's default playback start position to the new value; otherwise, it must
set the official playback position to the new value and then seek to the new value. The new value must be interpreted as being in
seconds.
If the media resource is a streaming resource, then the user agent might be unable to obtain certain parts of the resource after it has expired from its buffer. Similarly, some media resources might have a media timeline that doesn't start at zero. The earliest possible position is the earliest position in the stream or resource that the user agent can ever obtain again. It is also a time on the media timeline.
The earliest possible position is not explicitly exposed in the API;
it corresponds to the start time of the first range in the seekable attribute's TimeRanges object, if any, or
the current playback position otherwise.
When the earliest possible position changes, then: if the current playback
position is before the earliest possible position, the user agent must seek to the earliest possible position; otherwise, if
the user agent has not fired a timeupdate event at
the element in the past 15 to 250ms and is not still running event handlers for such an event,
then the user agent must queue a media element task given the media
element to fire an event named timeupdate at the element.
Because of the above requirement and the requirement in the resource fetch algorithm that kicks in when the metadata of the clip becomes known, the current playback position can never be less than the earliest possible position.
If at any time the user agent learns that an audio or video track has ended and all media data relating to that track corresponds to parts of the media timeline that are before the earliest possible position, the user agent may queue a media element task given the media element to run these steps:
Remove the track from the audioTracks
attribute's AudioTrackList object or the videoTracks attribute's VideoTrackList object
as appropriate.
Fire an event named removetrack at the media element's
aforementioned AudioTrackList or VideoTrackList object, using
TrackEvent, with the track attribute
initialized to the AudioTrack or VideoTrack object representing the
track.
The duration attribute must return the time of the end of the
media resource, in seconds, on the media timeline. If no media
data is available, then the attributes must return the Not-a-Number (NaN) value. If the
media resource is not known to be bounded (e.g. streaming radio, or a live event with
no announced end time), then the attribute must return the positive Infinity value.
The user agent must determine the duration of the media resource before playing
any part of the media data and before setting readyState to a value equal to or greater than HAVE_METADATA, even if doing so requires fetching multiple
parts of the resource.
When the length of the media resource changes to a known value
(e.g. from being unknown to known, or from a previously established length to a new length) the
user agent must queue a media element task given the media element to
fire an event named durationchange at the media element. (The
event is not fired when the duration is reset as part of loading a new media resource.) If the
duration is changed such that the current playback position ends up being greater
than the time of the end of the media resource, then the user agent must also seek to the time of the end of the media resource.
If an "infinite" stream ends for some reason, then the duration would change
from positive Infinity to the time of the last frame or sample in the stream, and the durationchange event would be fired. Similarly, if the
user agent initially estimated the media resource's duration instead of determining
it precisely, and later revises the estimate based on new information, then the duration would
change and the durationchange event would be
fired.
Some video files also have an explicit date and time corresponding to the zero time in the media timeline, known as the timeline offset. Initially, the timeline offset must be set to Not-a-Number (NaN).
The getStartDate() method must return a new Date object representing the current
timeline offset.
The loop attribute is
a boolean attribute that, if specified, indicates that the media element
is to seek back to the start of the media resource upon reaching the end.
Support in all current engines.
The loop IDL
attribute must reflect the content attribute of the same name.
readyStateSupport in all current engines.
Returns a value that expresses the current state of the element with respect to rendering the current playback position, from the codes in the list below.
Media elements have a ready state, which describes to what degree they are ready to be rendered at the current playback position. The possible values are as follows; the ready state of a media element at any particular time is the greatest value describing the state of the element:
HAVE_NOTHING (numeric value 0)No information regarding the media resource is available. No data for the
current playback position is available. Media
elements whose networkState attribute are set
to NETWORK_EMPTY are always in the HAVE_NOTHING state.
HAVE_METADATA (numeric value 1)Enough of the resource has been obtained that the duration of the resource is available.
In the case of a video element, the dimensions of the video are also available. No
media data is available for the immediate current playback
position.
HAVE_CURRENT_DATA (numeric value 2)Data for the immediate current playback position is available, but either not
enough data is available that the user agent could successfully advance the current
playback position in the direction of playback at all without immediately
reverting to the HAVE_METADATA state, or there is no
more data to obtain in the direction of playback. For example, in video this
corresponds to the user agent having data from the current frame, but not the next frame, when
the current playback position is at the end of the current frame; and to when playback has ended.
HAVE_FUTURE_DATA (numeric value 3)Data for the immediate current playback position is available, as well as
enough data for the user agent to advance the current playback position in the
direction of playback at least a little without immediately reverting to the HAVE_METADATA state, and the text tracks are
ready. For example, in video this corresponds to the user agent having data for at least
the current frame and the next frame when the current playback position is at the
instant in time between the two frames, or to the user agent having the video data for the
current frame and audio data to keep playing at least a little when the current playback
position is in the middle of a frame. The user agent cannot be in this state if playback has ended, as the current playback position
can never advance in this case.
HAVE_ENOUGH_DATA (numeric value 4)All the conditions described for the HAVE_FUTURE_DATA state are met, and, in addition,
either of the following conditions is also true:
playbackRate, would not overtake the available data
before playback reaches the end of the media resource.In practice, the difference between HAVE_METADATA and HAVE_CURRENT_DATA is negligible. Really the only time
the difference is relevant is when painting a video element onto a
canvas, where it distinguishes the case where something will be drawn (HAVE_CURRENT_DATA or greater) from the case where
nothing is drawn (HAVE_METADATA or less). Similarly,
the difference between HAVE_CURRENT_DATA (only
the current frame) and HAVE_FUTURE_DATA (at least
this frame and the next) can be negligible (in the extreme, only one frame). The only time that
distinction really matters is when a page provides an interface for "frame-by-frame"
navigation.
When the ready state of a media element whose networkState is not NETWORK_EMPTY changes, the user agent must follow the steps
given below:
Apply the first applicable set of substeps from the following list:
HAVE_NOTHING,
and the new ready state is HAVE_METADATAQueue a media element task given the media element to fire an event named loadedmetadata at the element.
Before this task is run, as part of the event loop mechanism, the
rendering will have been updated to resize the video element if appropriate.
HAVE_METADATA and the new ready state is HAVE_CURRENT_DATA or greaterIf this is the first time this occurs for this media
element since the load() algorithm was last
invoked, the user agent must queue a media element task given the media
element to fire an event named loadeddata at the element.
If the new ready state is HAVE_FUTURE_DATA
or HAVE_ENOUGH_DATA, then the relevant steps
below must then be run also.
HAVE_FUTURE_DATA or more, and the new ready state is
HAVE_CURRENT_DATA or lessIf the media element was potentially
playing before its readyState attribute
changed to a value lower than HAVE_FUTURE_DATA, and the element has not
ended playback, and playback has not stopped due to errors,
paused for user interaction, or paused for in-band content, the user
agent must queue a media element task given the media element to
fire an event named timeupdate at the element, and queue a media
element task given the media element to fire an event named waiting at the element.
HAVE_CURRENT_DATA or less, and the new ready state
is HAVE_FUTURE_DATAThe user agent must queue a media element task given the media
element to fire an event named canplay at the element.
If the element's paused attribute is false, the user
agent must notify about playing for the element.
HAVE_ENOUGH_DATAIf the previous ready state was HAVE_CURRENT_DATA or less, the user agent must
queue a media element task given the media element to fire an event named canplay at the element, and, if the element's paused attribute is false, notify about playing
for the element.
The user agent must queue a media element task given the media
element to fire an event named canplaythrough at the element.
If the element is not eligible for autoplay, then the user agent must abort these substeps.
The user agent may run the following substeps:
paused attribute to false.play at the element.Alternatively, if the element is a video element, the user agent may start
observing whether the element intersects the
viewport. When the element starts intersecting
the viewport, if the element is still eligible for autoplay, run the
substeps above. Optionally, when the element stops intersecting the viewport, if the can autoplay flag is still
true and the autoplay attribute is still specified,
run the following substeps:
pause at the element.The substeps for playing and pausing can run multiple times as the element starts or stops intersecting the viewport, as long as the can autoplay flag is true.
User agents do not need to support autoplay, and it is suggested that user
agents honor user preferences on the matter. Authors are urged to use the autoplay attribute rather than using script to force the
video to play, so as to allow the user to override the behavior if so desired.
It is possible for the ready state of a media element to jump between these states
discontinuously. For example, the state of a media element can jump straight from HAVE_METADATA to HAVE_ENOUGH_DATA without passing through the HAVE_CURRENT_DATA and HAVE_FUTURE_DATA states.
The readyState IDL attribute must, on
getting, return the value described above that describes the current ready state of the
media element.
The autoplay
attribute is a boolean attribute. When present, the user agent (as
described in the algorithm described herein) will automatically begin playback of the
media resource as soon as it can do so without stopping.
Authors are urged to use the autoplay
attribute rather than using script to trigger automatic playback, as this allows the user to
override the automatic playback when it is not desired, e.g. when using a screen reader. Authors
are also encouraged to consider not using the automatic playback behavior at all, and instead to
let the user agent wait for the user to start playback explicitly.
Support in all current engines.
The autoplay IDL attribute must reflect the
content attribute of the same name.
pausedSupport in all current engines.
Returns true if playback is paused; false otherwise.
endedSupport in all current engines.
Returns true if playback has reached the end of the media resource.
defaultPlaybackRate [ = value ]HTMLMediaElement/defaultPlaybackRate
Support in all current engines.
Returns the default rate of playback, for when the user is not fast-forwarding or reversing through the media resource.
Can be set, to change the default rate of playback.
The default rate has no direct effect on playback, but if the user switches to a fast-forward mode, when they return to the normal playback mode, it is expected that the rate of playback will be returned to the default rate of playback.
playbackRate [ = value ]Support in all current engines.
Returns the current rate playback, where 1.0 is normal speed.
Can be set, to change the rate of playback.
preservesPitchHTMLMediaElement/preservesPitch
Returns true if pitch-preserving algorithms are used when the playbackRate is not 1.0. The default value is true.
Can be set to false to have the media resource's audio pitch change up or down
depending on the playbackRate. This is useful for
aesthetic and performance reasons.
playedReturns a TimeRanges object that represents the ranges of the media
resource that the user agent has played.
play()Support in all current engines.
Sets the paused attribute to false, loading the
media resource and beginning playback if necessary. If the playback had ended, will
restart it from the start.
pause()Support in all current engines.
Sets the paused attribute to true, loading the
media resource if necessary.
The paused
attribute represents whether the media element is paused or not. The attribute must
initially be true.
A media element is a blocked media element if its readyState attribute is in the HAVE_NOTHING state, the HAVE_METADATA state, or the HAVE_CURRENT_DATA state, or if the element has
paused for user interaction or paused for in-band content.
A media element is said to be potentially playing when its paused attribute is false, the element has not ended
playback, playback has not stopped due to errors, and the element is not a
blocked media element.
A waiting DOM event can be fired as a result of an element that is
potentially playing stopping playback due to its readyState attribute changing to a value lower than HAVE_FUTURE_DATA.
A media element is said to be eligible for autoplay when all of the following conditions are met:
paused attribute is true.autoplay attribute specified.autoplay" feature.A media element is said to be allowed to play if the user agent and the system allow media playback in the current context.
For example, a user agent could allow playback only when the media
element's Window object has transient activation, but an
exception could be made to allow playback while muted.
A media element is said to have ended playback when:
readyState attribute is HAVE_METADATA or greater, and
Either:
loop
attribute specified.
Or:
The ended
attribute must return true if, the last time the event loop reached step 1, the media element had ended playback and the
direction of playback was forwards, and false otherwise.
A media element is said to have stopped due to errors when the
element's readyState attribute is HAVE_METADATA or greater, and the user agent encounters a non-fatal error during the processing of the
media data, and due to that error, is not able to play the content at the
current playback position.
A media element is said to have paused for user interaction when its
paused attribute is false, the readyState attribute is either HAVE_FUTURE_DATA or HAVE_ENOUGH_DATA and the user agent has reached a point
in the media resource where the user has to make a selection for the resource to
continue.
It is possible for a media element to have both ended playback and paused for user interaction at the same time.
When a media element that is potentially playing stops playing
because it has paused for user interaction, the user agent must queue a media
element task given the media element to fire
an event named timeupdate at the element.
A media element is said to have paused for in-band content when its
paused attribute is false, the readyState attribute is either HAVE_FUTURE_DATA or HAVE_ENOUGH_DATA and the user agent has suspended
playback of the media resource in order to play content that is temporally anchored
to the media resource and has a nonzero length, or to play content that is
temporally anchored to a segment of the media resource but has a length longer than
that segment.
One example of when a media element would be paused for in-band content is when the user agent is playing audio descriptions from an external WebVTT file, and the synthesized speech generated for a cue is longer than the time between the text track cue start time and the text track cue end time.
When the current playback position reaches the end of the media resource when the direction of playback is forwards, then the user agent must follow these steps:
If the media element has a loop
attribute specified, then seek to the earliest
possible position of the media resource and return.
As defined above, the ended IDL attribute starts
returning true once the event loop returns to step 1.
Queue a media element task given the media element and the following steps:
Fire an event named timeupdate at the media element.
If the media element has ended playback, the direction of playback is forwards, and paused is false, then:
Set the paused attribute to true.
Fire an event named pause at the media element.
Take pending play promises and reject pending play promises
with the result and an "AbortError"
DOMException.
Fire an event named ended at the media element.
When the current playback position reaches the earliest possible
position of the media resource when the direction of playback is
backwards, then the user agent must only queue a media element task given the
media element to fire an event named timeupdate at the element.
The word "reaches" here does not imply that the current playback position needs to have changed during normal playback; it could be via seeking, for instance.
The defaultPlaybackRate attribute
gives the desired speed at which the media resource is to play, as a multiple of its
intrinsic speed. The attribute is mutable: on getting it must return the last value it was set to,
or 1.0 if it hasn't yet been set; on setting the attribute must be set to the new value.
The defaultPlaybackRate is used
by the user agent when it exposes a user
interface to the user.
The playbackRate attribute gives the
effective playback rate, which is the speed at which the media resource plays, as a
multiple of its intrinsic speed. If it is not equal to the defaultPlaybackRate, then the implication is that
the user is using a feature such as fast forward or slow motion playback. The attribute is
mutable: on getting it must return the last value it was set to, or 1.0 if it hasn't yet been set;
on setting, the user agent must follow these steps:
If the given value is not supported by the user agent, then throw a
"NotSupportedError" DOMException.
Set playbackRate to the new value, and if the
element is potentially playing, change the playback speed.
When the defaultPlaybackRate or playbackRate attributes change value (either by being set
by script or by being changed directly by the user agent, e.g. in response to user control) the
user agent must queue a media element task given the media element to
fire an event named ratechange at the media element. The user
agent must process attribute changes smoothly and must not introduce any perceivable gaps or
muting of playback in response.
The preservesPitch getter steps are
to return true if a pitch-preserving algorithm is in effect during playback. The setter steps are
to correspondingly switch the pitch-preserving algorithm on or off, without any perceivable gaps
or muting of playback. By default, such a pitch-preserving algorithm must be in effect (i.e., the
getter will initially return true).
The played
attribute must return a new static normalized TimeRanges object that
represents the ranges of points on the media timeline of the media
resource reached through the usual monotonic increase of the current playback
position during normal playback, if any, at the time the attribute is evaluated.
Returning a new object each time is a bad pattern for attribute getters and is only enshrined here as it would be costly to change it. It is not to be copied to new APIs.
Each media element has a list of pending play promises, which must initially be empty.
To take pending play promises for a media element, the user agent must run the following steps:
Let promises be an empty list of promises.
Copy the media element's list of pending play promises to promises.
Clear the media element's list of pending play promises.
To resolve pending play promises for a media element with a list of promises promises, the user agent must resolve each promise in promises with undefined.
To reject pending play promises for a media element with a list of promise promises and an exception name error, the user agent must reject each promise in promises with error.
To notify about playing for a media element, the user agent must run the following steps:
Take pending play promises and let promises be the result.
Queue a media element task given the element and the following steps:
Fire an event named playing at the element.
Resolve pending play promises with promises.
When the play()
method on a media element is invoked, the user agent must run the following
steps.
If the media element is not allowed to play, return a promise
rejected with a "NotAllowedError" DOMException.
If the media element's error attribute is
not null and its code is MEDIA_ERR_SRC_NOT_SUPPORTED, return a
promise rejected with a "NotSupportedError" DOMException.
This means that the dedicated media source failure steps have run.
Playback is not possible until the media element load algorithm clears the error attribute.
Let promise be a new promise and append promise to the list of pending play promises.
Run the internal play steps for the media element.
Return promise.
The internal play steps for a media element are as follows:
If the media element's networkState attribute has the value NETWORK_EMPTY, invoke the media element's
resource selection algorithm.
If the playback has ended and the direction of playback is forwards, seek to the earliest possible position of the media resource.
This will cause the user agent to queue a media
element task given the media element to fire an event named timeupdate at the media element.
If the media element's paused attribute
is true, then:
Change the value of paused to false.
If the show poster flag is true, set the element's show poster flag to false and run the time marches on steps.
Queue a media element task given the media element to fire an event named play at the element.
If the media element's readyState
attribute has the value HAVE_NOTHING, HAVE_METADATA, or HAVE_CURRENT_DATA, queue a media element
task given the media element to fire an
event named waiting at the element.
Otherwise, the media element's readyState attribute has the value HAVE_FUTURE_DATA or HAVE_ENOUGH_DATA: notify about playing
for the element.
Otherwise, if the media element's readyState attribute has the value HAVE_FUTURE_DATA or HAVE_ENOUGH_DATA, take pending play
promises and queue a media element task given the media element
to resolve pending play promises with the result.
The media element is already playing. However, it's possible that promise will be rejected before the queued task is run.
Set the media element's can autoplay flag to false.
When the pause()
method is invoked, and when the user agent is required to pause the media element,
the user agent must run the following steps:
If the media element's networkState attribute has the value NETWORK_EMPTY, invoke the media element's
resource selection algorithm.
Run the internal pause steps for the media element.
The internal pause steps for a media element are as follows:
Set the media element's can autoplay flag to false.
If the media element's paused attribute
is false, run the following steps:
Change the value of paused to true.
Take pending play promises and let promises be the result.
Queue a media element task on the given the media element and the following steps:
Fire an event named timeupdate at the element.
Fire an event named pause at the element.
Reject pending play promises with promises and an
"AbortError" DOMException.
Set the official playback position to the current playback position.
If the element's playbackRate is positive or zero,
then the direction of playback is forwards. Otherwise, it is backwards.
When a media element is potentially playing and
its Document is a fully active Document, its current
playback position must increase monotonically at the element's playbackRate units of media time per unit time of the
media timeline's clock. (This specification always refers to this as an
increase, but that increase could actually be a decrease if the element's playbackRate is negative.)
The element's playbackRate can be
0.0, in which case the current playback position doesn't move, despite playback not
being paused (paused doesn't become true, and the pause event doesn't fire).
This specification doesn't define how the user agent achieves the appropriate playback rate — depending on the protocol and media available, it is plausible that the user agent could negotiate with the server to have the server provide the media data at the appropriate rate, so that (except for the period between when the rate is changed and when the server updates the stream's playback rate) the client doesn't actually have to drop or interpolate any frames.
Any time the user agent provides a stable state, the official playback position must be set to the current playback position.
While the direction of playback is backwards, any corresponding audio must be
muted. While the element's playbackRate is so low or so high that the user agent
cannot play audio usefully, the corresponding audio must also be muted. If the element's playbackRate is not 1.0 and preservesPitch is true, the user agent must apply pitch
adjustment to preserve the original pitch of the audio. Otherwise, the user agent must speed up
or slow down the audio without any pitch adjustment.
When a media element is potentially playing, its audio data played must be synchronized with the current playback position, at the element's effective media volume. The user agent must play the audio from audio tracks that were enabled when the event loop last reached step 1.
When a media element is not potentially playing, audio must not play for the element.
Media elements that are potentially playing while not in a document must not play any video, but should play any audio component. Media elements must not stop playing just because all references to them have been removed; only once a media element is in a state where no further audio could ever be played by that element may the element be garbage collected.
It is possible for an element to which no explicit references exist to play audio,
even if such an element is not still actively playing: for instance, it could be unpaused but
stalled waiting for content to buffer, or it could be still buffering, but with a
suspend event listener that begins playback. Even a
media element whose media resource has no audio tracks could eventually play audio
again if it had an event listener that changes the media resource.
Each media element has a list of newly introduced cues, which must be initially empty. Whenever a text track cue is added to the list of cues of a text track that is in the list of text tracks for a media element, that cue must be added to the media element's list of newly introduced cues. Whenever a text track is added to the list of text tracks for a media element, all of the cues in that text track's list of cues must be added to the media element's list of newly introduced cues. When a media element's list of newly introduced cues has new cues added while the media element's show poster flag is not set, then the user agent must run the time marches on steps.
When a text track cue is removed from the list of cues of a text track that is in the list of text tracks for a media element, and whenever a text track is removed from the list of text tracks of a media element, if the media element's show poster flag is not set, then the user agent must run the time marches on steps.
When the current playback position of a media element changes (e.g. due to playback or seeking), the user agent must run the time marches on steps. To support use cases that depend on the timing accuracy of cue event firing, such as synchronizing captions with shot changes in a video, user agents should fire cue events as close as possible to their position on the media timeline, and ideally within 20 milliseconds. If the current playback position changes while the steps are running, then the user agent must wait for the steps to complete, and then must immediately rerun the steps. These steps are thus run as often as possible or needed.
If one iteration takes a long time, this can cause short duration
cues to be skipped over as the user agent rushes ahead to
"catch up", so these cues will not appear in the activeCues list.
The time marches on steps are as follows:
Let current cues be a list of cues, initialized to contain all the cues of all the or showing text tracks of the media element (not the disabled ones) whose start times are less than or equal to the current playback position and whose end times are greater than the current playback position.
Let other cues be a list of cues, initialized to contain all the cues of and showing text tracks of the media element that are not present in current cues.
Let last time be the current playback position at the time this algorithm was last run for this media element, if this is not the first time it has run.
If the current playback position has, since the last time this algorithm was run, only changed through its usual monotonic increase during normal playback, then let missed cues be the list of cues in other cues whose start times are greater than or equal to last time and whose end times are less than or equal to the current playback position. Otherwise, let missed cues be an empty list.
Remove all the cues in missed cues that are also in the media element's list of newly introduced cues, and then empty the element's list of newly introduced cues.
If the time was reached through the usual monotonic increase of the current playback
position during normal playback, and if the user agent has not fired a timeupdate event at the element in the past 15 to 250ms
and is not still running event handlers for such an event, then the user agent must queue a
media element task given the media element to fire an event named timeupdate at the element. (In the other cases, such as
explicit seeks, relevant events get fired as part of the overall process of changing the
current playback position.)
The event thus is not to be fired faster than about 66Hz or slower than 4Hz (assuming the event handlers don't take longer than 250ms to run). User agents are encouraged to vary the frequency of the event based on the system load and the average cost of processing the event each time, so that the UI updates are not any more frequent than the user agent can comfortably handle while decoding the video.
If all of the cues in current cues have their text track cue active flag set, none of the cues in other cues have their text track cue active flag set, and missed cues is empty, then return.
If the time was reached through the usual monotonic increase of the current playback position during normal playback, and there are cues in other cues that have their text track cue pause-on-exit flag set and that either have their text track cue active flag set or are also in missed cues, then immediately pause the media element.
In the other cases, such as explicit seeks, playback is not paused by going past the end time of a cue, even if that cue has its text track cue pause-on-exit flag set.
Let events be a list of tasks, initially empty. Each task in this list will be associated with a text track, a text track cue, and a time, which are used to sort the list before the tasks are queued.
Let affected tracks be a list of text tracks, initially empty.
When the steps below say to prepare an event named event for a text track cue target with a time time, the user agent must run these steps:
Let track be the text track with which the text track cue target is associated.
Create a task to fire an event named event at target.
Add the newly created task to events, associated with the time time, the text track track, and the text track cue target.
Add track to affected tracks.
For each text track cue in missed
cues, prepare an event named enter for the
TextTrackCue object with the text track cue start time.
For each text track cue in other
cues that either has its text track cue active flag set or is in missed cues, prepare an event named exit for the TextTrackCue object with the later of the
text track cue end time and the text track cue start time.
For each text track cue in current
cues that does not have its text track cue active flag set, prepare an
event named enter for the TextTrackCue
object with the text track cue start time.
Sort the tasks in events in ascending time order (tasks with earlier times first).
Further sort tasks in events that have the same time by the relative text track cue order of the text track cues associated with these tasks.
Finally, sort tasks in events that have the
same time and same text track cue order by placing tasks that fire enter
events before those that fire exit events.
Queue a media element task given the media element for each task in events, in list order.
Sort affected tracks in the same order as the text tracks appear in the media element's list of text tracks, and remove duplicates.
For each text track in affected tracks, in the list order,
queue a media element task given the media element to fire an event named cuechange at the TextTrack object, and, if the
text track has a corresponding track element, to then fire an event named cuechange at the track element as well.
Set the text track cue active flag of all the cues in the current cues, and unset the text track cue active flag of all the cues in the other cues.
Run the rules for updating the text track rendering of each of the text tracks in affected tracks that are showing, providing the text track's text track language as the fallback language if it is not the empty string. For example, for text tracks based on WebVTT, the rules for updating the display of WebVTT text tracks. [WEBVTT]
For the purposes of the algorithm above, a text track cue is considered to be part of a text track only if it is listed in the text track list of cues, not merely if it is associated with the text track.
If the media element's node document stops being a fully active document, then the playback will stop until the document is active again.
When a media element is removed
from a Document, the user agent must run the following steps:
Await a stable state, allowing the task that removed the media element from the
Document to continue. The synchronous section consists of all the
remaining steps of this algorithm. (Steps in the synchronous section are marked with
⌛.)
⌛ If the media element is in a document, return.
⌛ Run the internal pause steps for the media element.
seekingReturns true if the user agent is currently seeking.
seekableSupport in all current engines.
Returns a TimeRanges object that represents the ranges of the media
resource to which it is possible for the user agent to seek.
fastSeek( time )Seeks to near the given time as fast as possible, trading precision for
speed. (To seek to a precise time, use the currentTime attribute.)
This does nothing if the media resource has not been loaded.
The seeking
attribute must initially have the value false.
The fastSeek() method must seek to the time given by the method's argument, with the
approximate-for-speed flag set.
When the user agent is required to seek to a particular new playback position in the media resource, optionally with the approximate-for-speed flag set, it means that the user agent must run the following steps. This algorithm interacts closely with the event loop mechanism; in particular, it has a synchronous section (which is triggered as part of the event loop algorithm). Steps in that section are marked with ⌛.
Set the media element's show poster flag to false.
If the media element's readyState
is HAVE_NOTHING, return.
If the element's seeking IDL attribute is true,
then another instance of this algorithm is already running. Abort that other instance of the
algorithm without waiting for the step that it is running to complete.
Set the seeking IDL attribute to true.
If the seek was in response to a DOM method call or setting of an IDL attribute, then continue the script. The remainder of these steps must be run in parallel. With the exception of the steps marked with ⌛, they could be aborted at any time by another instance of this algorithm being invoked.
If the new playback position is later than the end of the media resource, then let it be the end of the media resource instead.
If the new playback position is less than the earliest possible position, let it be that position instead.
If the (possibly now changed) new playback position is not in one of
the ranges given in the seekable attribute, then let it
be the position in one of the ranges given in the seekable attribute that is the nearest to the new
playback position. If two positions both satisfy that constraint (i.e. the new playback position is exactly in the middle between two ranges in the seekable attribute) then use the position that is closest to
the current playback position. If there are no ranges given in the seekable attribute then set the seeking IDL attribute to false and return.
If the approximate-for-speed flag is set, adjust the new playback position to a value that will allow for playback to resume promptly. If new playback position before this step is before current playback position, then the adjusted new playback position must also be before the current playback position. Similarly, if the new playback position before this step is after current playback position, then the adjusted new playback position must also be after the current playback position.
For example, the user agent could snap to a nearby key frame, so that it doesn't have to spend time decoding then discarding intermediate frames before resuming playback.
Queue a media element task given the media element to fire an event named seeking at the element.
Set the current playback position to the new playback position.
If the media element was potentially playing
immediately before it started seeking, but seeking caused its readyState attribute to change to a value lower than HAVE_FUTURE_DATA, then a waiting event will be
fired at the element.
This step sets the current playback position, and thus can immediately trigger other conditions, such as the rules regarding when playback "reaches the end of the media resource" (part of the logic that handles looping), even before the user agent is actually able to render the media data for that position (as determined in the next step).
The currentTime attribute returns
the official playback position, not the current playback position, and
therefore gets updated before script execution, separate from this algorithm.
Wait until the user agent has established whether or not the media data for the new playback position is available, and, if it is, until it has decoded enough data to play back that position.
Await a stable state. The synchronous section consists of all the remaining steps of this algorithm. (Steps in the synchronous section are marked with ⌛.)
⌛ Set the seeking IDL attribute to
false.
⌛ Run the time marches on steps.
⌛ Queue a media element task given the media
element to fire an event named timeupdate at the element.
⌛ Queue a media element task given the media element to
fire an event named seeked at the element.
The seekable attribute must return a new static
normalized TimeRanges object that represents the ranges of the
media resource, if any, that the user agent is able to seek to, at the time the
attribute is evaluated.
If the user agent can seek to anywhere in the media resource, e.g.
because it is a simple movie file and the user agent and the server support HTTP Range requests,
then the attribute would return an object with one range, whose start is the time of the first
frame (the earliest possible position, typically zero), and whose end is the same as
the time of the first frame plus the duration attribute's
value (which would equal the time of the last frame, and might be positive Infinity).
The range might be continuously changing, e.g. if the user agent is buffering a sliding window on an infinite stream. This is the behavior seen with DVRs viewing live TV, for instance.
Returning a new object each time is a bad pattern for attribute getters and is only enshrined here as it would be costly to change it. It is not to be copied to new APIs.
User agents should adopt a very liberal and optimistic view of what is seekable. User agents should also buffer recent content where possible to enable seeking to be fast.
For instance, consider a large video file served on an HTTP server without support for HTTP Range requests. A browser could implement this by only buffering the current frame and data obtained for subsequent frames, never allow seeking, except for seeking to the very start by restarting the playback. However, this would be a poor implementation. A high quality implementation would buffer the last few minutes of content (or more, if sufficient storage space is available), allowing the user to jump back and rewatch something surprising without any latency, and would in addition allow arbitrary seeking by reloading the file from the start if necessary, which would be slower but still more convenient than having to literally restart the video and watch it all the way through just to get to an earlier unbuffered spot.
Media resources might be internally scripted or interactive. Thus, a media element could play in a non-linear fashion. If this happens, the user agent must act as if the algorithm for seeking was used whenever the current playback position changes in a discontinuous fashion (so that the relevant events fire).
媒体资源 可以内嵌多个音视频轨道。例如， 除了主音视频轨道外，媒体资源 可以有外语配音的对话， 导演评论，语音描述，另外的拍摄角度，符号语言，或者手语叠加层。
audioTracksSupport in all current engines.
返回一个表示该 媒体资源 可用的
音频轨道的 AudioTrackList 对象。
videoTracksSupport in all current engines.
返回一个表示该 媒体资源 可用的
视频轨道的 AudioTrackList 对象。
媒体元素 的
audioTracks 属性必须返回
活的 AudioTrackList 对象，表示
在 媒体元素 的 媒体资源
中可用的音频轨道。
媒体元素 的
videoTracks 属性必须返回
活的 VideoTrackList 对象，表示
在 媒体元素 的 媒体资源
中可用的视频轨道。
每个 媒体元素 永远都只有一个
AudioTrackList 对象和一个 VideoTrackList 对象，
即使元素中加载了另一个 媒体资源：
这两个对象会被复用。（但 AudioTrack 和 VideoTrack 对象不会。）
AudioTrackList and VideoTrackList objectsSupport in all current engines.
Support in all current engines.
Support in all current engines.
The AudioTrackList and VideoTrackList interfaces are used by
attributes defined in the previous section.
Support in all current engines.
Support in all current engines.
[Exposed=Window]
interface AudioTrackList : EventTarget {
readonly attribute unsigned long length;
getter AudioTrack (unsigned long index);
AudioTrack? getTrackById(DOMString id);
attribute EventHandler onchange;
attribute EventHandler onaddtrack;
attribute EventHandler onremovetrack;
};
[Exposed=Window]
interface AudioTrack {
readonly attribute DOMString id;
readonly attribute DOMString kind;
readonly attribute DOMString label;
readonly attribute DOMString language;
attribute boolean enabled;
};
[Exposed=Window]
interface VideoTrackList : EventTarget {
readonly attribute unsigned long length;
getter VideoTrack (unsigned long index);
VideoTrack? getTrackById(DOMString id);
readonly attribute long selectedIndex;
attribute EventHandler onchange;
attribute EventHandler onaddtrack;
attribute EventHandler onremovetrack;
};
[Exposed=Window]
interface VideoTrack {
readonly attribute DOMString id;
readonly attribute DOMString kind;
readonly attribute DOMString label;
readonly attribute DOMString language;
attribute boolean selected;
};audioTracks . lengthSupport in all current engines.
videoTracks . lengthSupport in all current engines.
Returns the number of tracks in the list.
audioTracks[index]videoTracks[index]Returns the specified AudioTrack or VideoTrack object.
audioTracks . getTrackById( id )Support in all current engines.
videoTracks . getTrackById( id )Support in all current engines.
Returns the AudioTrack or VideoTrack object with the given identifier, or null if no track has that identifier.
idSupport in all current engines.
idSupport in all current engines.
Returns the ID of the given track. This is the ID that can be used with a fragment if the format supports media fragment
syntax, and that can be used with the getTrackById() method.
kindSupport in all current engines.
kindSupport in all current engines.
Returns the category the given track falls into. The possible track categories are given below.
labelSupport in all current engines.
labelSupport in all current engines.
Returns the label of the given track, if known, or the empty string otherwise.
languageSupport in all current engines.
languageSupport in all current engines.
Returns the language of the given track, if known, or the empty string otherwise.
enabled [ = value ]Support in all current engines.
Returns true if the given track is active, and false otherwise.
Can be set, to change whether the track is enabled or not. If multiple audio tracks are enabled simultaneously, they are mixed.
videoTracks . selectedIndexSupport in all current engines.
Returns the index of the currently selected track, if any, or −1 otherwise.
selected [ = value ]Support in all current engines.
Returns true if the given track is active, and false otherwise.
Can be set, to change whether the track is selected or not. Either zero or one video track is selected; selecting a new track while a previous one is selected will unselect the previous one.
An AudioTrackList object represents a dynamic list of zero or more audio tracks,
of which zero or more can be enabled at a time. Each audio track is represented by an
AudioTrack object.
A VideoTrackList object represents a dynamic list of zero or more video tracks, of
which zero or one can be selected at a time. Each video track is represented by a
VideoTrack object.
Tracks in AudioTrackList and VideoTrackList objects must be
consistently ordered. If the media resource is in a format that defines an order,
then that order must be used; otherwise, the order must be the relative order in which the tracks
are declared in the media resource. The order used is called the natural order
of the list.
Each track in one of these objects thus has an index; the first has the index 0, and each subsequent track is numbered one higher than the previous one. If a media resource dynamically adds or removes audio or video tracks, then the indices of the tracks will change dynamically. If the media resource changes entirely, then all the previous tracks will be removed and replaced with new tracks.
The AudioTrackList length and VideoTrackList
length attribute getters must return
the number of tracks represented by their objects at the time of getting.
The supported property indices of AudioTrackList and
VideoTrackList objects at any instant are the numbers from zero to the number of
tracks represented by the respective object minus one, if any tracks are represented. If an
AudioTrackList or VideoTrackList object represents no tracks, it has no
supported property indices.
To determine the value of an indexed property for a
given index index in an AudioTrackList or VideoTrackList
object list, the user agent must return the AudioTrack or
VideoTrack object that represents the indexth track in list.
The AudioTrackList getTrackById(id) and
VideoTrackList getTrackById(id) methods must
return the first AudioTrack or VideoTrack object (respectively) in the
AudioTrackList or VideoTrackList object (respectively) whose identifier
is equal to the value of the id argument (in the natural order of the list, as defined
above). When no tracks match the given argument, the methods must return null.
The AudioTrack and VideoTrack objects represent specific tracks of a
media resource. Each track can have an identifier, category, label, and language.
These aspects of a track are permanent for the lifetime of the track; even if a track is removed
from a media resource's AudioTrackList or VideoTrackList
objects, those aspects do not change.
In addition, AudioTrack objects can each be enabled or disabled; this is the audio
track's enabled state. When an AudioTrack is created, its enabled state
must be set to false (disabled). The resource fetch
algorithm can override this.
Similarly, a single VideoTrack object per VideoTrackList object can
be selected, this is the video track's selection state. When a VideoTrack is
created, its selection state must be set to false (not selected). The resource fetch algorithm can override this.
The AudioTrack id and
VideoTrack id attributes must
return the identifier of the track, if it has one, or the empty string otherwise. If the
media resource is in a format that supports media fragment syntax, the
identifier returned for a particular track must be the same identifier that would enable the track
if used as the name of a track in the track dimension of such a fragment. [INBAND]
For example, in Ogg files, this would be the Name header field of the track. [OGGSKELETONHEADERS]
The AudioTrack kind and
VideoTrack kind attributes must
return the category of the track, if it has one, or the empty string otherwise.
The category of a track is the string given in the first column of the table below that is the
most appropriate for the track based on the definitions in the table's second and third columns,
as determined by the metadata included in the track in the media resource. The cell
in the third column of a row says what the category given in the cell in the first column of that
row applies to; a category is only appropriate for an audio track if it applies to audio tracks,
and a category is only appropriate for video tracks if it applies to video tracks. Categories must
only be returned for AudioTrack objects if they are appropriate for audio, and must
only be returned for VideoTrack objects if they are appropriate for video.
For Ogg files, the Role header field of the track gives the relevant metadata. For DASH media
resources, the Role element conveys the information. For WebM, only the
FlagDefault element currently maps to a value. Sourcing In-band
Media Resource Tracks from Media Containers into HTML has further details.
[OGGSKELETONHEADERS] [DASH] [WEBMCG] [INBAND]
| Category | Definition | Applies to... | Examples |
|---|---|---|---|
"alternative"
| A possible alternative to the main track, e.g. a different take of a song (audio), or a different angle (video). | Audio and video. | Ogg: "audio/alternate" or "video/alternate"; DASH: "alternate" without "main" and "commentary" roles, and, for audio, without the "dub" role (other roles ignored). |
"captions"
| A version of the main video track with captions burnt in. (For legacy content; new content would use text tracks.) | Video only. | DASH: "caption" and "main" roles together (other roles ignored). |
"descriptions"
| An audio description of a video track. | Audio only. | Ogg: "audio/audiodesc". |
"main"
| The primary audio or video track. | Audio and video. | Ogg: "audio/main" or "video/main"; WebM: the "FlagDefault" element is set; DASH: "main" role without "caption", "subtitle", and "dub" roles (other roles ignored). |
"main-desc"
| The primary audio track, mixed with audio descriptions. | Audio only. | AC3 audio in MPEG-2 TS: bsmod=2 and full_svc=1. |
"sign"
| A sign-language interpretation of an audio track. | Video only. | Ogg: "video/sign". |
"subtitles"
| A version of the main video track with subtitles burnt in. (For legacy content; new content would use text tracks.) | Video only. | DASH: "subtitle" and "main" roles together (other roles ignored). |
"translation"
| A translated version of the main audio track. | Audio only. | Ogg: "audio/dub". DASH: "dub" and "main" roles together (other roles ignored). |
"commentary"
| Commentary on the primary audio or video track, e.g. a director's commentary. | Audio and video. | DASH: "commentary" role without "main" role (other roles ignored). |
" | No explicit kind, or the kind given by the track's metadata is not recognized by the user agent. | Audio and video. |
The AudioTrack label and
VideoTrack label attributes
must return the label of the track, if it has one, or the empty string otherwise. [INBAND]
The AudioTrack language
and VideoTrack language
attributes must return the BCP 47 language tag of the language of the track, if it has one, or the
empty string otherwise. If the user agent is not able to express that language as a BCP 47
language tag (for example because the language information in the media resource's
format is a free-form string without a defined interpretation), then the method must return the
empty string, as if the track had no language. [INBAND]
The AudioTrack enabled
attribute, on getting, must return true if the track is currently enabled, and false otherwise. On
setting, it must enable the track if the new value is true, and disable it otherwise. (If the
track is no longer in an AudioTrackList object, then the track being enabled or
disabled has no effect beyond changing the value of the attribute on the AudioTrack
object.)
Whenever an audio track in an AudioTrackList that was
disabled is enabled, and whenever one that was enabled is disabled, the user agent must
queue a media element task given the media element to fire an event named change at the AudioTrackList object.
An audio track that has no data for a particular position on the media timeline, or that does not exist at that position, must be interpreted as being silent at that point on the timeline.
The VideoTrackList selectedIndex attribute must return the
index of the currently selected track, if any. If the VideoTrackList object does not
currently represent any tracks, or if none of the tracks are selected, it must instead return
−1.
The VideoTrack selected
attribute, on getting, must return true if the track is currently selected, and false otherwise.
On setting, it must select the track if the new value is true, and unselect it otherwise. If the
track is in a VideoTrackList, then all the other VideoTrack objects in
that list must be unselected. (If the track is no longer in a VideoTrackList object,
then the track being selected or unselected has no effect beyond changing the value of the
attribute on the VideoTrack object.)
Whenever a track in a VideoTrackList that was previously
not selected is selected, and whenever the selected track in a VideoTrackList is
unselected without a new track being selected in its stead, the user agent must queue a
media element task given the media element to fire an event named change at the VideoTrackList object. This task must be queued
before the task that fires the resize event, if any.
A video track that has no data for a particular position on the media timeline must be interpreted as being transparent black at that point on the timeline, with the same dimensions as the last frame before that position, or, if the position is before all the data for that track, the same dimensions as the first frame for that track. A track that does not exist at all at the current position must be treated as if it existed but had no data.
For instance, if a video has a track that is only introduced after one hour of playback, and the user selects that track then goes back to the start, then the user agent will act as if that track started at the start of the media resource but was simply transparent until one hour in.
The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes,
by all objects implementing the AudioTrackList and VideoTrackList
interfaces:
| Event handler | Event handler event type |
|---|---|
onchange Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo Support in all current engines. Firefox31+Safari7+Chrome33+ Opera20+Edge79+ Edge (Legacy)18Internet ExplorerNo Firefox Android31+Safari iOS7+Chrome Android33+WebView Android4.4+Samsung Internet2.0+Opera Android20+ Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo | change
|
onaddtrack Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo Support in all current engines. Firefox31+Safari6+Chrome23+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer11 Firefox Android31+Safari iOS7+Chrome Android25+WebView Android4.4+Samsung Internet1.5+Opera Android12.1+ Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo | addtrack
|
onremovetrack AudioTrackList/removetrack_event Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo TextTrackList/removetrack_event Support in all current engines. Firefox31+Safari7+Chrome33+ Opera20+Edge79+ Edge (Legacy)18Internet ExplorerNo Firefox Android31+Safari iOS7+Chrome Android33+WebView Android4.4+Samsung Internet2.0+Opera Android20+ VideoTrackList/removetrack_event Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo | removetrack
|
audioTracks 和 videoTracks 属性使得脚本可以选择应该播放哪个轨道，
但也可以声明式地选择特定的轨道：
在 媒体资源 的 URL 的
fragment 上指定特定的轨道。
fragment 的格式取决于
资源 的 MIME 类型。
[RFC2046] [URL]
下面例子中嵌入的这个视频使用了支持 媒体片段语法 的媒体格式，启用了标注为 "Alternative" 的轨道，而不是默认视频轨道。
<video src="myvideo#track=Alternative"></video>
A media element can have a group of associated text tracks, known as the media element's list of text tracks. The text tracks are sorted as follows:
track element
children of the media element, in tree order.addTextTrack() method, in the order they were added, oldest
first.A text track consists of:
This decides how the track is handled by the user agent. The kind is represented by a string. The possible strings are:
subtitles
captions
descriptions
chapters
metadata
The kind of track can change dynamically, in the case of
a text track corresponding to a track element.
This is a human-readable string intended to identify the track for the user.
The label of a track can change dynamically, in the
case of a text track corresponding to a track element.
When a text track label is the empty string, the user agent should automatically generate an appropriate label from the text track's other properties (e.g. the kind of text track and the text track's language) for use in its user interface. This automatically-generated label is not exposed in the API.
This is a string extracted from the media resource specifically for in-band metadata tracks to enable such tracks to be dispatched to different scripts in the document.
For example, a traditional TV station broadcast streamed on the web and augmented with web-specific interactive features could include text tracks with metadata for ad targeting, trivia game data during game shows, player states during sports games, recipe information during food programs, and so forth. As each program starts and ends, new tracks might be added or removed from the stream, and as each one is added, the user agent could bind them to dedicated script modules using the value of this attribute.
Other than for in-band metadata text tracks, the in-band metadata track dispatch type is the empty string. How this value is populated for different media formats is described in steps to expose a media-resource-specific text track.
This is a string (a BCP 47 language tag) representing the language of the text track's cues. [BCP47]
The language of a text track can change dynamically,
in the case of a text track corresponding to a track element.
One of the following:
Indicates that the text track's cues have not been obtained.
Indicates that the text track is loading and there have been no fatal errors encountered so far. Further cues might still be added to the track by the parser.
Indicates that the text track has been loaded with no fatal errors.
Indicates that the text track was enabled, but when the user agent attempted to obtain it, this failed in some way (e.g. URL could not be parsed, network error, unknown text track format). Some or all of the cues are likely missing and will not be obtained.
The readiness state of a text track changes dynamically as the track is obtained.
One of the following:
Indicates that the text track is not active. Other than for the purposes of exposing the track in the DOM, the user agent is ignoring the text track. No cues are active, no events are fired, and the user agent will not attempt to obtain the track's cues.
Indicates that the text track is active, but that the user agent is not actively displaying the cues. If no attempt has yet been made to obtain the track's cues, the user agent will perform such an attempt momentarily. The user agent is maintaining a list of which cues are active, and events are being fired accordingly.
Indicates that the text track is active. If no attempt has yet been made to obtain the
track's cues, the user agent will perform such an attempt momentarily. The user agent is
maintaining a list of which cues are active, and events are being fired accordingly. In
addition, for text tracks whose kind is subtitles or captions, the cues are being overlaid on the video
as appropriate; for text tracks whose kind is descriptions, the user agent is making the
cues available to the user in a non-visual fashion; and for text tracks whose kind is chapters, the user agent is making available to
the user a mechanism by which the user can navigate to any point in the media
resource by selecting a cue.
A list of text track cues, along with rules for updating the text track rendering. For example, for WebVTT, the rules for updating the display of WebVTT text tracks. [WEBVTT]
The list of cues of a text track can change dynamically, either because the text track has not yet been loaded or is still loading, or due to DOM manipulation.
Each text track has a corresponding TextTrack object.
Each media element has a list of pending text tracks, which must initially be empty, a blocked-on-parser flag, which must initially be false, and a did-perform-automatic-track-selection flag, which must also initially be false.
When the user agent is required to populate the list of pending text tracks of a media element, the user agent must add to the element's list of pending text tracks each text track in the element's list of text tracks whose text track mode is not disabled and whose text track readiness state is loading.
Whenever a track element's parent node changes, the user agent must remove the
corresponding text track from any list of pending text tracks that it is
in.
Whenever a text track's text track readiness state changes to either loaded or failed to load, the user agent must remove it from any list of pending text tracks that it is in.
When a media element is created by an HTML parser or XML parser, the user agent must set the element's blocked-on-parser flag to true. When a media element is popped off the stack of open elements of an HTML parser or XML parser, the user agent must honor user preferences for automatic text track selection, populate the list of pending text tracks, and set the element's blocked-on-parser flag to false.
The text tracks of a media element are ready when both the element's list of pending text tracks is empty and the element's blocked-on-parser flag is false.
Each media element has a pending text track change notification flag, which must initially be unset.
Whenever a text track that is in a media element's list of text tracks has its text track mode change value, the user agent must run the following steps for the media element:
If the media element's pending text track change notification flag is set, return.
Set the media element's pending text track change notification flag.
Queue a media element task given the media element to run these steps:
Unset the media element's pending text track change notification flag.
Fire an event named change at the media element's textTracks attribute's TextTrackList
object.
If the media element's show poster flag is not set, run the time marches on steps.
The task source for the tasks listed in this section is the DOM manipulation task source.
A text track cue is the unit of time-sensitive data in a text track, corresponding for instance for subtitles and captions to the text that appears at a particular time and disappears at another time.
Each text track cue consists of:
An arbitrary string.
The time, in seconds and fractions of a second, that describes the beginning of the range of the media data to which the cue applies.
The time, in seconds and fractions of a second, that describes the end of the range of the media data to which the cue applies.
A boolean indicating whether playback of the media resource is to pause when the end of the range to which the cue applies is reached.
Additional fields, as needed for the format, including the actual data of the cue. For example, WebVTT has a text track cue writing direction and so forth. [WEBVTT]
The text track cue start time and text track cue end time can be negative. (The current playback position can never be negative, though, so cues entirely before time zero cannot be active.)
Each text track cue has a corresponding TextTrackCue object (or more
specifically, an object that inherits from TextTrackCue — for example, WebVTT
cues use the VTTCue interface). A text track cue's in-memory
representation can be dynamically changed through this TextTrackCue API. [WEBVTT]
A text track cue is associated with rules for updating the text track
rendering, as defined by the specification for the specific kind of text track
cue. These rules are used specifically when the object representing the cue is added to a
TextTrack object using the addCue()
method.
In addition, each text track cue has two pieces of dynamic information:
This flag must be initially unset. The flag is used to ensure events are fired appropriately when the cue becomes active or inactive, and to make sure the right cues are rendered.
The user agent must synchronously unset this flag whenever the text track cue is
removed from its text track's text track list of cues; whenever the
text track itself is removed from its media element's list of
text tracks or has its text track mode changed to disabled; and whenever the media element's readyState is changed back to HAVE_NOTHING. When the flag is unset in this way for one
or more cues in text tracks that were showing prior to the relevant incident, the user agent must, after having unset
the flag for all the affected cues, apply the rules for updating the text track
rendering of those text tracks. For example, for text tracks based on WebVTT, the rules for updating the display
of WebVTT text tracks. [WEBVTT]
This is used as part of the rendering model, to keep cues in a consistent position. It must initially be empty. Whenever the text track cue active flag is unset, the user agent must empty the text track cue display state.
The text track cues of a media element's text tracks are ordered relative to each other in the text track cue order, which is determined as follows: first group the cues by their text track, with the groups being sorted in the same order as their text tracks appear in the media element's list of text tracks; then, within each group, cues must be sorted by their start time, earliest first; then, any cues with the same start time must be sorted by their end time, latest first; and finally, any cues with identical end times must be sorted in the order they were last added to their respective text track list of cues, oldest first (so e.g. for cues from a WebVTT file, that would initially be the order in which the cues were listed in the file). [WEBVTT]
A media-resource-specific text track is a text track that corresponds to data found in the media resource.
Rules for processing and rendering such data are defined by the relevant specifications, e.g. the specification of the video format if the media resource is a video. Details for some legacy formats can be found in Sourcing In-band Media Resource Tracks from Media Containers into HTML. [INBAND]
When a media resource contains data that the user agent recognizes and supports as being equivalent to a text track, the user agent runs the steps to expose a media-resource-specific text track with the relevant data, as follows.
Associate the relevant data with a new text track and its corresponding new
TextTrack object. The text track is a media-resource-specific
text track.
Set the new text track's kind, label, and language based on the semantics of the relevant data, as defined by the relevant specification. If there is no label in that data, then the label must be set to the empty string.
Associate the text track list of cues with the rules for updating the text track rendering appropriate for the format in question.
If the new text track's kind is chapters or metadata, then set the text track in-band
metadata track dispatch type as follows, based on the type of the media
resource:
CodecID element. [WEBMCG]stsd box of the
first stbl box of the
first minf box of the
first mdia box of the
text track's trak box in the
first moov box
of the file be the stsd box, if any.
If the file has no stsd box, or if the stsd box has neither a mett box nor a metx box, then the text track
in-band metadata track dispatch type must be set to the empty string.
Otherwise, if the stsd box has a mett box then the text
track in-band metadata track dispatch type must be set to the concatenation of the
string "mett", a U+0020 SPACE character, and the value of the first mime_format field of the first mett box of the stsd
box, or the empty string if that field is absent in that box.
Otherwise, if the stsd box has no mett box but has a metx box then the text track in-band metadata track dispatch type
must be set to the concatenation of the string "metx", a U+0020 SPACE
character, and the value of the first namespace field of the first metx box of the stsd box, or the empty string if that field is absent in
that box.
[MPEG4]
Populate the new text track's list of cues with the cues parsed so far, following the guidelines for exposing cues, and begin updating it dynamically as necessary.
Set the new text track's readiness state to loaded.
Set the new text track's mode to the mode consistent with the user's preferences and the requirements of the relevant specification for the data.
For instance, if there are no other active subtitles, and this is a forced subtitle track (a subtitle track giving subtitles in the audio track's primary language, but only for audio that is actually in another language), then those subtitles might be activated here.
Add the new text track to the media element's list of text tracks.
Fire an event named addtrack at the media element's textTracks attribute's TextTrackList object,
using TrackEvent, with the track
attribute initialized to the text track's TextTrack object.
When a track element is created, it must be associated with a new text
track (with its value set as defined below) and its corresponding new
TextTrack object.
The text track kind is determined from the state of the element's kind attribute according to the following table; for a state given
in a cell of the first column, the kind is the string given
in the second column:
| State | String |
|---|---|
| Subtitles | subtitles
|
| Captions | captions
|
| Descriptions | descriptions
|
| Chapters metadata | chapters
|
| Metadata | metadata
|
The text track label is the element's track label.
The text track language is the element's track language, if any, or the empty string otherwise.
As the kind, label,
and srclang attributes are set, changed, or removed, the
text track must update accordingly, as per the definitions above.
Changes to the track URL are handled in the algorithm below.
The text track readiness state is initially not loaded, and the text track mode is initially disabled.
The text track list of cues is initially empty. It is dynamically modified when the referenced file is parsed. Associated with the list are the rules for updating the text track rendering appropriate for the format in question; for WebVTT, this is the rules for updating the display of WebVTT text tracks. [WEBVTT]
When a track element's parent element changes and the new parent is a media
element, then the user agent must add the track element's corresponding
text track to the media element's list of text tracks, and
then queue a media element task given the media element to fire an event named addtrack at the media element's textTracks attribute's TextTrackList object,
using TrackEvent, with the track attribute
initialized to the text track's TextTrack object.
When a track element's parent element changes and the old parent was a media
element, then the user agent must remove the track element's corresponding
text track from the media element's list of text tracks,
and then queue a media element task given the media element to fire an event named removetrack at the media element's textTracks attribute's TextTrackList object,
using TrackEvent, with the track attribute
initialized to the text track's TextTrack object.
When a text track corresponding to a track element is added to a
media element's list of text tracks, the user agent must queue a
media element task given the media element to run the following steps for the
media element:
If the element's blocked-on-parser flag is true, then return.
If the element's did-perform-automatic-track-selection flag is true, then return.
Honor user preferences for automatic text track selection for this element.
When the user agent is required to honor user preferences for automatic text track selection for a media element, the user agent must run the following steps:
Perform automatic text track selection for subtitles and captions.
If there are any text tracks in the media
element's list of text tracks whose text track kind is chapters or metadata that correspond to track
elements with a default attribute set whose text
track mode is set to disabled, then set the
text track mode of all such tracks to
Set the element's did-perform-automatic-track-selection flag to true.
When the steps above say to perform automatic text track selection for one or more text track kinds, it means to run the following steps:
Let candidates be a list consisting of the text tracks in the media element's list of text tracks whose text track kind is one of the kinds that were passed to the algorithm, if any, in the order given in the list of text tracks.
If candidates is empty, then return.
If any of the text tracks in candidates have a text track mode set to showing, return.
If the user has expressed an interest in having a track from candidates enabled based on its text track kind, text track language, and text track label, then set its text track mode to showing.
For example, the user could have set a browser preference to the effect of "I want French captions whenever possible", or "If there is a subtitle track with 'Commentary' in the title, enable it", or "If there are audio description tracks available, enable one, ideally in Swiss German, but failing that in Standard Swiss German or Standard German".
Otherwise, if there are any text tracks in candidates that correspond to track elements with a default attribute set whose text track mode is
set to disabled, then set the text track
mode of the first such track to showing.
When a text track corresponding to a track element experiences any of
the following circumstances, the user agent must start the track processing
model for that text track and its track element:
track element is created.track element's parent element changes and the new parent is a media
element.When a user agent is to start the track processing model for a
text track and its track element, it must run the following algorithm.
This algorithm interacts closely with the event loop mechanism; in particular, it has
a synchronous section (which is triggered as part of the event loop
algorithm). The steps in that section are marked with ⌛.
If another occurrence of this algorithm is already running for this text
track and its track element, return, letting that other algorithm
take care of this element.
If the text track's text track mode is not set to one of or showing, then return.
If the text track's track element does not have a media
element as a parent, return.
Run the remainder of these steps in parallel, allowing whatever caused these steps to run to continue.
Top: Await a stable state. The synchronous section consists of the following steps. (The steps in the synchronous section are marked with ⌛.)
⌛ Set the text track readiness state to loading.
⌛ If the track element's parent is a media element then
let corsAttributeState be the state of the parent media element's crossorigin content attribute. Otherwise, let
corsAttributeState be No CORS.
End the synchronous section, continuing the remaining steps in parallel.
If URL is not the empty string, then:
Let request be the result of creating a potential-CORS request given
URL, "track", and corsAttributeState, and with the
same-origin fallback flag set.
Set request's client to the
track element's node document's relevant settings
object.
Fetch request.
The tasks queued by the fetching algorithm on the networking task source to process the data as it is being fetched must determine the type of the resource. If the type of the resource is not a supported text track format, the load will fail, as described below. Otherwise, the resource's data must be passed to the appropriate parser (e.g., the WebVTT parser) as it is received, with the text track list of cues being used for that parser's output. [WEBVTT]
The appropriate parser will incrementally update the text track list of cues during these networking task source tasks, as each such task is run with whatever data has been received from the network).
This specification does not currently say whether or how to check the MIME types of text tracks, or whether or how to perform file type sniffing using the actual file data. Implementors differ in their intentions on this matter and it is therefore unclear what the right solution is. In the absence of any requirement here, the HTTP specifications' strict requirement to follow the Content-Type header prevails ("Content-Type specifies the media type of the underlying data." ... "If and only if the media type is not given by a Content-Type field, the recipient MAY attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the resource.").
If fetching fails for any reason (network error, the server returns an error code, CORS
fails, etc), or if URL is the empty string, then queue an element task
on the DOM manipulation task source given the media element to first
change the text track readiness state to failed to load and then fire an event
named error at the track element.
If fetching does not fail, but the type of the resource is not a supported
text track format, or the file was not successfully processed (e.g., the format in question is
an XML format and the file contained a well-formedness error that XML requires be
detected and reported to the application), then the task that
is queued on the networking task source
in which the aforementioned problem is found must change the text track readiness
state to failed to load and fire an event named error at the track element.
If fetching does not fail, and the file was successfully processed, then the final task that is queued by
the networking task source, after it has finished parsing the data, must change the
text track readiness state to loaded, and
fire an event named load at the track element.
If, while fetching is ongoing, either:
...then the user agent must abort fetching, discarding
any pending tasks generated by that algorithm (and in
particular, not adding any cues to the text track list of cues after the moment the
URL changed), and then queue an element task on the DOM manipulation task
source given the track element that first changes the text track
readiness state to failed to load and
then fires an event named error at the track element.
Wait until the text track readiness state is no longer set to loading.
Wait until the track URL is no longer equal to URL, at the same time as the text track mode is set to or showing.
Jump to the step labeled top.
Whenever a track element has its src attribute
set, changed, or removed, the user agent must immediately empty the element's text
track's text track list of cues. (This also causes the algorithm above to stop
adding cues from the resource being obtained using the previously given URL, if any.)
HTML 用户代理如何处理和解释字幕提示取决于特定格式的定义。 如果没有这样一个规范，那么本小节提供了一些约束，实现方可以尝试一致地显示这种格式。
为了支持 HTML 的 字幕 模型，每个计时数据单元都被转化为一个 字幕提示。 在本规范中定义的格式特征与 字幕提示 的各方面的映射没有被定义的情况下， 实现必须确保映射与上面定义的 字幕提示 的各方面的定义一致，并满足以下约束：
如果格式没有为每个字幕提示提供明显的 ID 类似物的话，应该设为空字符串。
应该设为 false。
Support in all current engines.
[Exposed=Window]
interface TextTrackList : EventTarget {
readonly attribute unsigned long length;
getter TextTrack (unsigned long index);
TextTrack? getTrackById(DOMString id);
attribute EventHandler onchange;
attribute EventHandler onaddtrack;
attribute EventHandler onremovetrack;
};textTracks . lengthSupport in all current engines.
Returns the number of text tracks associated with the media element (e.g. from track elements). This is the number of text tracks in the media element's list of text tracks.
textTracks[ n ]Returns the TextTrack object representing the nth text track in the media element's list of text tracks.
textTracks . getTrackById( id )Support in all current engines.
Returns the TextTrack object with the given identifier, or null if no track has that identifier.
A TextTrackList object represents a dynamically updating list of text tracks in a given order.
The textTracks attribute of media elements must return a TextTrackList object
representing the TextTrack objects of the text tracks
in the media element's list of text tracks, in the same order as in the
list of text tracks.
Support in all current engines.
The length attribute of a
TextTrackList object must return the number of text
tracks in the list represented by the TextTrackList object.
The supported property indices of a TextTrackList object at any
instant are the numbers from zero to the number of text tracks in
the list represented by the TextTrackList object minus one, if any. If there are no
text tracks in the list, there are no supported property
indices.
To determine the value of an indexed property of a
TextTrackList object for a given index index, the user agent must return
the indexth text track in the list represented by the
TextTrackList object.
The getTrackById(id) method must return the first TextTrack in the
TextTrackList object whose id IDL attribute
would return a value equal to the value of the id argument. When no tracks
match the given argument, the method must return null.
Support in all current engines.
enum TextTrackMode { "disabled", "hidden", "showing" };
enum TextTrackKind { "subtitles", "captions", "descriptions", "chapters", "metadata" };
[Exposed=Window]
interface TextTrack : EventTarget {
readonly attribute TextTrackKind kind;
readonly attribute DOMString label;
readonly attribute DOMString language;
readonly attribute DOMString id;
readonly attribute DOMString inBandMetadataTrackDispatchType;
attribute TextTrackMode mode;
readonly attribute TextTrackCueList? cues;
readonly attribute TextTrackCueList? activeCues;
undefined addCue(TextTrackCue cue);
undefined removeCue(TextTrackCue cue);
attribute EventHandler oncuechange;
};addTextTrack( kind [, label [, language ] ] )Creates and returns a new TextTrack object, which is also added to the
media element's list of text tracks.
kindReturns the text track kind string.
labelReturns the text track label, if there is one, or the empty string otherwise (indicating that a custom label probably needs to be generated from the other attributes of the object if the object is exposed to the user).
languageReturns the text track language string.
idReturns the ID of the given track.
For in-band tracks, this is the ID that can be used with a fragment if the format supports media fragment
syntax, and that can be used with the getTrackById() method.
For TextTrack objects corresponding to track elements, this is the
ID of the track element.
inBandMetadataTrackDispatchTypeReturns the text track in-band metadata track dispatch type string.
mode [ = value ]Support in all current engines.
Returns the text track mode, represented by a string from the following list:
disabled"The text track disabled mode.
The mode.
showing"The text track showing mode.
Can be set, to change the mode.
cuesReturns the text track list of cues, as a TextTrackCueList object.
activeCuesReturns the text track cues from the text track
list of cues that are currently active (i.e. that start before the current playback
position and end after it), as a TextTrackCueList object.
addCue( cue )Adds the given cue to textTrack's text track list of cues.
removeCue( cue )Removes the given cue from textTrack's text track list of cues.
The addTextTrack(kind, label, language) method of media elements, when invoked, must run the following steps:
Create a new TextTrack object.
Create a new text track corresponding to the new object, and set its text track kind to kind, its text track label to label, its text track language to language, its text track readiness state to the text track loaded state, its text track mode to the mode, and its text track list of cues to an empty list.
Initially, the text track list of cues is not associated with any rules for updating the text track rendering. When a text track cue is added to it, the text track list of cues has its rules permanently set accordingly.
Add the new text track to the media element's list of text tracks.
Queue a media element task given the media element to fire an event named addtrack at the media element's textTracks attribute's TextTrackList object,
using TrackEvent, with the track
attribute initialized to the new text track's TextTrack object.
Return the new TextTrack object.
The kind attribute must return the
text track kind of the text track that the TextTrack object
represents.
The label attribute must return the
text track label of the text track that the TextTrack
object represents.
The language attribute must return the
text track language of the text track that the TextTrack
object represents.
The id attribute returns the track's
identifier, if it has one, or the empty string otherwise. For tracks that correspond to
track elements, the track's identifier is the value of the element's id attribute, if any. For in-band tracks, the track's identifier is
specified by the media resource. If the media resource is in a format
that supports media fragment syntax, the identifier returned for a particular
track must be the same identifier that would enable the track if used as the name of a track in
the track dimension of such a fragment.
The inBandMetadataTrackDispatchType
attribute must return the text track in-band metadata track dispatch type of the
text track that the TextTrack object represents.
The mode attribute, on getting, must return
the string corresponding to the text track mode of the text track that
the TextTrack object represents, as defined by the following list:
disabled"hidden"showing"On setting, if the new value isn't equal to what the attribute would currently return, the new value must be processed as follows:
disabled"Set the text track mode of the text track that the
TextTrack object represents to the text track disabled mode.
Set the text track mode of the text track that the
TextTrack object represents to the mode.
showing"Set the text track mode of the text track that the
TextTrack object represents to the text track showing mode.
If the text track mode of the text track that the
TextTrack object represents is not the text track disabled mode, then
the cues attribute must return a
live TextTrackCueList object that represents the subset of the
text track list of cues of the text track that the
TextTrack object represents whose end
times occur at or after the earliest possible position when the script
started, in text track cue order. Otherwise, it must return null. For each TextTrack object, when an
object is returned, the same TextTrackCueList object must be returned each time.
The earliest possible position when the script started is whatever the earliest possible position was the last time the event loop reached step 1.
If the text track mode of the text track that the
TextTrack object represents is not the text track disabled mode, then
the activeCues attribute must return a
live TextTrackCueList object that represents the subset of the
text track list of cues of the text track that the
TextTrack object represents whose active flag was set when the script
started, in text track cue order. Otherwise, it must return null. For each TextTrack object, when an
object is returned, the same TextTrackCueList object must be returned each time.
A text track cue's active flag was set when the script started if its text track cue active flag was set the last time the event loop reached step 1.
The addCue(cue) method
of TextTrack objects, when invoked, must run the following steps:
If the text track list of cues does not yet have any associated rules for updating the text track rendering, then associate the text track list of cues with the rules for updating the text track rendering appropriate to cue.
If text track list of cues' associated rules for updating the text
track rendering are not the same rules for updating the text track rendering
as appropriate for cue, then throw an "InvalidStateError"
DOMException.
If the given cue is in a text track list of cues, then remove cue from that text track list of cues.
Add cue to the TextTrack object's text track's
text track list of cues.
The removeCue(cue)
method of TextTrack objects, when invoked, must run the following steps:
If the given cue is not in the TextTrack object's text
track's text track list of cues, then throw a
"NotFoundError" DOMException.
Remove cue from the TextTrack object's text track's
text track list of cues.
In this example, an audio element is used to play a specific sound-effect from a
sound file containing many sound effects. A cue is used to pause the audio, so that it ends
exactly at the end of the clip, even if the browser is busy running some script. If the page had
relied on script to pause the audio, then the start of the next clip might be heard if the
browser was not able to run the script at the exact time specified.
var sfx = new Audio('sfx.wav');
var sounds = sfx.addTextTrack('metadata');
// add sounds we care about
function addFX(start, end, name) {
var cue = new VTTCue(start, end, '');
cue.id = name;
cue.pauseOnExit = true;
sounds.addCue(cue);
}
addFX(12.783, 13.612, 'dog bark');
addFX(13.612, 15.091, 'kitten mew'))
function playSound(id) {
sfx.currentTime = sounds.getCueById(id).startTime;
sfx.play();
}
// play a bark as soon as we can
sfx.oncanplaythrough = function () {
playSound('dog bark');
}
// meow when the user tries to leave,
// and have the browser ask them to stay
window.onbeforeunload = function (e) {
playSound('kitten mew');
e.preventDefault();
}Support in all current engines.
[Exposed=Window]
interface TextTrackCueList {
readonly attribute unsigned long length;
getter TextTrackCue (unsigned long index);
TextTrackCue? getCueById(DOMString id);
};lengthReturns the number of cues in the list.
Returns the text track cue with index index in the list. The cues are sorted in text track cue order.
getCueById( id )Returns the first text track cue (in text track cue order) with text track cue identifier id.
Returns null if none of the cues have the given identifier or if the argument is the empty string.
A TextTrackCueList object represents a dynamically updating list of text track cues in a given order.
Support in all current engines.
The length attribute must return
the number of cues in the list represented by the
TextTrackCueList object.
The supported property indices of a TextTrackCueList object at any
instant are the numbers from zero to the number of cues in the
list represented by the TextTrackCueList object minus one, if any. If there are no
cues in the list, there are no supported property
indices.
To determine the value of an indexed property for a
given index index, the user agent must return the indexth text track
cue in the list represented by the TextTrackCueList object.
Support in all current engines.
The getCueById(id) method, when called with an argument other than the empty string,
must return the first text track cue in the list represented by the
TextTrackCueList object whose text track cue identifier is id, if any, or null otherwise. If the argument is the empty string, then the method
must return null.
Support in all current engines.
[Exposed=Window]
interface TextTrackCue : EventTarget {
readonly attribute TextTrack? track;
attribute DOMString id;
attribute double startTime;
attribute double endTime;
attribute boolean pauseOnExit;
attribute EventHandler onenter;
attribute EventHandler onexit;
};Returns the TextTrack object to which this
text track cue belongs, if any, or null
otherwise.
Returns the text track cue identifier.
Can be set.
Returns the text track cue start time, in seconds.
Can be set.
Returns the text track cue end time, in seconds.
Can be set.
Returns true if the text track cue pause-on-exit flag is set, false otherwise.
Can be set.
Support in all current engines.
The track attribute, on getting, must
return the TextTrack object of the text track in whose list of cues the text track cue that the
TextTrackCue object represents finds itself, if any; or null otherwise.
Support in all current engines.
The id attribute, on getting, must return
the text track cue identifier of the text track cue that the
TextTrackCue object represents. On setting, the text track cue
identifier must be set to the new value.
Support in all current engines.
The startTime attribute, on
getting, must return the text track cue start time of the text track cue
that the TextTrackCue object represents, in seconds. On setting, the text track
cue start time must be set to the new value, interpreted in seconds; then, if the
TextTrackCue object's text track cue is in a text track's
list of cues, and that text track is in
a media element's list of text tracks, and the media
element's show poster flag is not set, then run the time marches on steps for that media element.
Support in all current engines.
The endTime attribute, on getting,
must return the text track cue end time of the text track cue that the
TextTrackCue object represents, in seconds. On setting, the text track cue end
time must be set to the new value, interpreted in seconds; then, if the
TextTrackCue object's text track cue is in a text track's
list of cues, and that text track is in
a media element's list of text tracks, and the media
element's show poster flag is not set, then run the time marches on steps for that media element.
Support in all current engines.
The pauseOnExit attribute, on
getting, must return true if the text track cue pause-on-exit flag of the text
track cue that the TextTrackCue object represents is set; or false otherwise.
On setting, the text track cue pause-on-exit flag must be set if the new value is
true, and must be unset otherwise.
The following are the event handlers that (and their corresponding event handler event types) that must
be supported, as event handler IDL attributes, by all objects implementing the
TextTrackList interface:
| Event handler | Event handler event type |
|---|---|
onchange | change
|
onaddtrack | addtrack
|
onremovetrack | removetrack
|
The following are the event handlers that (and their corresponding event handler event types) that must
be supported, as event handler IDL attributes, by all objects implementing the
TextTrack interface:
| Event handler | Event handler event type |
|---|---|
oncuechange Support in all current engines. Firefox31+Safari6+Chrome23+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer10+ Firefox Android31+Safari iOS7+Chrome Android25+WebView Android4.4+Samsung Internet1.5+Opera Android12.1+ | cuechange
|
The following are the event handlers (and their corresponding event handler event types) that must
be supported, as event handler IDL attributes, by all objects implementing the
TextTrackCue interface:
| Event handler | Event handler event type |
|---|---|
onenter Support in all current engines. Firefox31+Safari6+Chrome23+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer10+ Firefox Android31+Safari iOS7+Chrome Android25+WebView Android4.4+Samsung Internet1.5+Opera Android12.1+ | enter
|
onexit Support in all current engines. Firefox31+Safari6+Chrome23+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer10+ Firefox Android31+Safari iOS7+Chrome Android25+WebView Android4.4+Samsung Internet1.5+Opera Android12.1+ | exit
|
This section is non-normative.
字幕可用于存储与多媒体数据相关的数据，用来做交互式的，增强的视图。
例如，一个显示体育播报的页面可能会包含当前分数信息。 假设正在直播一个机器人比赛。图片上可能会覆盖上分数，像这样：
为了让用户回放到视频任意位置时分数都能正确地呈现，元数据字幕提示需要达到那个分数适用的长度。 例如在上面这一帧，显示比赛序号的字幕提示可能持续整场比赛，显示蓝队分数的字幕提示持续到蓝队得分变化， 显示红队分数的字幕提示持续到红队得分变化。如果这个视频只是事件的直播流， 右下角的时间可能会自动从当前视频时间得出，而不是通过字幕提示显示。 但如果视频只是精彩回放，那么它可能也会通过字幕给出。
在 WebVTT 文件中，这些字幕提示的片段可能像这样：
WEBVTT ... 05:10:00.000 --> 05:12:15.000 matchtype:qual matchnumber:37 ... 05:11:02.251 --> 05:11:17.198 red:78 05:11:03.672 --> 05:11:54.198 blue:66 05:11:17.198 --> 05:11:25.912 red:80 05:11:25.912 --> 05:11:26.522 red:83 05:11:26.522 --> 05:11:26.982 red:86 05:11:26.982 --> 05:11:27.499 red:89 ...
这里的关键在于字幕提示给出的信息覆盖了相关事件的时间长度。 如果得分变化时得分以 0 长度（或非常短的，近乎 0 的长度）的字幕给出， 比如在 05:11:17.198 时 "red+2"，在 05:11:25.912 时 "red+3"，问题就会出现： 回放寻找会很难实现，因为脚本必须遍历整个字幕提示列表来确保没有错过任何通知； 但同时，如果字幕提示很短，脚本也可能永远不会看到它们被激活，除非脚本明确地监听了它。
当这样使用字幕时，鼓励作者使用 cuechange 事件来更新当前提示。
（典型地，使用 timeupdate 事件不太合适，
因为即使字幕提示没变也要进行计算，更重要的是这会在元数据字幕提示激活与显示更新之间引入更高的延迟，
因为 timeupdate 事件是限制速率的。）
需要通过 URL 来识别
AudioTrack.kind 或
VideoTrack.kind IDL 属性的返回值，
或识别 字幕类型 的其他规范或格式，
必须使用 about:html-kind URL。
The controls
attribute is a boolean attribute. If present, it indicates that the author has not
provided a scripted controller and would like the user agent to provide its own set of
controls.
If the attribute is present, or if scripting is disabled for the media element, then the user agent should expose a user interface to the user. This user interface should include features to begin playback, pause playback, seek to an arbitrary position in the content (if the content supports arbitrary seeking), change the volume, change the display of closed captions or embedded sign-language tracks, select different audio tracks or turn on audio descriptions, and show the media content in manners more suitable to the user (e.g. fullscreen video or in an independent resizable window). Other controls may also be made available.
Even when the attribute is absent, however, user agents may provide controls to affect playback
of the media resource (e.g. play, pause, seeking, track selection, and volume controls), but such
features should not interfere with the page's normal rendering. For example, such features could
be exposed in the media element's context menu, platform media keys, or a remote
control. The user agent may implement this simply by exposing a user interface to the user as described above (as if the controls attribute was present).
If the user agent exposes a user interface to
the user by displaying controls over the media element, then the user agent
should suppress any user interaction events while the user agent is interacting with this
interface. (For example, if the user clicks on a video's playback control, mousedown events and so forth would not simultaneously be fired at
elements on the page.)
Where possible (specifically, for starting, stopping, pausing, and unpausing playback, for seeking, for changing the rate of playback, for fast-forwarding or rewinding, for listing, enabling, and disabling text tracks, and for muting or changing the volume of the audio), user interface features exposed by the user agent must be implemented in terms of the DOM API described above, so that, e.g., all the same events fire.
Features such as fast-forward or rewind must be implemented by only changing the playbackRate attribute (and not the defaultPlaybackRate
attribute).
Seeking must be implemented in terms of seeking to the requested position in the media element's media timeline. For media resources where seeking to an arbitrary position would be slow, user agents are encouraged to use the approximate-for-speed flag when seeking in response to the user manipulating an approximate position interface such as a seek bar.
Support in all current engines.
The controls IDL attribute must reflect the
content attribute of the same name.
volume [ = value ]Support in all current engines.
Returns the current playback volume, as a number in the range 0.0 to 1.0, where 0.0 is the quietest and 1.0 the loudest.
Can be set, to change the volume.
Throws an "IndexSizeError" DOMException if the new
value is not in the range 0.0 .. 1.0.
muted [ = value ]Support in all current engines.
Returns true if audio is muted, overriding the volume
attribute, and false if the volume attribute is being
honored.
Can be set, to change whether the audio is muted or not.
A media element has a playback volume, which is a fraction in the range 0.0 (silent) to 1.0 (loudest). Initially, the volume should be 1.0, but user agents may remember the last set value across sessions, on a per-site basis or otherwise, so the volume may start at other values.
The volume
IDL attribute must return the playback volume of any
audio portions of the media element. On setting, if the new value is in the range 0.0
to 1.0 inclusive, the media element's playback
volume must be set to the new value. If the new value is outside the range 0.0 to 1.0
inclusive, then, on setting, an "IndexSizeError"
DOMException must be thrown instead.
A media element can also be muted. If anything is muting the element, then it is muted. (For example, when the direction of playback is backwards, the element is muted.)
The muted IDL
attribute must return the value to which it was last set. When a media element is
created, if the element has a muted content attribute
specified, then the muted IDL attribute should be set to
true; otherwise, the user agents may set the value to the user's preferred value (e.g. remembering
the last set value across sessions, on a per-site basis or otherwise). While the muted IDL attribute is set to true, the media element
must be muted.
Whenever either of the values that would be returned by the volume and muted IDL
attributes change, the user agent must queue a media element task given the
media element to fire an event named volumechange at the media element. Then, if
the media element is not allowed to play, the user agent must run the
internal pause steps for the media element.
An element's effective media volume is determined as follows:
If the user has indicated that the user agent is to override the volume of the element, then return the volume desired by the user.
If the element's audio output is muted, then return zero.
Let volume be the playback volume of the audio portions of the media element, in range 0.0 (silent) to 1.0 (loudest).
Return volume, interpreted relative to the range 0.0 to 1.0, with 0.0 being silent, and 1.0 being the loudest setting, values in between increasing in loudness. The range need not be linear. The loudest setting may be lower than the system's loudest possible setting; for example the user could have set a maximum volume.
The muted content
attribute on media elements is a boolean
attribute that controls the default state of the audio output of the media
resource, potentially overriding user preferences.
Support in all current engines.
The defaultMuted IDL attribute must
reflect the muted content attribute.
This attribute has no dynamic effect (it only controls the default state of the element).
This video (an advertisement) autoplays, but to avoid annoying users, it does so without sound, and allows the user to turn the sound on. The user agent can pause the video if it's unmuted without a user interaction.
<video src="adverts.cgi?kind=video" controls autoplay loop muted></video>Support in all current engines.
Objects implementing the TimeRanges interface
represent a list of ranges (periods) of time.
[Exposed=Window]
interface TimeRanges {
readonly attribute unsigned long length;
double start(unsigned long index);
double end(unsigned long index);
};lengthSupport in all current engines.
Returns the number of ranges in the object.
start(index)Support in all current engines.
Returns the time for the start of the range with the given index.
Throws an "IndexSizeError" DOMException if the index
is out of range.
end(index)Support in all current engines.
Returns the time for the end of the range with the given index.
Throws an "IndexSizeError" DOMException if the index
is out of range.
The length IDL attribute must return the
number of ranges represented by the object.
The start(index)
method must return the position of the start of the indexth range represented
by the object, in seconds measured from the start of the timeline that the object covers.
The end(index) method
must return the position of the end of the indexth range represented by the
object, in seconds measured from the start of the timeline that the object covers.
These methods must throw "IndexSizeError" DOMExceptions
if called with an index argument greater than or equal to the number of ranges
represented by the object.
When a TimeRanges object is said to be a
normalized TimeRanges
object, the ranges it represents must obey the following criteria:
In other words, the ranges in such an object are ordered, don't overlap, and don't touch (adjacent ranges are folded into one bigger range). A range can be empty (referencing just a single moment in time), e.g. to indicate that only one frame is currently buffered in the case that the user agent has discarded the entire media resource except for the current frame, when a media element is paused.
Ranges in a TimeRanges object must be inclusive.
Thus, the end of a range would be equal to the start of a following adjacent (touching but not overlapping) range. Similarly, a range covering a whole timeline anchored at zero would have a start equal to zero and an end equal to the duration of the timeline.
The timelines used by the objects returned by the buffered, seekable and
played IDL attributes of media
elements must be that element's media timeline.
TrackEvent interfaceSupport in all current engines.
[Exposed=Window]
interface TrackEvent : Event {
constructor(DOMString type, optional TrackEventInit eventInitDict = {});
readonly attribute (VideoTrack or AudioTrack or TextTrack)? track;
};
dictionary TrackEventInit : EventInit {
(VideoTrack or AudioTrack or TextTrack)? track = null;
};trackSupport in all current engines.
Returns the track object (TextTrack, AudioTrack, or
VideoTrack) to which the event relates.
The track attribute must return the value
it was initialized to. It represents the context information for the event.
This section is non-normative.
The following events fire on media elements as part of the processing model described above:
| Event name | Interface | Fired when... | Preconditions |
|---|---|---|---|
loadstart
HTMLMediaElement/loadstart_event Support in all current engines. Firefox6+Safari4+Chrome1+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android6+Safari iOS3+Chrome Android18+WebView Android1+Samsung Internet1.0+Opera Android12.1+ | Event
| The user agent begins looking for media data, as part of the resource selection algorithm. | networkState equals NETWORK_LOADING
|
progress
HTMLMediaElement/progress_event Support in all current engines. Firefox6+Safari3.1+Chrome1+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android6+Safari iOS3+Chrome Android18+WebView Android1+Samsung Internet1.0+Opera Android12.1+ | Event
| The user agent is fetching media data. | networkState equals NETWORK_LOADING
|
suspend
HTMLMediaElement/suspend_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The user agent is intentionally not currently fetching media data. | networkState equals NETWORK_IDLE
|
abort
Support in all current engines. Firefox9+Safari3.1+Chrome1+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android9+Safari iOS3+Chrome Android18+WebView Android1+Samsung Internet1.0+Opera Android12.1+ | Event
| The user agent stops fetching the media data before it is completely downloaded, but not due to an error. | error is an object with the code MEDIA_ERR_ABORTED. networkState equals either NETWORK_EMPTY or NETWORK_IDLE, depending on when the download was aborted.
|
error
Support in all current engines. Firefox6+Safari3.1+Chrome1+ Opera11.6+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android6+Safari iOS3+Chrome Android18+WebView Android1+Samsung Internet1.0+Opera Android12+ | Event
| An error occurs while fetching the media data or the type of the resource is not supported media format. | error is an object with the code MEDIA_ERR_NETWORK or higher. networkState equals either NETWORK_EMPTY or NETWORK_IDLE, depending on when the download was aborted.
|
emptied
HTMLMediaElement/emptied_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| A media element whose networkState
was previously not in the NETWORK_EMPTY state has
just switched to that state (either because of a fatal error during load that's about to be
reported, or because the load() method was invoked while
the resource selection algorithm was already
running).
| networkState is NETWORK_EMPTY; all the IDL attributes are in their
initial states.
|
stalled
HTMLMediaElement/stalled_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The user agent is trying to fetch media data, but data is unexpectedly not forthcoming. | networkState is NETWORK_LOADING.
|
loadedmetadata
HTMLMediaElement/loadedmetadata_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The user agent has just determined the duration and dimensions of the media resource and the text tracks are ready. | readyState is newly equal to HAVE_METADATA or greater for the first time.
|
loadeddata
HTMLMediaElement/loadeddata_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The user agent can render the media data at the current playback position for the first time. | readyState newly increased to HAVE_CURRENT_DATA or greater for the first time.
|
canplay
HTMLMediaElement/canplay_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The user agent can resume playback of the media data, but estimates that if playback were to be started now, the media resource could not be rendered at the current playback rate up to its end without having to stop for further buffering of content. | readyState newly increased to HAVE_FUTURE_DATA or greater.
|
canplaythrough
HTMLMediaElement/canplaythrough_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The user agent estimates that if playback were to be started now, the media resource could be rendered at the current playback rate all the way to its end without having to stop for further buffering. | readyState is newly equal to HAVE_ENOUGH_DATA.
|
playing
HTMLMediaElement/playing_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| Playback is ready to start after having been paused or delayed due to lack of media data. | readyState is newly equal to or greater than
HAVE_FUTURE_DATA and paused is false, or paused is newly false and readyState is equal to or greater than HAVE_FUTURE_DATA. Even if this event fires, the
element might still not be potentially playing, e.g. if the element is
paused for user interaction or paused for in-band content.
|
waiting
HTMLMediaElement/waiting_event Support in all current engines. Firefox6+Safari3.1+Chrome1+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android6+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android12.1+ | Event
| Playback has stopped because the next frame is not available, but the user agent expects that frame to become available in due course. | readyState is equal to or less than HAVE_CURRENT_DATA, and paused is false. Either seeking is true, or the current playback position
is not contained in any of the ranges in buffered. It
is possible for playback to stop for other reasons without paused being false, but those reasons do not fire this event
(and when those situations resolve, a separate playing
event is not fired either): e.g., playback has ended, or
playback stopped due to errors, or the element has paused for user
interaction or paused for in-band content.
|
seeking
HTMLMediaElement/seeking_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The seeking IDL attribute changed to true, and the user agent has started seeking to a new position.
| |
seeked
Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The seeking IDL attribute changed to false after the current playback position was changed.
| |
ended
Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| Playback has stopped because the end of the media resource was reached. | currentTime equals the end of the media
resource; ended is true.
|
durationchange
HTMLMediaElement/durationchange_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The duration attribute has just been updated.
| |
timeupdate
HTMLMediaElement/timeupdate_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The current playback position changed as part of normal playback or in an especially interesting way, for example discontinuously. | |
play
Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The element is no longer paused. Fired after the play()
method has returned, or when the autoplay attribute
has caused playback to begin.
| paused is newly false.
|
pause
Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| The element has been paused. Fired after the pause()
method has returned.
| paused is newly true.
|
ratechange
HTMLMediaElement/ratechange_event Support in all current engines. Firefox3.5+Safari3.1+Chrome3+ Opera10.5+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+ | Event
| Either the defaultPlaybackRate or the
playbackRate attribute has just been updated.
| |
resize
| Event
| One or both of the videoWidth and videoHeight attributes have just been updated.
| Media element is a video element; readyState is not HAVE_NOTHING
|
volumechange
HTMLMediaElement/volumechange_event Support in all current engines. Firefox6+Safari3.1+Chrome1+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer9+ Firefox Android6+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android12.1+ | Event
| Either the volume attribute or the muted attribute has changed. Fired after the relevant
attribute's setter has returned.
|
The following event fires on source element:
| Event name | Interface | Fired when... |
|---|---|---|
error
| Event
| An error occurs while fetching the media data or the type of the resource is not supported media format. |
The following events fire on AudioTrackList, VideoTrackList, and
TextTrackList objects:
| Event name | Interface | Fired when... |
|---|---|---|
change
Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo Support in all current engines. Firefox31+Safari7+Chrome33+ Opera20+Edge79+ Edge (Legacy)18Internet ExplorerNo Firefox Android31+Safari iOS7+Chrome Android33+WebView Android4.4+Samsung Internet2.0+Opera Android20+ Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo | Event
| One or more tracks in the track list have been enabled or disabled. |
addtrack
Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo Support in all current engines. Firefox31+Safari6+Chrome23+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer11 Firefox Android31+Safari iOS7+Chrome Android25+WebView Android4.4+Samsung Internet1.5+Opera Android12.1+ Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo | TrackEvent
| A track has been added to the track list. |
removetrack
AudioTrackList/removetrack_event Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo TextTrackList/removetrack_event Support in all current engines. Firefox31+Safari7+Chrome33+ Opera20+Edge79+ Edge (Legacy)18Internet ExplorerNo Firefox Android31+Safari iOS7+Chrome Android33+WebView Android4.4+Samsung Internet2.0+Opera Android20+ VideoTrackList/removetrack_event Support in all current engines. Firefox🔰 33+Safari7+Chrome🔰 37+ Opera🔰 24+Edge🔰 79+ Edge (Legacy)NoInternet Explorer10+ Firefox AndroidNoSafari iOS7+Chrome Android🔰 37+WebView AndroidNoSamsung InternetNoOpera AndroidNo | TrackEvent
| A track has been removed from the track list. |
The following event fires on TextTrack objects and track elements:
| Event name | Interface | Fired when... |
|---|---|---|
cuechange
HTMLTrackElement/cuechange_event Support in all current engines. Firefox68+Safari10+Chrome32+ Opera19+Edge79+ Edge (Legacy)14+Internet ExplorerNo Firefox Android68+Safari iOS10+Chrome Android32+WebView Android4.4.3+Samsung Internet2.0+Opera Android19+ Support in all current engines. Firefox31+Safari6+Chrome23+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer10+ Firefox Android31+Safari iOS7+Chrome Android25+WebView Android4.4+Samsung Internet1.5+Opera Android12.1+ | Event
| One or more cues in the track have become active or stopped being active. |
The following events fire on track elements:
| Event name | Interface | Fired when... |
|---|---|---|
error
| Event
| An error occurs while fetching the track data or the type of the resource is not supported text track format. |
load
| Event
| A track data has been fetched and successfully processed. |
The following events fire on TextTrackCue objects:
| Event name | Interface | Fired when... |
|---|---|---|
enter
Support in all current engines. Firefox31+Safari6+Chrome23+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer10+ Firefox Android31+Safari iOS7+Chrome Android25+WebView Android4.4+Samsung Internet1.5+Opera Android12.1+ | Event
| The cue has become active. |
exit
Support in all current engines. Firefox31+Safari6+Chrome23+ Opera12.1+Edge79+ Edge (Legacy)12+Internet Explorer10+ Firefox Android31+Safari iOS7+Chrome Android25+WebView Android4.4+Samsung Internet1.5+Opera Android12.1+ | Event
| The cue has stopped being active. |
The main security and privacy implications of the video and audio
elements come from the ability to embed media cross-origin. There are two directions that threats
can flow: from hostile content to a victim page, and from a hostile page to victim content.
If a victim page embeds hostile content, the threat is that the content might contain scripted
code that attempts to interact with the Document that embeds the content. To avoid
this, user agents must ensure that there is no access from the content to the embedding page. In
the case of media content that uses DOM concepts, the embedded content must be treated as if it
was in its own unrelated top-level browsing context.
For instance, if an SVG animation was embedded in a video element,
the user agent would not give it access to the DOM of the outer page. From the perspective of
scripts in the SVG resource, the SVG file would appear to be in a lone top-level browsing context
with no parent.
If a hostile page embeds victim content, the threat is that the embedding page could obtain
information from the content that it would not otherwise have access to. The API does expose some
information: the existence of the media, its type, its duration, its size, and the performance
characteristics of its host. Such information is already potentially problematic, but in practice
the same information can more or less be obtained using the img element, and so it
has been deemed acceptable.
However, significantly more sensitive information could be obtained if the user agent further
exposes metadata within the content, such as subtitles. That information is therefore only exposed
if the video resource uses CORS. The crossorigin
attribute allows authors to enable CORS. [FETCH]
Without this restriction, an attacker could trick a user running within a corporate network into visiting a site that attempts to load a video from a previously leaked location on the corporation's intranet. If such a video included confidential plans for a new product, then being able to read the subtitles would present a serious confidentiality breach.
This section is non-normative.
在机顶盒和手机的小型设备上播放音视频资源经常会受到有限的硬件资源的限制，
比如一个机器可能只支持同时播放三个视频。
因此好的实践是在 媒体元素 播放完后立即释放资源。
做法可以是移除所有对这个元素的引用让它被垃圾回收，或者更好的方式是，
移除元素的的 src 属性和所有后代
source 元素，然后调用元素的 load() 方法。
类似地，当回放速率不是 1.0 时，软硬件和格式限制会造成视频丢帧或音频卡顿和静音。
This section is non-normative.
How accurately various aspects of the media element API are implemented is considered a quality-of-implementation issue.
For example, when implementing the buffered attribute,
how precise an implementation reports the ranges that have been buffered depends on how carefully
the user agent inspects the data. Since the API reports ranges as times, but the data is obtained
in byte streams, a user agent receiving a variable-bitrate stream might only be able to determine
precise times by actually decoding all of the data. User agents aren't required to do this,
however; they can instead return estimates (e.g. based on the average bitrate seen so far) which
get revised as more information becomes available.
As a general rule, user agents are urged to be conservative rather than optimistic. For example, it would be bad to report that everything had been buffered when it had not.
Another quality-of-implementation issue would be playing a video backwards when the codec is designed only for forward playback (e.g. there aren't many key frames, and they are far apart, and the intervening frames only have deltas from the previous frame). User agents could do a poor job, e.g. only showing key frames; however, better implementations would do more work and thus do a better job, e.g. actually decoding parts of the video forwards, storing the complete frames, and then playing the frames backwards.
Similarly, while implementations are allowed to drop buffered data at any time (there is no requirement that a user agent keep all the media data obtained for the lifetime of the media element), it is again a quality of implementation issue: user agents with sufficient resources to keep all the data around are encouraged to do so, as this allows for a better user experience. For example, if the user is watching a live stream, a user agent could allow the user only to view the live video; however, a better user agent would buffer everything and allow the user to seek through the earlier material, pause it, play it forwards and backwards, etc.
When a media element that is paused is removed from a document and not reinserted before the next time the event loop reaches step 1, implementations that are resource constrained are encouraged to take that opportunity to release all hardware resources (like video planes, networking resources, and data buffers) used by the media element. (User agents still have to keep track of the playback position and so forth, though, in case playback is later restarted.)
map elementSupport in all current engines.
Support in all current engines.
name — Name of image map to reference from the usemap attribute[Exposed=Window]
interface HTMLMapElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString name;
[SameObject] readonly attribute HTMLCollection areas;
};The map element, in conjunction with an img element and any
area element descendants, defines an image map. The element
represents its children.
The name attribute gives
the map a name so that it can be referenced. The attribute must be present and must
have a non-empty value with no ASCII whitespace. The value of the name attribute must not be equal to the value of the name attribute of another map element in the same
tree. If the id attribute is also specified, both
attributes must have the same value.
areasReturns an HTMLCollection of the area elements in the
map.
The areas
attribute must return an HTMLCollection rooted at the map element, whose
filter matches only area elements.
The IDL attribute name must reflect the content attribute of the
same name.
Image maps can be defined in conjunction with other content on the page, to ease maintenance. This example is of a page with an image map at the top of the page and a corresponding set of text links at the bottom.
<!DOCTYPE HTML>
<HTML LANG="EN">
<TITLE>Babies™: Toys</TITLE>
<HEADER>
<H1>Toys</H1>
<IMG SRC="/images/menu.gif"
ALT="Babies™ navigation menu. Select a department to go to its page."
USEMAP="#NAV">
</HEADER>
...
<FOOTER>
<MAP NAME="NAV">
<P>
<A HREF="/clothes/">Clothes</A>
<AREA ALT="Clothes" COORDS="0,0,100,50" HREF="/clothes/"> |
<A HREF="/toys/">Toys</A>
<AREA ALT="Toys" COORDS="100,0,200,50" HREF="/toys/"> |
<A HREF="/food/">Food</A>
<AREA ALT="Food" COORDS="200,0,300,50" HREF="/food/"> |
<A HREF="/books/">Books</A>
<AREA ALT="Books" COORDS="300,0,400,50" HREF="/books/">
</P>
</MAP>
</FOOTER>area elementSupport in all current engines.
Support in all current engines.
map element ancestor.alt — Replacement text for use when images are not availablecoords — Coordinates for the shape to be created in an image mapshape — The kind of shape to be created in an image maphref — Address of the hyperlinktarget — Browsing context for hyperlink navigationdownload — Whether to download the resource instead of navigating to it, and its file name if soping — URLs to pingrel — Relationship between the location in the document containing the hyperlink and the destination resourcereferrerpolicy — Referrer policy for fetches initiated by the elementhref attribute: for authors; for implementers.[Exposed=Window]
interface HTMLAreaElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString alt;
[CEReactions] attribute DOMString coords;
[CEReactions] attribute DOMString shape;
[CEReactions] attribute DOMString target;
[CEReactions] attribute DOMString download;
[CEReactions] attribute USVString ping;
[CEReactions] attribute DOMString rel;
[SameObject, PutForwards=value] readonly attribute DOMTokenList relList;
[CEReactions] attribute DOMString referrerPolicy;
// also has obsolete members
};
HTMLAreaElement includes HTMLHyperlinkElementUtils;The area element represents either a hyperlink with some text and a
corresponding area on an image map, or a dead area on an image map.
An area element with a parent node must have a map element
ancestor.
If the area element has an href
attribute, then the area element represents a hyperlink. In this case,
the alt attribute must be
present. It specifies the text of the hyperlink. Its value must be text that, when presented with
the texts specified for the other hyperlinks of the image map, and with the
alternative text of the image, but without the image itself, provides the user with the same kind
of choice as the hyperlink would when used without its text but with its shape applied to the
image. The alt attribute may be left blank if there is another
area element in the same image map that points to the same resource and
has a non-blank alt attribute.
If the area element has no href
attribute, then the area represented by the element cannot be selected, and the alt attribute must be omitted.
In both cases, the shape and coords attributes specify the area.
The shape attribute is
an enumerated attribute. The following table lists the keywords defined for this
attribute. The states given in the first cell of the rows with keywords give the states to which
those keywords map. Some of the keywords are non-conforming, as noted in the last
column.
| State | Keywords | Notes |
|---|---|---|
| Circle state | circle
| |
circ
| Non-conforming | |
| Default state | default
| |
| Polygon state | poly
| |
polygon
| Non-conforming | |
| Rectangle state | rect
| |
rectangle
| Non-conforming |
The attribute may be omitted. The missing value default and invalid value default are the rectangle state.
The coords attribute
must, if specified, contain a valid list of floating-point numbers. This attribute
gives the coordinates for the shape described by the shape
attribute. The processing for this attribute is described as part of the image
map processing model.
In the circle state, area elements must
have a coords attribute present, with three integers, the
last of which must be non-negative. The first integer must be the distance in CSS pixels from the left edge of the image to the center of the circle, the
second integer must be the distance in CSS pixels from the top edge of
the image to the center of the circle, and the third integer must be the radius of the circle,
again in CSS pixels.
In the default state state, area
elements must not have a coords attribute. (The area is the
whole image.)
In the polygon state, area elements must
have a coords attribute with at least six integers, and the
number of integers must be even. Each pair of integers must represent a coordinate given as the
distances from the left and the top of the image in CSS pixels
respectively, and all the coordinates together must represent the points of the polygon, in
order.
In the rectangle state, area elements
must have a coords attribute with exactly four integers,
the first of which must be less than the third, and the second of which must be less than the
fourth. The four points must represent, respectively, the distance from the left edge of the image
to the left side of the rectangle, the distance from the top edge to the top side, the distance
from the left edge to the right side, and the distance from the top edge to the bottom side, all
in CSS pixels.
When user agents allow users to follow hyperlinks or
download hyperlinks created using the
area element, as described in the next section, the href, target, download, and ping
attributes decide how the link is followed. The rel
attribute may be used to indicate to the user the likely nature of the target resource before the
user follows the link.
The target, download, ping,
rel, and referrerpolicy attributes must be omitted if the
href attribute is not present.
If the itemprop attribute is specified on an
area element, then the href attribute must
also be specified.
The activation behavior of area elements is to follow the hyperlink or download the hyperlink created by the area element, if any, and as
determined by the download attribute and any
expressed user preference.
Support in all current engines.
The IDL attributes alt, coords,
target, download, ping, and rel, each must reflect the respective content
attributes of the same name.
The IDL attribute shape must reflect the shape content attribute.
Support in all current engines.
The IDL attribute relList must reflect the rel content attribute.
HTMLAreaElement/referrerPolicy
Support in all current engines.
The IDL attribute referrerPolicy must
reflect the referrerpolicy
content attribute, limited to only known values.
图片映射 可以让图片的几何区域与 超链接 相关联。
img 元素或 object 元素表示的图片可以通过在该元素上指定一个
usemap 属性来与一个图片映射相关联
（以 map 元素的形式）。如果指定了 usemap
属性，它必须是一个对 map 元素的 合法的哈希名引用。
考虑下面这样的图片：
我们想只让有颜色的区域可点，可以这样做：
<p>
Please select a shape:
<img src="shapes.png" usemap="#shapes"
alt="Four shapes are available: a red hollow box, a green circle, a blue triangle, and a yellow four-pointed star.">
<map name="shapes">
<area shape=rect coords="50,50,100,100"> <!-- the hole in the red box -->
<area shape=rect coords="25,25,125,125" href="red.html" alt="Red box.">
<area shape=circle coords="200,75,50" href="green.html" alt="Green circle.">
<area shape=poly coords="325,25,262,125,388,125" href="blue.html" alt="Blue triangle.">
<area shape=poly coords="450,25,435,60,400,75,435,90,450,125,465,90,500,75,465,60"
href="yellow.html" alt="Yellow star.">
</map>
</p>
If an img element or an object element representing an image has a
usemap attribute specified, user agents must process it
as follows:
Parse the attribute's value using the rules for parsing a hash-name reference
to a map element, with the element as the context node. This will return either an
element (the map) or null.
If that returned null, then return. The image is not associated with an image map after all.
Otherwise, the user agent must collect all the area elements that are
descendants of the map. Let those be the areas.
Having obtained the list of area elements that form the image map (the areas), interactive user agents must process the list in one of two ways.
If the user agent intends to show the text that the img element represents, then
it must use the following steps.
In user agents that do not support images, or that have images disabled,
object elements cannot represent images, and thus this section never applies (the
fallback content is shown instead). The following steps therefore only apply to
img elements.
Remove all the area elements in areas that have no href attribute.
Remove all the area elements in areas that have no alt attribute, or whose alt
attribute's value is the empty string, if there is another area element in
areas with the same value in the href attribute and with a non-empty alt attribute.
Each remaining area element in areas represents a
hyperlink. Those hyperlinks should all be made available to the user in a manner
associated with the text of the img.
In this context, user agents may represent area and img elements
with no specified alt attributes, or whose alt
attributes are the empty string or some other non-visible text, in an
implementation-defined fashion intended to indicate the lack of suitable
author-provided text.
If the user agent intends to show the image and allow interaction with the image to select
hyperlinks, then the image must be associated with a set of layered shapes, taken from the
area elements in areas, in reverse tree order (so the last
specified area element in the map is the bottom-most shape, and
the first element in the map, in tree order, is the top-most shape).
Each area element in areas must be processed as follows to
obtain a shape to layer onto the image:
Find the state that the element's shape attribute
represents.
Use the rules for parsing a list of floating-point numbers to parse the
element's coords attribute, if it is present, and let the
result be the coords list. If the attribute is absent, let the coords list
be the empty list.
If the number of items in the coords list is less than the minimum number
given for the area element's current state, as per the following table, then the
shape is empty; return.
| State | Minimum number of items |
|---|---|
| Circle state | 3 |
| Default state | 0 |
| Polygon state | 6 |
| Rectangle state | 4 |
Check for excess items in the coords list as per the entry in the
following list corresponding to the shape attribute's
state:
If the shape attribute represents the rectangle state, and the first number in the list is
numerically greater than the third number in the list, then swap those two numbers around.
If the shape attribute represents the rectangle state, and the second number in the list is
numerically greater than the fourth number in the list, then swap those two numbers around.
If the shape attribute represents the circle state, and the third number in the list is less than
or equal to zero, then the shape is empty; return.
Now, the shape represented by the element is the one described for the entry in the list
below corresponding to the state of the shape attribute:
Let x be the first number in coords, y be the second number, and r be the third number.
The shape is a circle whose center is x CSS pixels from the left edge of the image and y CSS pixels from the top edge of the image, and whose radius is r CSS pixels.
The shape is a rectangle that exactly covers the entire image.
Let xi be the (2i)th entry in coords, and yi be the (2i+1)th entry in coords (the first entry in coords being the one with index 0).
Let the coordinates be (xi, yi), interpreted in CSS pixels measured from the top left of the image, for all integer values of i from 0 to (N/2)-1, where N is the number of items in coords.
The shape is a polygon whose vertices are given by the coordinates, and whose interior is established using the even-odd rule. [GRAPHICS]
Let x1 be the first number in coords, y1 be the second number, x2 be the third number, and y2 be the fourth number.
The shape is a rectangle whose top-left corner is given by the coordinate (x1, y1) and whose bottom right corner is given by the coordinate (x2, y2), those coordinates being interpreted as CSS pixels from the top left corner of the image.
For historical reasons, the coordinates must be interpreted relative to the
displayed image after any stretching caused by the CSS 'width' and
'height' properties (or, for non-CSS browsers, the image element's width and height attributes — CSS browsers map
those attributes to the aforementioned CSS properties).
Browser zoom features and transforms applied using CSS or SVG do not affect the coordinates.
Pointing device interaction with an image associated with a set of layered shapes per the above
algorithm must result in the relevant user interaction events being first fired to the top-most
shape covering the point that the pointing device indicated, if any, or to the image element
itself, if there is no shape covering that point. User agents may also allow individual
area elements representing hyperlinks to be selected
and activated (e.g. using a keyboard).
Because a map element (and its area elements) can be
associated with multiple img and object elements, it is possible for an
area element to correspond to multiple focusable areas
of the document.
Image maps are live; if the DOM is mutated, then the user agent must act as if it had rerun the algorithms for image maps.
HTML/HTML5/HTML5_Parser#Inline_SVG_and_MathML_support
Support in all current engines.
The MathML math element falls into the embedded content,
phrasing content, flow content, and palpable content
categories for the purposes of the content models in this specification.
When the MathML annotation-xml element contains elements from the
HTML namespace, such elements must all be flow content.
When the MathML token elements (mi, mo, mn, ms, and mtext) are descendants of HTML elements, they may contain
phrasing content elements from the HTML namespace.
User agents must handle text other than inter-element whitespace found in MathML
elements whose content models do not allow straight text by pretending for the purposes of MathML
content models, layout, and rendering that the text is actually wrapped in a MathML
mtext element. (Such text is not, however, conforming.)
User agents must act as if any MathML element whose contents does not match the element's
content model was replaced, for the purposes of MathML layout and rendering, by a MathML
merror element containing some appropriate error message.
The semantics of MathML elements are defined by MathML and other applicable specifications. [MATHML]
Here is an example of the use of MathML in an HTML document:
<!DOCTYPE html>
<html lang="en">
<head>
<title>The quadratic formula</title>
</head>
<body>
<h1>The quadratic formula</h1>
<p>
<math>
<mi>x</mi>
<mo>=</mo>
<mfrac>
<mrow>
<mo form="prefix">−</mo> <mi>b</mi>
<mo>±</mo>
<msqrt>
<msup> <mi>b</mi> <mn>2</mn> </msup>
<mo>−</mo>
<mn>4</mn> <mo></mo> <mi>a</mi> <mo></mo> <mi>c</mi>
</msqrt>
</mrow>
<mrow>
<mn>2</mn> <mo></mo> <mi>a</mi>
</mrow>
</mfrac>
</math>
</p>
</body>
</html>HTML/HTML5/HTML5_Parser#Inline_SVG_and_MathML_support
Support in all current engines.
The SVG svg element falls into the embedded content,
phrasing content, flow content, and palpable content
categories for the purposes of the content models in this specification.
When the SVG foreignObject element contains elements from the
HTML namespace, such elements must all be flow content.
The content model for the SVG title element inside HTML
documents is phrasing content. (This further constrains the requirements given
in SVG 2.)
The semantics of SVG elements are defined by SVG 2 and other applicable specifications. [SVG]
getSVGDocument()getSVGDocument()getSVGDocument()Returns the Document object, in the case of iframe,
embed, or object elements being used to embed SVG.
The getSVGDocument() method must run
the following steps:
Let document be this element's content document.
If document is non-null and was created by the page
load processing model for XML files section because the computed type of the resource in the navigate algorithm was
image/svg+xml, then return document.
Return null.
Author requirements: The width and height attributes on img, iframe,
embed, object, video, and, when their type attribute is in the Image Button state, input elements may be
specified to give the dimensions of the visual content of the element (the width and height
respectively, relative to the nominal direction of the output medium), in CSS
pixels. The attributes, if specified, must have values that are valid non-negative integers.
The specified dimensions given may differ from the dimensions specified in the resource itself, since the resource may have a resolution that differs from the CSS pixel resolution. (On screens, CSS pixels have a resolution of 96ppi, but in general the CSS pixel resolution depends on the reading distance.) If both attributes are specified, then one of the following statements must be true:
The target ratio is the ratio of the intrinsic width to the
intrinsic height in the resource. The specified width and specified
height are the values of the width and height attributes respectively.
The two attributes must be omitted if the resource in question does not have both an intrinsic width and an intrinsic height.
If the two attributes are both zero, it indicates that the element is not intended for the user (e.g. it might be a part of a service to count page views).
The dimension attributes are not intended to be used to stretch the image.
User agent requirements: User agents are expected to use these attributes as hints for the rendering.
Support in all current engines.
Support in all current engines.
The width and height IDL attributes on the iframe,
embed, object, and video elements must reflect
the respective content attributes of the same name.
For iframe, embed, and object the IDL
attributes are DOMString; for video the IDL attributes are unsigned long.
The corresponding IDL attributes for img and
input elements are defined in those respective elements'
sections, as they are slightly more specific to those elements' other behaviors.
table elementSupport in all current engines.
Support in all current engines.
caption element, followed by zero or more
colgroup elements, followed optionally by a thead element, followed by
either zero or more tbody elements or one or more tr elements, followed
optionally by a tfoot element, optionally intermixed with one or more
script-supporting elements.[Exposed=Window]
interface HTMLTableElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute HTMLTableCaptionElement? caption;
HTMLTableCaptionElement createCaption();
[CEReactions] undefined deleteCaption();
[CEReactions] attribute HTMLTableSectionElement? tHead;
HTMLTableSectionElement createTHead();
[CEReactions] undefined deleteTHead();
[CEReactions] attribute HTMLTableSectionElement? tFoot;
HTMLTableSectionElement createTFoot();
[CEReactions] undefined deleteTFoot();
[SameObject] readonly attribute HTMLCollection tBodies;
HTMLTableSectionElement createTBody();
[SameObject] readonly attribute HTMLCollection rows;
HTMLTableRowElement insertRow(optional long index = -1);
[CEReactions] undefined deleteRow(long index);
// also has obsolete members
};The table element represents data with more than one dimension, in
the form of a table.
The table element takes part in the table
model. Tables have rows, columns, and cells given by their descendants. The rows and
columns form a grid; a table's cells must completely cover that grid without overlap.
Precise rules for determining whether this conformance requirement is met are described in the description of the table model.
Authors are encouraged to provide information describing how to interpret complex tables. Guidance on how to provide such information is given below.
Tables must not be used as layout aids. Historically, some web authors have misused tables in HTML as a way to control their page layout. This usage is non-conforming, because tools attempting to extract tabular data from such documents would obtain very confusing results. In particular, users of accessibility tools like screen readers are likely to find it very difficult to navigate pages with tables used for layout.
There are a variety of alternatives to using HTML tables for layout, primarily using CSS positioning and the CSS table model. [CSS]
Tables can be complicated to understand and navigate. To help users with this, user agents should clearly delineate cells in a table from each other, unless the user agent has classified the table as a (non-conforming) layout table.
Authors and implementers are encouraged to consider using some of the table design techniques described below to make tables easier to navigate for users.
User agents, especially those that do table analysis on arbitrary content, are encouraged to find heuristics to determine which tables actually contain data and which are merely being used for layout. This specification does not define a precise heuristic, but the following are suggested as possible indicators:
| Feature | Indication |
|---|---|
The use of the role attribute with the value presentation
| Probably a layout table |
The use of the non-conforming border attribute with the non-conforming value 0
| Probably a layout table |
The use of the non-conforming cellspacing and
cellpadding attributes with the value 0
| Probably a layout table |
The use of caption, thead, or th elements
| Probably a non-layout table |
The use of the headers and scope attributes
| Probably a non-layout table |
The use of the non-conforming border attribute with a value other than 0
| Probably a non-layout table |
| Explicit visible borders set using CSS | Probably a non-layout table |
The use of the summary attribute
| Not a good indicator (both layout and non-layout tables have historically been given this attribute) |
It is quite possible that the above suggestions are wrong. Implementors are urged to provide feedback elaborating on their experiences with trying to create a layout table detection heuristic.
If a table element has a (non-conforming) summary attribute, and the user agent has not classified the
table as a layout table, the user agent may report the contents of that attribute to the user.
caption [ = value ]Support in all current engines.
Returns the table's caption element.
Can be set, to replace the caption element.
createCaption()HTMLTableElement/createCaption
Support in all current engines.
Ensures the table has a caption element, and returns it.
deleteCaption()HTMLTableElement/deleteCaption
Support in all current engines.
Ensures the table does not have a caption element.
tHead [ = value ]Support in all current engines.
Returns the table's thead element.
Can be set, to replace the thead element. If the new value is not a
thead element, throws a "HierarchyRequestError"
DOMException.
createTHead()Support in all current engines.
Ensures the table has a thead element, and returns it.
deleteTHead()Support in all current engines.
Ensures the table does not have a thead element.
tFoot [ = value ]Support in all current engines.
Returns the table's tfoot element.
Can be set, to replace the tfoot element. If the new value is not a
tfoot element, throws a "HierarchyRequestError"
DOMException.
createTFoot()Support in all current engines.
Ensures the table has a tfoot element, and returns it.
deleteTFoot()Support in all current engines.
Ensures the table does not have a tfoot element.
tBodiesSupport in all current engines.
Returns an HTMLCollection of the tbody elements of the table.
createTBody()Support in all current engines.
Creates a tbody element, inserts it into the table, and returns it.
rowsSupport in all current engines.
Returns an HTMLCollection of the tr elements of the table.
insertRow( [ index ] )Support in all current engines.
Creates a tr element, along with a tbody if required, inserts them
into the table at the position given by the argument, and returns the tr.
The position is relative to the rows in the table. The index −1, which is the default if the argument is omitted, is equivalent to inserting at the end of the table.
If the given position is less than −1 or greater than the number of rows, throws an
"IndexSizeError" DOMException.
deleteRow(index)Support in all current engines.
Removes the tr element with the given position in the table.
The position is relative to the rows in the table. The index −1 is equivalent to deleting the last row of the table.
If the given position is less than −1 or greater than the index of the last row, or if
there are no rows, throws an "IndexSizeError"
DOMException.
In all of the following attribute and method definitions, when an element is to be
table-created, that means to create an element given the
table element's node document, the given local name, and the HTML
namespace.
The caption
IDL attribute must return, on getting, the first caption element child of the
table element, if any, or null otherwise. On setting, the first caption
element child of the table element, if any, must be removed, and the new value, if
not null, must be inserted as the first node of the table element.
The createCaption() method must return
the first caption element child of the table element, if any; otherwise
a new caption element must be table-created, inserted as the first node
of the table element, and then returned.
The deleteCaption() method must remove
the first caption element child of the table element, if any.
The tHead IDL attribute must return, on
getting, the first thead element child of the table element, if any, or
null otherwise. On setting, if the new value is null or a thead element, the first
thead element child of the table element, if any, must be removed, and
the new value, if not null, must be inserted immediately before the first element in the
table element that is neither a caption element nor a
colgroup element, if any, or at the end of the table if there are no such elements.
If the new value is neither null nor a thead element, then a
"HierarchyRequestError" DOMException must be thrown
instead.
The createTHead() method must return the
first thead element child of the table element, if any; otherwise a new
thead element must be table-created and inserted immediately before the
first element in the table element that is neither a caption element nor
a colgroup element, if any, or at the end of the table if there are no such elements,
and then that new element must be returned.
The deleteTHead() method must remove the
first thead element child of the table element, if any.
The tFoot IDL attribute must return, on
getting, the first tfoot element child of the table element, if any, or
null otherwise. On setting, if the new value is null or a tfoot element, the first
tfoot element child of the table element, if any, must be removed, and
the new value, if not null, must be inserted at the end of the table. If the new value is neither
null nor a tfoot element, then a "HierarchyRequestError"
DOMException must be thrown instead.
The createTFoot() method must return the
first tfoot element child of the table element, if any; otherwise a new
tfoot element must be table-created and inserted at the end of the
table, and then that new element must be returned.
The deleteTFoot() method must remove the
first tfoot element child of the table element, if any.
The tBodies attribute must return an
HTMLCollection rooted at the table node, whose filter matches only
tbody elements that are children of the table element.
The createTBody() method must
table-create a new tbody element, insert it
immediately after the last tbody element child in the table element, if
any, or at the end of the table element if the table element has no
tbody element children, and then must return the new tbody element.
The rows
attribute must return an HTMLCollection rooted at the table node, whose
filter matches only tr elements that are either children of the table
element, or children of thead, tbody, or tfoot elements
that are themselves children of the table element. The elements in the collection
must be ordered such that those elements whose parent is a thead are included first,
in tree order, followed by those elements whose parent is either a table
or tbody element, again in tree order, followed finally by those
elements whose parent is a tfoot element, still in tree order.
The behavior of the insertRow(index) method depends on the state of the table. When it is called,
the method must act as required by the first item in the following list of conditions that
describes the state of the table and the index argument:
rows collection:IndexSizeError"
DOMException.rows collection has zero elements in it, and the
table has no tbody elements in it:tbody
element, then table-create a tr element, then
append the tr element to the tbody element, then append the
tbody element to the table element, and finally return the
tr element.rows collection has zero elements in it:tr element,
append it to the last tbody element in the table, and return the tr
element.rows collection:tr element,
and append it to the parent of the last tr element in the rows collection. Then, the newly created tr element
must be returned.tr element,
insert it immediately before the indexth tr element in the rows collection, in the same parent, and finally must return the
newly created tr element.When the deleteRow(index) method is called, the user agent must run the following
steps:
If index is less than −1 or greater than or equal to the number of
elements in the rows collection, then throw an
"IndexSizeError" DOMException.
If index is −1, then remove
the last element in the rows collection from its parent, or
do nothing if the rows collection is empty.
Otherwise, remove the indexth element
in the rows collection from its parent.
Here is an example of a table being used to mark up a Sudoku puzzle. Observe the lack of headers, which are not necessary in such a table.
<style>
#sudoku { border-collapse: collapse; border: solid thick; }
#sudoku colgroup, table#sudoku tbody { border: solid medium; }
#sudoku td { border: solid thin; height: 1.4em; width: 1.4em; text-align: center; padding: 0; }
</style>
<h1>Today's Sudoku</h1>
<table id="sudoku">
<colgroup><col><col><col>
<colgroup><col><col><col>
<colgroup><col><col><col>
<tbody>
<tr> <td> 1 <td> <td> 3 <td> 6 <td> <td> 4 <td> 7 <td> <td> 9
<tr> <td> <td> 2 <td> <td> <td> 9 <td> <td> <td> 1 <td>
<tr> <td> 7 <td> <td> <td> <td> <td> <td> <td> <td> 6
<tbody>
<tr> <td> 2 <td> <td> 4 <td> <td> 3 <td> <td> 9 <td> <td> 8
<tr> <td> <td> <td> <td> <td> <td> <td> <td> <td>
<tr> <td> 5 <td> <td> <td> 9 <td> <td> 7 <td> <td> <td> 1
<tbody>
<tr> <td> 6 <td> <td> <td> <td> 5 <td> <td> <td> <td> 2
<tr> <td> <td> <td> <td> <td> 7 <td> <td> <td> <td>
<tr> <td> 9 <td> <td> <td> 8 <td> <td> 2 <td> <td> <td> 5
</table>如果一个表格不以第一行和第一列作为标题，或着其他难以理解的情况，作者引入介绍该表格的解释性信息。 该信息对所有用户都很有用，尤其是对于无法看到该表的用户，例如屏幕阅读器的用户。
这样的解释性信息应该介绍表格的目的，概述其基本单元格结构， 突出显示任何趋势或模式，通常用来教会用户如何使用这个表格。
例如下面的表格：
| Negative | Characteristic | Positive |
|---|---|---|
| Sad | Mood | Happy |
| Failing | Grade | Passing |
如果有一段描述来解释该表格的布局就很有用，比如 “性格在第二列给出，左侧一列是缺点，右侧一列是优点”。
有很多种方式引入这个信息，比如：
<p>In the following table, characteristics are given in the second column, with the negative side in the left column and the positive side in the right column.</p> <table> <caption>Characteristics with positive and negative sides</caption> <thead> <tr> <th id="n"> Negative <th> Characteristic <th> Positive <tbody> <tr> <td headers="n r1"> Sad <th id="r1"> Mood <td> Happy <tr> <td headers="n r2"> Failing <th id="r2"> Grade <td> Passing </table>
caption 中<table> <caption> <strong>Characteristics with positive and negative sides.</strong> <p>Characteristics are given in the second column, with the negative side in the left column and the positive side in the right column.</p> </caption> <thead> <tr> <th id="n"> Negative <th> Characteristic <th> Positive <tbody> <tr> <td headers="n r1"> Sad <th id="r1"> Mood <td> Happy <tr> <td headers="n r2"> Failing <th id="r2"> Grade <td> Passing </table>
caption 中，用一个 details 元素<table> <caption> <strong>Characteristics with positive and negative sides.</strong> <details> <summary>Help</summary> <p>Characteristics are given in the second column, with the negative side in the left column and the positive side in the right column.</p> </details> </caption> <thead> <tr> <th id="n"> Negative <th> Characteristic <th> Positive <tbody> <tr> <td headers="n r1"> Sad <th id="r1"> Mood <td> Happy <tr> <td headers="n r2"> Failing <th id="r2"> Grade <td> Passing </table>
figure 中在表格的旁边<figure>
<figcaption>Characteristics with positive and negative sides</figcaption>
<p>Characteristics are given in the second column, with the
negative side in the left column and the positive side in the right
column.</p>
<table>
<thead>
<tr>
<th id="n"> Negative
<th> Characteristic
<th> Positive
<tbody>
<tr>
<td headers="n r1"> Sad
<th id="r1"> Mood
<td> Happy
<tr>
<td headers="n r2"> Failing
<th id="r2"> Grade
<td> Passing
</table>
</figure>figure 的 figcaption 中，表格的旁边<figure>
<figcaption>
<strong>Characteristics with positive and negative sides</strong>
<p>Characteristics are given in the second column, with the
negative side in the left column and the positive side in the right
column.</p>
</figcaption>
<table>
<thead>
<tr>
<th id="n"> Negative
<th> Characteristic
<th> Positive
<tbody>
<tr>
<td headers="n r1"> Sad
<th id="r1"> Mood
<td> Happy
<tr>
<td headers="n r2"> Failing
<th id="r2"> Grade
<td> Passing
</table>
</figure>作者也可以使用其他技术，或适当地结合上述技术。
相比于编写一段描述来解释表格的布局，最佳的选择当然还是调整好表格使得不再有解释的必要。
对于上面示例中使用的表格，对表格进行简单的重新排列使标题位于顶部和左侧。
这样就消除了解释的必要，也不需要使用 headers 属性了：
<table> <caption>Characteristics with positive and negative sides</caption> <thead> <tr> <th> Characteristic <th> Negative <th> Positive <tbody> <tr> <th> Mood <td> Sad <td> Happy <tr> <th> Grade <td> Failing <td> Passing </table>
好的表格设计是让表格更加可读和可用的关键
在视觉媒体中，提供行列边框和修改行的背景对于制作可读性强的表格非常有效。
对于具有大量数字内容的表格，使用等宽字体可以帮助用户看到其中的模式， 特别是在用户代理未呈现边界的情况下。（不幸的是，由于历史原因，不在表格上渲染边界是常见的默认设置。）
在语音媒体中，单元格使得媒体能够在读取单元格的内容之前报告相应的标题， 以及允许用户以网格方式来浏览表格，而不需要按源格式序列化表格的全部内容。
鼓励作者使用 CSS 来实现这些效果。
鼓励用户代理在页面不使用 CSS 并且该表格不是用来布局的时候，使用这些技术来呈现表格。
caption elementSupport in all current engines.
Support in all current engines.
table element.table elements.caption element's end tag can be omitted if
the caption element is not immediately followed by ASCII whitespace or a
comment.[Exposed=Window]
interface HTMLTableCaptionElement : HTMLElement {
[HTMLConstructor] constructor();
// also has obsolete members
};The caption element represents the title of the table
that is its parent, if it has a parent and that is a table element.
The caption element takes part in the table model.
When a table element is the only content in a figure element other
than the figcaption, the caption element should be omitted in favor of
the figcaption.
A caption can introduce context for a table, making it significantly easier to understand.
Consider, for instance, the following table:
| 1 | 2 | 3 | 4 | 5 | 6 | |
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| 5 | 6 | 7 | 8 | 9 | 10 | 11 |
| 6 | 7 | 8 | 9 | 10 | 11 | 12 |
In the abstract, this table is not clear. However, with a caption giving the table's number (for reference in the main prose) and explaining its use, it makes more sense:
<caption>
<p>Table 1.
<p>This table shows the total score obtained from rolling two
six-sided dice. The first row represents the value of the first die,
the first column the value of the second die. The total is given in
the cell that corresponds to the values of the two dice.
</caption>This provides the user with more context:
| 1 | 2 | 3 | 4 | 5 | 6 | |
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| 5 | 6 | 7 | 8 | 9 | 10 | 11 |
| 6 | 7 | 8 | 9 | 10 | 11 | 12 |
colgroup elementSupport in all current engines.
Support in all current engines.
table element, after any
caption elements and before any thead,
tbody, tfoot, and tr
elements.span attribute is present: Nothing.span attribute is absent: Zero or more col and template elements.colgroup element's start tag can be
omitted if the first thing inside the colgroup element is a col element,
and if the element is not immediately preceded by another colgroup element whose
end tag has been omitted. (It can't be omitted if the element
is empty.)colgroup element's end tag can be omitted
if the colgroup element is not immediately followed by ASCII whitespace
or a comment.span — Number of columns spanned by the element[Exposed=Window]
interface HTMLTableColElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute unsigned long span;
// also has obsolete members
};The colgroup element represents a group of one or more columns in the table that is its parent, if it has a
parent and that is a table element.
If the colgroup element contains no col elements, then the element
may have a span
content attribute specified, whose value must be a valid non-negative integer greater
than zero and less than or equal to 1000.
The colgroup element and its span
attribute take part in the table model.
The span
IDL attribute must reflect the content attribute of the same name. It is
clamped to the range [1, 1000], and its default value is 1.
col elementSupport in all current engines.
colgroup element that doesn't have
a span attribute.span — Number of columns spanned by the elementHTMLTableColElement, as defined for colgroup elements.If a col element has a parent and that is a colgroup element that
itself has a parent that is a table element, then the col element
represents one or more columns in the column group represented by that colgroup.
The element may have a span content attribute specified, whose value must be a
valid non-negative integer greater than zero and less than or equal to 1000.
The col element and its span attribute take
part in the table model.
The span IDL
attribute must reflect the content attribute of the same name. It is clamped to
the range [1, 1000], and its default value is 1.
tbody elementSupport in all current engines.
Support in all current engines.
table element, after any
caption, colgroup, and
thead elements, but only if there are no
tr elements that are children of the
table element.tr and script-supporting elements.tbody element's start tag can be omitted
if the first thing inside the tbody element is a tr element, and if the
element is not immediately preceded by a tbody, thead, or
tfoot element whose end tag has been omitted. (It
can't be omitted if the element is empty.)tbody element's end tag can be omitted if
the tbody element is immediately followed by a tbody or
tfoot element, or if there is no more content in the parent element.[Exposed=Window]
interface HTMLTableSectionElement : HTMLElement {
[HTMLConstructor] constructor();
[SameObject] readonly attribute HTMLCollection rows;
HTMLTableRowElement insertRow(optional long index = -1);
[CEReactions] undefined deleteRow(long index);
// also has obsolete members
};The
HTMLTableSectionElement interface is also used for thead and
tfoot elements.
The tbody element represents a block of rows that consist of a
body of data for the parent table element, if the tbody element has a
parent and it is a table.
The tbody element takes part in the table model.
rowsReturns an HTMLCollection of the tr elements of the table
section.
insertRow( [ index ] )Creates a tr element, inserts it into the table section at the position given by
the argument, and returns the tr.
The position is relative to the rows in the table section. The index −1, which is the default if the argument is omitted, is equivalent to inserting at the end of the table section.
If the given position is less than −1 or greater than the number of rows, throws an
"IndexSizeError" DOMException.
deleteRow(index)Removes the tr element with the given position in the table section.
The position is relative to the rows in the table section. The index −1 is equivalent to deleting the last row of the table section.
If the given position is less than −1 or greater than the index of the last row, or if
there are no rows, throws an "IndexSizeError"
DOMException.
The rows attribute must return an HTMLCollection
rooted at this element, whose filter matches only tr elements that are children of
this element.
The insertRow(index)
method must act as follows:
If index is less than −1 or greater than the number of elements in the
rows collection, throw an
"IndexSizeError" DOMException.
Let table row be the result of creating an
element given this element's node document, tr, and the
HTML namespace.
If index is −1 or equal to the number of items in the rows collection, then append table row to this element.
Otherwise, insert table row as a
child of this element, immediately before the indexth tr element in the
rows collection.
Return table row.
The deleteRow(index) method
must, when invoked, act as follows:
If index is less than −1 or greater than or equal to the number of
elements in the rows collection, then throw an
"IndexSizeError" DOMException.
If index is −1, then remove
the last element in the rows collection from this
element, or do nothing if the rows collection is
empty.
Otherwise, remove the indexth element
in the rows collection from this element.
thead elementSupport in all current engines.
table element, after any
caption, and colgroup
elements and before any tbody, tfoot, and
tr elements, but only if there are no other
thead elements that are children of the
table element.tr and script-supporting elements.thead element's end tag can be omitted if
the thead element is immediately followed by a tbody or
tfoot element.HTMLTableSectionElement, as defined for tbody elements.The thead element represents the block of rows that consist of
the column labels (headers) for the parent table element, if the thead
element has a parent and it is a table.
The thead element takes part in the table model.
This example shows a thead element being used. Notice the use of both
th and td elements in the thead element: the first row is
the headers, and the second row is an explanation of how to fill in the table.
<table>
<caption> School auction sign-up sheet </caption>
<thead>
<tr>
<th><label for=e1>Name</label>
<th><label for=e2>Product</label>
<th><label for=e3>Picture</label>
<th><label for=e4>Price</label>
<tr>
<td>Your name here
<td>What are you selling?
<td>Link to a picture
<td>Your reserve price
<tbody>
<tr>
<td>Ms Danus
<td>Doughnuts
<td><img src="https://example.com/mydoughnuts.png" title="Doughnuts from Ms Danus">
<td>$45
<tr>
<td><input id=e1 type=text name=who required form=f>
<td><input id=e2 type=text name=what required form=f>
<td><input id=e3 type=url name=pic form=f>
<td><input id=e4 type=number step=0.01 min=0 value=0 required form=f>
</table>
<form id=f action="/auction.cgi">
<input type=button name=add value="Submit">
</form>tfoot elementSupport in all current engines.
table element, after any
caption, colgroup, thead,
tbody, and tr elements, but only if there
are no other tfoot elements that are children of the
table element.tr and script-supporting elements.tfoot element's end tag can be omitted if
there is no more content in the parent element.HTMLTableSectionElement, as defined for tbody elements.The tfoot element represents the block of rows that consist of
the column summaries (footers) for the parent table element, if the
tfoot element has a parent and it is a table.
The tfoot element takes part in the table
model.
tr elementSupport in all current engines.
Support in all current engines.
thead element.tbody element.tfoot element.table element, after any
caption, colgroup, and thead
elements, but only if there are no tbody elements that
are children of the table element.td, th, and script-supporting elements.tr element's end tag can be omitted if the
tr element is immediately followed by another tr element, or if there is
no more content in the parent element.[Exposed=Window]
interface HTMLTableRowElement : HTMLElement {
[HTMLConstructor] constructor();
readonly attribute long rowIndex;
readonly attribute long sectionRowIndex;
[SameObject] readonly attribute HTMLCollection cells;
HTMLTableCellElement insertCell(optional long index = -1);
[CEReactions] undefined deleteCell(long index);
// also has obsolete members
};The tr element represents a row of
cells in a table.
The tr element takes part in the table model.
rowIndexSupport in all current engines.
Returns the position of the row in the table's rows
list.
Returns −1 if the element isn't in a table.
sectionRowIndexReturns the position of the row in the table section's rows list.
Returns −1 if the element isn't in a table section.
cellsReturns an HTMLCollection of the td and th elements of
the row.
insertCell( [ index ] )HTMLTableRowElement/insertCell
Support in all current engines.
Creates a td element, inserts it into the table row at the position given by the
argument, and returns the td.
The position is relative to the cells in the row. The index −1, which is the default if the argument is omitted, is equivalent to inserting at the end of the row.
If the given position is less than −1 or greater than the number of cells, throws an
"IndexSizeError" DOMException.
deleteCell(index)Removes the td or th element with the given position in the
row.
The position is relative to the cells in the row. The index −1 is equivalent to deleting the last cell of the row.
If the given position is less than −1 or greater than the index of the last cell, or
if there are no cells, throws an "IndexSizeError"
DOMException.
The rowIndex attribute must, if this element has
a parent table element, or a parent tbody, thead, or
tfoot element and a grandparent table element, return the index
of this tr element in that table element's rows collection. If there is no such table element,
then the attribute must return −1.
The sectionRowIndex attribute must, if
this element has a parent table, tbody, thead, or
tfoot element, return the index of the tr element in the parent
element's rows collection (for tables, that's
HTMLTableElement's rows collection; for table
sections, that's HTMLTableSectionElement's rows
collection). If there is no such parent element, then the attribute must return −1.
The cells
attribute must return an HTMLCollection rooted at this tr element, whose
filter matches only td and th elements that are children of the
tr element.
The insertCell(index)
method must act as follows:
If index is less than −1 or greater than the number of elements in
the cells collection, then throw an
"IndexSizeError" DOMException.
Let table cell be the result of creating an
element given this tr element's node document, td,
and the HTML namespace.
If index is equal to −1 or equal to the number of items in cells collection, then append table cell to this tr
element.
Otherwise, insert table cell as a
child of this tr element, immediately before the indexth td
or th element in the cells collection.
Return table cell.
The deleteCell(index)
method must act as follows:
If index is less than −1 or greater than or equal to the number of
elements in the cells collection, then throw an
"IndexSizeError" DOMException.
If index is −1, then remove
the last element in the cells collection from its
parent, or do nothing if the cells collection is
empty.
Otherwise, remove the indexth element
in the cells collection from its parent.
td elementSupport in all current engines.
Support in all current engines.
tr element.td element's end tag can be omitted if the
td element is immediately followed by a td or th element,
or if there is no more content in the parent element.colspan — Number of columns that the cell is to spanrowspan — Number of rows that the cell is to spanheaders — The header cells for this cell[Exposed=Window]
interface HTMLTableCellElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute unsigned long colSpan;
[CEReactions] attribute unsigned long rowSpan;
[CEReactions] attribute DOMString headers;
readonly attribute long cellIndex;
[CEReactions] attribute DOMString scope; // only conforming for th elements
[CEReactions] attribute DOMString abbr; // only conforming for th elements
// also has obsolete members
};The
HTMLTableCellElement interface is also used for th elements.
The td element represents a data cell in a table.
The td element and its colspan, rowspan, and headers
attributes take part in the table model.
User agents, especially in non-visual environments or where displaying the table as a 2D grid
is impractical, may give the user context for the cell when rendering the contents of a cell; for
instance, giving its position in the table model, or listing the cell's header cells
(as determined by the algorithm for assigning header cells). When a cell's header
cells are being listed, user agents may use the value of abbr
attributes on those header cells, if any, instead of the contents of the header cells
themselves.
In this example, we see a snippet of a web application consisting of a grid of editable cells
(essentially a simple spreadsheet). One of the cells has been configured to show the sum of the
cells above it. Three have been marked as headings, which use th elements instead of
td elements. A script would attach event handlers to these elements to maintain the
total.
<table>
<tr>
<th><input value="Name">
<th><input value="Paid ($)">
<tr>
<td><input value="Jeff">
<td><input value="14">
<tr>
<td><input value="Britta">
<td><input value="9">
<tr>
<td><input value="Abed">
<td><input value="25">
<tr>
<td><input value="Shirley">
<td><input value="2">
<tr>
<td><input value="Annie">
<td><input value="5">
<tr>
<td><input value="Troy">
<td><input value="5">
<tr>
<td><input value="Pierce">
<td><input value="1000">
<tr>
<th><input value="Total">
<td><output value="1060">
</table>th elementSupport in all current engines.
tr element.header, footer,
sectioning content, or heading content descendants.th element's end tag can be omitted if the
th element is immediately followed by a td or th element,
or if there is no more content in the parent element.colspan — Number of columns that the cell is to spanrowspan — Number of rows that the cell is to spanheaders — The header cells for this cellscope — Specifies which cells the header cell applies toabbr — Alternative label to use for the header cell when referencing the cell in other contextsHTMLTableCellElement, as defined for td elements.The th element represents a header cell in a table.
The th element may have a scope content attribute specified. The scope attribute is an enumerated attribute with five
states, four of which have explicit keywords:
row keyword, which maps to the
row statecol keyword, which maps to the
column staterowgroup keyword, which maps to
the row group stateth element's scope attribute must
not be in the row group state if the element is not
anchored in a row group.colgroup keyword, which maps to
the column group stateth element's scope attribute must
not be in the column group state if the element is
not anchored in a column group.The scope attribute's missing value default and invalid value default
are the auto state.
The th element may have an abbr content attribute specified. Its value must be an
alternative label for the header cell, to be used when referencing the cell in other contexts
(e.g. when describing the header cells that apply to a data cell). It is typically an abbreviated
form of the full header cell, but can also be an expansion, or merely a different phrasing.
The th element and its colspan, rowspan, headers, and
scope attributes take part in the table model.
The following example shows how the scope attribute's rowgroup value affects which data cells a header cell
applies to.
Here is a markup fragment showing a table:
<table>
<thead>
<tr> <th> ID <th> Measurement <th> Average <th> Maximum
<tbody>
<tr> <td> <th scope=rowgroup> Cats <td> <td>
<tr> <td> 93 <th scope=row> Legs <td> 3.5 <td> 4
<tr> <td> 10 <th scope=row> Tails <td> 1 <td> 1
<tbody>
<tr> <td> <th scope=rowgroup> English speakers <td> <td>
<tr> <td> 32 <th scope=row> Legs <td> 2.67 <td> 4
<tr> <td> 35 <th scope=row> Tails <td> 0.33 <td> 1
</table>This would result in the following table:
| ID | Measurement | Average | Maximum |
|---|---|---|---|
| Cats | |||
| 93 | Legs | 3.5 | 4 |
| 10 | Tails | 1 | 1 |
| English speakers | |||
| 32 | Legs | 2.67 | 4 |
| 35 | Tails | 0.33 | 1 |
The headers in the first row all apply directly down to the rows in their column.
The headers with the explicit scope attributes apply to all
the cells in their row group other than the cells in the first column.
The remaining headers apply just to the cells to the right of them.
td and th elementsThe td and th elements may have a colspan content attribute specified, whose value must be a
valid non-negative integer greater than zero and less than or equal to 1000.
The td and th elements may also have a rowspan content attribute specified,
whose value must be a valid non-negative integer less than or equal to 65534. For
this attribute, the value zero means that the cell is to span all the remaining rows in the row
group.
These attributes give the number of columns and rows respectively that the cell is to span. These attributes must not be used to overlap cells, as described in the description of the table model.
The td and th element may have a headers content attribute specified. The headers attribute, if specified, must contain a string
consisting of an unordered set of unique space-separated tokens, none of which are
identical to another token and each of which must have the value of an ID of a th element taking part in the same table as the td or th element (as defined by the table model).
A th element with ID id is
said to be directly targeted by all td and th elements in the
same table that have headers attributes whose values include as one of their tokens
the ID id. A th element A is said to be targeted by a th or td element
B if either A is directly targeted by B or if there exists an element C that is itself
targeted by the element B and A is directly
targeted by C.
A th element must not be targeted by itself.
The colspan, rowspan, and headers
attributes take part in the table model.
cellIndexReturns the position of the cell in the row's cells list.
This does not necessarily correspond to the x-position of the cell in the
table, since earlier cells might cover multiple rows or columns.
Returns −1 if the element isn't in a row.
The colSpan IDL attribute must
reflect the colspan content attribute. It is
clamped to the range [1, 1000], and its default value is 1.
The rowSpan IDL attribute must
reflect the rowspan content attribute. It is
clamped to the range [0, 65534], and its default value is 1.
The headers IDL attribute must reflect the content
attribute of the same name.
The cellIndex IDL attribute must, if the
element has a parent tr element, return the index of the cell's element in the parent
element's cells collection. If there is no such parent element,
then the attribute must return −1.
The scope
IDL attribute must reflect the content attribute of the same name, limited to
only known values.
The abbr IDL
attribute must reflect the content attribute of the same name.
The various table elements and their content attributes together define the table model.
A table consists of cells aligned on a two-dimensional grid of
slots with coordinates (x, y). The grid is finite, and is either empty or has one or more slots. If the grid
has one or more slots, then the x coordinates are always in the range 0 ≤ x < xwidth, and the y coordinates are always in the
range 0 ≤ y < yheight. If one or both of xwidth and yheight are zero, then the
table is empty (has no slots). Tables correspond to table elements.
A cell is a set of slots anchored at a slot (cellx, celly), and with
a particular width and height such that the cell covers
all the slots with coordinates (x, y) where cellx ≤ x < cellx+width and celly ≤ y < celly+height. Cells can either be data cells
or header cells. Data cells correspond to td elements, and header cells
correspond to th elements. Cells of both types can have zero or more associated
header cells.
It is possible, in certain error cases, for two cells to occupy the same slot.
A row is a complete set of slots from x=0 to x=xwidth-1, for a particular value of y. Rows usually
correspond to tr elements, though a row group
can have some implied rows at the end in some cases involving
cells spanning multiple rows.
A column is a complete set of slots from y=0 to y=yheight-1, for a particular value of x. Columns can
correspond to col elements. In the absence of col elements, columns are
implied.
A row group is a set of rows anchored at a slot (0, groupy) with a particular height such that the row group
covers all the slots with coordinates (x, y) where 0 ≤ x < xwidth and groupy ≤ y < groupy+height. Row groups correspond to
tbody, thead, and tfoot elements. Not every row is
necessarily in a row group.
A column group is a set of columns anchored at a slot (groupx, 0) with a particular width such that the column group
covers all the slots with coordinates (x, y) where groupx ≤ x < groupx+width and 0 ≤ y < yheight. Column
groups correspond to colgroup elements. Not every column is necessarily in a column
group.
Row groups cannot overlap each other. Similarly, column groups cannot overlap each other.
A cell cannot cover slots that are from two or more row groups. It is, however, possible for a cell to be in multiple column groups. All the slots that form part of one cell are part of zero or one row groups and zero or more column groups.
In addition to cells, columns, rows, row groups, and column
groups, tables can have a caption element
associated with them. This gives the table a heading, or legend.
A table model error is an error with the data represented by table
elements and their descendants. Documents must not have table model errors.
To determine which elements correspond to which slots in a table associated with a table element, to determine the
dimensions of the table (xwidth and yheight), and to determine if there are any table model errors, user agents must use the following algorithm:
Let xwidth be zero.
Let yheight be zero.
Let pending tfoot elements be a list of tfoot
elements, initially empty.
Let the table be the table represented
by the table element. The xwidth and yheight variables give the table's
dimensions. The table is initially empty.
If the table element has no children elements, then return the
table (which will be empty).
Associate the first caption element child of the table element with
the table. If there are no such children, then it has no associated
caption element.
Let the current element be the first element child of the
table element.
If a step in this algorithm ever requires the current element to be advanced to the next child of the table when
there is no such next child, then the user agent must jump to the step labeled end, near
the end of this algorithm.
While the current element is not one of the following elements, advance the current element to the next
child of the table:
If the current element is a colgroup, follow these
substeps:
Column groups: Process the current element according to the appropriate case below:
col element childrenFollow these steps:
Let xstart have the value of xwidth.
Let the current column be the first col element child
of the colgroup element.
Columns: If the current column col element has
a span attribute, then parse its value using the
rules for parsing non-negative integers.
If the result of parsing the value is not an error or zero, then let span be that value.
Otherwise, if the col element has no span attribute, or if trying to parse the attribute's value
resulted in an error or zero, then let span be 1.
If span is greater than 1000, let it be 1000 instead.
Increase xwidth by span.
Let the last span columns in
the table correspond to the current column
col element.
If current column is not the last col element child of
the colgroup element, then let the current column be the
next col element child of the colgroup element, and return to
the step labeled columns.
Let all the last columns in the
table from x=xstart to
x=xwidth-1 form a new column group, anchored at the slot (xstart, 0), with width xwidth-xstart, corresponding to the colgroup element.
col element childrenIf the colgroup element has a span
attribute, then parse its value using the rules for parsing non-negative
integers.
If the result of parsing the value is not an error or zero, then let span be that value.
Otherwise, if the colgroup element has no span attribute, or if trying to parse the attribute's
value resulted in an error or zero, then let span be 1.
If span is greater than 1000, let it be 1000 instead.
Increase xwidth by span.
Let the last span columns in
the table form a new column
group, anchored at the slot (xwidth-span, 0), with width span, corresponding to the colgroup element.
While the current element is not one of the following elements, advance the current element to the
next child of the table:
If the current element is a colgroup element, jump to the
step labeled column groups above.
Let ycurrent be zero.
Let the list of downward-growing cells be an empty list.
Rows: While the current element is not one of the following
elements, advance the current
element to the next child of the table:
If the current element is a tr, then run the algorithm
for processing rows, advance the current element to the next child of the table, and return to the
step labeled rows.
Run the algorithm for ending a row group.
If the current element is a tfoot, then add that element to
the list of pending tfoot elements, advance the current element to the next
child of the table, and return to the step labeled rows.
The current element is either a thead or a
tbody.
Run the algorithm for processing row groups.
Return to the step labeled rows.
End: For each tfoot element in the list of pending
tfoot elements, in tree order, run the algorithm for processing row
groups.
If there exists a row or column in the table containing only slots that do not have a cell anchored to them, then this is a table model error.
Return the table.
The algorithm for processing row groups, which is invoked by the set of steps above
for processing thead, tbody, and tfoot elements, is:
Let ystart have the value of yheight.
For each tr element that is a child of the element being processed, in tree
order, run the algorithm for processing rows.
If yheight > ystart, then let all the last rows in the table from y=ystart to y=yheight-1 form a new row group, anchored at the slot with coordinate (0, ystart), with height yheight-ystart, corresponding to the element being processed.
Run the algorithm for ending a row group.
The algorithm for ending a row group, which is invoked by the set of steps above when starting and ending a block of rows, is:
While ycurrent is less than yheight, follow these steps:
Increase ycurrent by 1.
Empty the list of downward-growing cells.
The algorithm for processing rows, which is invoked by the set of steps above for
processing tr elements, is:
If yheight is equal to ycurrent, then increase yheight by 1. (ycurrent is never greater than yheight.)
Let xcurrent be 0.
If the tr element being processed has no td or th
element children, then increase ycurrent by 1, abort
this set of steps, and return to the algorithm above.
Let current cell be the first td or th element child
in the tr element being processed.
Cells: While xcurrent is less than xwidth and the slot with coordinate (xcurrent, ycurrent) already has a cell assigned to it, increase xcurrent by 1.
If xcurrent is equal to xwidth, increase xwidth by 1. (xcurrent is never greater than xwidth.)
If the current cell has a colspan
attribute, then parse that attribute's
value, and let colspan be the result.
If parsing that value failed, or returned zero, or if the attribute is absent, then let colspan be 1, instead.
If colspan is greater than 1000, let it be 1000 instead.
If the current cell has a rowspan
attribute, then parse that attribute's
value, and let rowspan be the result.
If parsing that value failed or if the attribute is absent, then let rowspan be 1, instead.
If rowspan is greater than 65534, let it be 65534 instead.
If rowspan is zero and the table element's
node document is not set to quirks mode, then let cell grows
downward be true, and set rowspan to 1. Otherwise, let cell grows downward be false.
If xwidth < xcurrent+colspan, then let xwidth be xcurrent+colspan.
If yheight < ycurrent+rowspan, then let yheight be ycurrent+rowspan.
Let the slots with coordinates (x, y) such that xcurrent ≤ x < xcurrent+colspan and ycurrent ≤ y < ycurrent+rowspan be covered by a new cell c, anchored at (xcurrent, ycurrent), which has width colspan and height rowspan, corresponding to the current cell element.
If the current cell element is a th element, let this new
cell c be a header cell; otherwise, let it be a data cell.
To establish which header cells apply to the current cell element, use the algorithm for assigning header cells described in the next section.
If any of the slots involved already had a cell covering them, then this is a table model error. Those slots now have two cells overlapping.
If cell grows downward is true, then add the tuple {c, xcurrent, colspan} to the list of downward-growing cells.
Increase xcurrent by colspan.
If current cell is the last td or th element child in
the tr element being processed, then increase ycurrent by 1, abort this set of steps, and return to the algorithm
above.
Let current cell be the next td or th element child
in the tr element being processed.
Return to the step labeled cells.
When the algorithms above require the user agent to run the algorithm for growing downward-growing cells, the user agent must, for each {cell, cellx, width} tuple in the list of downward-growing cells, if any, extend the cell cell so that it also covers the slots with coordinates (x, ycurrent), where cellx ≤ x < cellx+width.
Each cell can be assigned zero or more header cells. The algorithm for assigning header cells to a cell principal cell is as follows.
Let header list be an empty list of cells.
Let (principalx, principaly) be the coordinate of the slot to which the principal cell is anchored.
headers attribute specifiedTake the value of the principal cell's headers attribute and split it on ASCII whitespace, letting id list be the
list of tokens obtained.
For each token in the id list, if the
first element in the Document with an ID equal to
the token is a cell in the same table, and that cell is not the
principal cell, then add that cell to header list.
headers attribute specifiedLet principalwidth be the width of the principal cell.
Let principalheight be the height of the principal cell.
For each value of y from principaly to principaly+principalheight-1, run the internal algorithm for scanning and assigning header cells, with the principal cell, the header list, the initial coordinate (principalx,y), and the increments Δx=−1 and Δy=0.
For each value of x from principalx to principalx+principalwidth-1, run the internal algorithm for scanning and assigning header cells, with the principal cell, the header list, the initial coordinate (x,principaly), and the increments Δx=0 and Δy=−1.
If the principal cell is anchored in a row group, then add all header cells that are row group headers and are anchored in the same row group with an x-coordinate less than or equal to principalx+principalwidth-1 and a y-coordinate less than or equal to principaly+principalheight-1 to header list.
If the principal cell is anchored in a column group, then add all header cells that are column group headers and are anchored in the same column group with an x-coordinate less than or equal to principalx+principalwidth-1 and a y-coordinate less than or equal to principaly+principalheight-1 to header list.
Remove all the empty cells from the header list.
Remove any duplicates from the header list.
Remove principal cell from the header list if it is there.
Assign the headers in the header list to the principal cell.
The internal algorithm for scanning and assigning header cells, given a principal cell, a header list, an initial coordinate (initialx, initialy), and Δx and Δy increments, is as follows:
Let x equal initialx.
Let y equal initialy.
Let opaque headers be an empty list of cells.
Let in header block be true, and let headers from current header block be a list of cells containing just the principal cell.
Let in header block be false and let headers from current header block be an empty list of cells.
Loop: Increment x by Δx; increment y by Δy.
For each invocation of this algorithm, one of Δx and Δy will be −1, and the other will be 0.
If either x or y are less than 0, then abort this internal algorithm.
If there is no cell covering slot (x, y), or if there is more than one cell covering slot (x, y), return to the substep labeled loop.
Let current cell be the cell covering slot (x, y).
Set in header block to true.
Add current cell to headers from current header block.
Let blocked be false.
If there are any cells in the opaque headers list anchored with the same x-coordinate as the current cell, and with the same width as current cell, then let blocked be true.
If the current cell is not a column header, then let blocked be true.
If there are any cells in the opaque headers list anchored with the same y-coordinate as the current cell, and with the same height as current cell, then let blocked be true.
If the current cell is not a row header, then let blocked be true.
If blocked is false, then add the current cell to the headers list.
Set in header block to false. Add all the cells in headers from current header block to the opaque headers list, and empty the headers from current header block list.
Return to the step labeled loop.
A header cell anchored at the slot with coordinate (x, y) with width width and height height is said to be a column header if any of the following conditions are true:
scope attribute is in the column state, orscope attribute is in the auto state, and there are no data cells in any of the cells
covering slots with y-coordinates y .. y+height-1.A header cell anchored at the slot with coordinate (x, y) with width width and height height is said to be a row header if any of the following conditions are true:
scope attribute is in the row state, orscope attribute is in the auto state, the cell is not a column header, and
there are no data cells in any of the cells covering slots with x-coordinates
x .. x+width-1.A header cell is said to be a column group header if its scope attribute is in the column
group state.
A header cell is said to be a row group header if its scope attribute is in the row
group state.
A cell is said to be an empty cell if it contains no elements and its child text content, if any, consists only of ASCII whitespace.
This section is non-normative.
下面的例子是 Smithsonian physical tables, Volume 71 中的表 45 后半部分的标记：
<table> <caption>Specification values: <b>Steel</b>, <b>Castings</b>, Ann. A.S.T.M. A27-16, Class B;* P max. 0.06; S max. 0.05.</caption> <thead> <tr> <th rowspan=2>Grade.</th> <th rowspan=2>Yield Point.</th> <th colspan=2>Ultimate tensile strength</th> <th rowspan=2>Per cent elong. 50.8mm or 2 in.</th> <th rowspan=2>Per cent reduct. area.</th> </tr> <tr> <th>kg/mm<sup>2</sup></th> <th>lb/in<sup>2</sup></th> </tr> </thead> <tbody> <tr> <td>Hard</td> <td>0.45 ultimate</td> <td>56.2</td> <td>80,000</td> <td>15</td> <td>20</td> </tr> <tr> <td>Medium</td> <td>0.45 ultimate</td> <td>49.2</td> <td>70,000</td> <td>18</td> <td>25</td> </tr> <tr> <td>Soft</td> <td>0.45 ultimate</td> <td>42.2</td> <td>60,000</td> <td>22</td> <td>30</td> </tr> </tbody> </table>
这个表格看起来可能是这样：
| Grade. | Yield Point. | Ultimate tensile strength | Per cent elong. 50.8 mm or 2 in. | Per cent reduct. area. | |
|---|---|---|---|---|---|
| kg/mm2 | lb/in2 | ||||
| Hard | 0.45 ultimate | 56.2 | 80,000 | 15 | 20 |
| Medium | 0.45 ultimate | 49.2 | 70,000 | 18 | 25 |
| Soft | 0.45 ultimate | 42.2 | 60,000 | 22 | 30 |
以下显示的是 Apple 公司 2008 财年 10-K 文件的第 46 页的毛利润表的标记：
<table> <thead> <tr> <th> <th>2008 <th>2007 <th>2006 <tbody> <tr> <th>Net sales <td>$ 32,479 <td>$ 24,006 <td>$ 19,315 <tr> <th>Cost of sales <td> 21,334 <td> 15,852 <td> 13,717 <tbody> <tr> <th>Gross margin <td>$ 11,145 <td>$ 8,154 <td>$ 5,598 <tfoot> <tr> <th>Gross margin percentage <td>34.3% <td>34.0% <td>29.0% </table>
这个表格看起来可能是这样：
| 2008 | 2007 | 2006 | |
|---|---|---|---|
| Net sales | $ 32,479 | $ 24,006 | $ 19,315 |
| Cost of sales | 21,334 | 15,852 | 13,717 |
| Gross margin | $ 11,145 | $ 8,154 | $ 5,598 |
| Gross margin percentage | 34.3% | 34.0% | 29.0% |
下面是该文档中同一页下方的经营费用表的标记：
<table>
<colgroup> <col>
<colgroup> <col> <col> <col>
<thead>
<tr> <th> <th>2008 <th>2007 <th>2006
<tbody>
<tr> <th scope=rowgroup> Research and development
<td> $ 1,109 <td> $ 782 <td> $ 712
<tr> <th scope=row> Percentage of net sales
<td> 3.4% <td> 3.3% <td> 3.7%
<tbody>
<tr> <th scope=rowgroup> Selling, general, and administrative
<td> $ 3,761 <td> $ 2,963 <td> $ 2,433
<tr> <th scope=row> Percentage of net sales
<td> 11.6% <td> 12.3% <td> 12.6%
</table>
这个表格看起来可能是这样：
| 2008 | 2007 | 2006 | |
|---|---|---|---|
| Research and development | $ 1,109 | $ 782 | $ 712 |
| Percentage of net sales | 3.4% | 3.3% | 3.7% |
| Selling, general, and administrative | $ 3,761 | $ 2,963 | $ 2,433 |
| Percentage of net sales | 11.6% | 12.3% | 12.6% |
Support in all current engines.
This section is non-normative.
表单是一种网页组件，可以包含表单控件如文本，按钮，复选框，范围或颜色选择器控件。 用户可以与这样的表单进行交互并提供数据，随后这些数据被发送到服务器用于进一步处理 （例如返回搜索或计算结果）。在许多情况下都不需要客户端脚本，尽管提供了这样的 API， 脚本可以用这些 API 增强用户体验，或者将表单用于提交数据之外的用途。
编写一个表单包含几个步骤，可以按任意顺序进行： 编写用户界面，实现服务器端处理以及配置用户界面以与服务器进行通信。
This section is non-normative.
为了这个简短的介绍，我们将创建一个 Pizza 订购表单。
表单都以 form 元素开始，其中可以放置一些控件。
多数控件都由 input 元素表示，它默认提供一个文本控件。
使用 label 元素来描述控件；标签文本和控件自己都会进入
label 元素内。表单的每一部分都是一个
段落，一般会用 p 元素来与其他部分分隔。
放到一起，可以这样询问顾客的名字：
<form> <p><label>Customer name: <input></label></p> </form>
为了让用户选择 Pizza 的大小，我们可以使用一组单选按钮。
单选按钮也是用 input 元素，这次设置一个 type 属性，值为 radio。
为了让单选按钮成组，要使用 name 属性给它们一样的名字。
为了让一批控件成为一组，比如这个例子中的单选按钮，可以使用
fieldset 元素。这样的控件组的标题由 fieldset 中的第一个元素给出，
这个元素必须是一个 legend 元素。
<form> <p><label>Customer name: <input></label></p> <fieldset> <legend> Pizza Size </legend> <p><label> <input type=radio name=size> Small </label></p> <p><label> <input type=radio name=size> Medium </label></p> <p><label> <input type=radio name=size> Large </label></p> </fieldset> </form>
我们高亮了上一个步骤中的改变。
我们可以使用复选框来选择配料。这些复选框也使用 input 元素，
它们的 type 属性值为 checkbox：
<form> <p><label>Customer name: <input></label></p> <fieldset> <legend> Pizza Size </legend> <p><label> <input type=radio name=size> Small </label></p> <p><label> <input type=radio name=size> Medium </label></p> <p><label> <input type=radio name=size> Large </label></p> </fieldset> <fieldset> <legend> Pizza Toppings </legend> <p><label> <input type=checkbox> Bacon </label></p> <p><label> <input type=checkbox> Extra Cheese </label></p> <p><label> <input type=checkbox> Onion </label></p> <p><label> <input type=checkbox> Mushroom </label></p> </fieldset> </form>
这个 pizza 店总会犯一些错误，因此需要顾客的联系方式。
为此我们可以使用电话号码的表单控件（type 属性设为
tel 的 input 元素），
以及 e-mail 地址（type 属性设为
email 的 input 元素）：
<form> <p><label>Customer name: <input></label></p> <p><label>Telephone: <input type=tel></label></p> <p><label>E-mail address: <input type=email></label></p> <fieldset> <legend> Pizza Size </legend> <p><label> <input type=radio name=size> Small </label></p> <p><label> <input type=radio name=size> Medium </label></p> <p><label> <input type=radio name=size> Large </label></p> </fieldset> <fieldset> <legend> Pizza Toppings </legend> <p><label> <input type=checkbox> Bacon </label></p> <p><label> <input type=checkbox> Extra Cheese </label></p> <p><label> <input type=checkbox> Onion </label></p> <p><label> <input type=checkbox> Mushroom </label></p> </fieldset> </form>
我们可以使用 type 属性设为
time 的 input 元素来询问配送时间。
很多控件都可以设置可选值的范围；这个例子中我们关心的属性是
min, max 和
step。这些属性分别用来设置最小时间，最大时间，
以及允许的值之间的间隔（单位为秒）。这个 pizza 店只在 11am 到 9pm 之间配送，
而且不保证精确到 15 分钟以内，我们可以这样写：
<form> <p><label>Customer name: <input></label></p> <p><label>Telephone: <input type=tel></label></p> <p><label>E-mail address: <input type=email></label></p> <fieldset> <legend> Pizza Size </legend> <p><label> <input type=radio name=size> Small </label></p> <p><label> <input type=radio name=size> Medium </label></p> <p><label> <input type=radio name=size> Large </label></p> </fieldset> <fieldset> <legend> Pizza Toppings </legend> <p><label> <input type=checkbox> Bacon </label></p> <p><label> <input type=checkbox> Extra Cheese </label></p> <p><label> <input type=checkbox> Onion </label></p> <p><label> <input type=checkbox> Mushroom </label></p> </fieldset> <p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900"></label></p> </form>
textarea 元素可以用来提供多行文本。例如我们要用它来给顾客提供一个地方，编写配送备注：
<form> <p><label>Customer name: <input></label></p> <p><label>Telephone: <input type=tel></label></p> <p><label>E-mail address: <input type=email></label></p> <fieldset> <legend> Pizza Size </legend> <p><label> <input type=radio name=size> Small </label></p> <p><label> <input type=radio name=size> Medium </label></p> <p><label> <input type=radio name=size> Large </label></p> </fieldset> <fieldset> <legend> Pizza Toppings </legend> <p><label> <input type=checkbox> Bacon </label></p> <p><label> <input type=checkbox> Extra Cheese </label></p> <p><label> <input type=checkbox> Onion </label></p> <p><label> <input type=checkbox> Mushroom </label></p> </fieldset> <p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900"></label></p> <p><label>Delivery instructions: <textarea></textarea></label></p> </form>
最后，为了让表单可以提交我们要用一个 button 元素：
<form> <p><label>Customer name: <input></label></p> <p><label>Telephone: <input type=tel></label></p> <p><label>E-mail address: <input type=email></label></p> <fieldset> <legend> Pizza Size </legend> <p><label> <input type=radio name=size> Small </label></p> <p><label> <input type=radio name=size> Medium </label></p> <p><label> <input type=radio name=size> Large </label></p> </fieldset> <fieldset> <legend> Pizza Toppings </legend> <p><label> <input type=checkbox> Bacon </label></p> <p><label> <input type=checkbox> Extra Cheese </label></p> <p><label> <input type=checkbox> Onion </label></p> <p><label> <input type=checkbox> Mushroom </label></p> </fieldset> <p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900"></label></p> <p><label>Delivery instructions: <textarea></textarea></label></p> <p><button>Submit order</button></p> </form>
This section is non-normative.
编写服务器端处理程序的具体的细节超出了本规范的范围。
这个概述中我们会假设
https://pizza.example.com/order.cgi 处的脚本已经配置为接受
application/x-www-form-urlencoded 格式的表单提交，
而且 HTTP POST 消息体中会包含以下参数：
custnamecusttelcustemailsizesmall, medium
或 largetoppingbacon, cheese, onion
和 mushroomdeliverycommentsThis section is non-normative.
表单提交以多种方式暴露给服务器，通常是 HTTP GET 或 POST 请求。
在 form 元素上使用 method 属性来指定使用哪种方法。
这还没有指定表单数据的编码方式，指定编码方式要使用 enctype
属性。你还需要使用 action 属性来指定
处理提交的数据的服务的 URL。
每个需要提交的表单控件都要有一个名字来对应提交的数据。
我们已经为单选按钮组指定了名字；这个属性（name）也指定了提交的名字。
单选按钮可以之间可以通过（使用 value 属性）
给它们不同的值来区分。
多个控件可以有同样的名字；例如我们给所有复选框一样的名字，服务器检查那个名字的哪些值提交了上来。
— 像单选按钮一样，也通过 value 属性给它们不重复的值。
鉴于上一小节的设置，表单现在变成了：
<form method="post"
enctype="application/x-www-form-urlencoded"
action="https://pizza.example.com/order.cgi">
<p><label>Customer name: <input name="custname"></label></p>
<p><label>Telephone: <input type=tel name="custtel"></label></p>
<p><label>E-mail address: <input type=email name="custemail"></label></p>
<fieldset>
<legend> Pizza Size </legend>
<p><label> <input type=radio name=size value="small"> Small </label></p>
<p><label> <input type=radio name=size value="medium"> Medium </label></p>
<p><label> <input type=radio name=size value="large"> Large </label></p>
</fieldset>
<fieldset>
<legend> Pizza Toppings </legend>
<p><label> <input type=checkbox name="topping" value="bacon"> Bacon </label></p>
<p><label> <input type=checkbox name="topping" value="cheese"> Extra Cheese </label></p>
<p><label> <input type=checkbox name="topping" value="onion"> Onion </label></p>
<p><label> <input type=checkbox name="topping" value="mushroom"> Mushroom </label></p>
</fieldset>
<p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900" name="delivery"></label></p>
<p><label>Delivery instructions: <textarea name="comments"></textarea></label></p>
<p><button>Submit order</button></p>
</form>
有些属性的值用引号包含，有些没有，但这没有特别的含义。 HTML 语法允许很多种合法的方式来指定属性，如 语法部分 所述。
例如，如果顾客输入 "Denise Lawrence" 作为名字，"555-321-8642" 作为电话号，没有指定 e-mail 地址， 要一个中等大小的 Pizza，选择了 Extra Cheese 和 Mushroom 配料， 输入的配送时间是 7pm 并在配送备注部分留空，用户代理会提交下面内容到在线 Web 服务：
custname=Denise+Lawrence&custtel=555-321-8642&custemail=&size=medium&topping=cheese&topping=mushroom&delivery=19%3A00&comments=
Support in all current engines.
This section is non-normative.
可以通过表单标记使得用户代理可以在表单提交之前检查用户输入。 服务器仍然必须校验输入是有效的（因为恶意用户可以很容易绕过表单验证）， 但这使我们可以避免因为只能在服务器上检查而造成用户等待。
最简单的标记是 required 属性，
可以在 input 元素上指定，表示只有给出值之后表单才可以提交。
把这个属性添加到顾客名字、Pizza 大小和配送时间字段，在用户没填写这些字段就提交表单时，
我们可以让用户代理提示用户。
<form method="post"
enctype="application/x-www-form-urlencoded"
action="https://pizza.example.com/order.cgi">
<p><label>Customer name: <input name="custname" required></label></p>
<p><label>Telephone: <input type=tel name="custtel"></label></p>
<p><label>E-mail address: <input type=email name="custemail"></label></p>
<fieldset>
<legend> Pizza Size </legend>
<p><label> <input type=radio name=size required value="small"> Small </label></p>
<p><label> <input type=radio name=size required value="medium"> Medium </label></p>
<p><label> <input type=radio name=size required value="large"> Large </label></p>
</fieldset>
<fieldset>
<legend> Pizza Toppings </legend>
<p><label> <input type=checkbox name="topping" value="bacon"> Bacon </label></p>
<p><label> <input type=checkbox name="topping" value="cheese"> Extra Cheese </label></p>
<p><label> <input type=checkbox name="topping" value="onion"> Onion </label></p>
<p><label> <input type=checkbox name="topping" value="mushroom"> Mushroom </label></p>
</fieldset>
<p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900" name="delivery" required></label></p>
<p><label>Delivery instructions: <textarea name="comments"></textarea></label></p>
<p><button>Submit order</button></p>
</form>
也可以使用 maxlength 属性来限制输入的长度。
把这个属性添加到 textarea 元素上，我们可以限制用户输入到 1000 字符，
防止他们给配送员编写长文而不聚焦重点。
<form method="post"
enctype="application/x-www-form-urlencoded"
action="https://pizza.example.com/order.cgi">
<p><label>Customer name: <input name="custname" required></label></p>
<p><label>Telephone: <input type=tel name="custtel"></label></p>
<p><label>E-mail address: <input type=email name="custemail"></label></p>
<fieldset>
<legend> Pizza Size </legend>
<p><label> <input type=radio name=size required value="small"> Small </label></p>
<p><label> <input type=radio name=size required value="medium"> Medium </label></p>
<p><label> <input type=radio name=size required value="large"> Large </label></p>
</fieldset>
<fieldset>
<legend> Pizza Toppings </legend>
<p><label> <input type=checkbox name="topping" value="bacon"> Bacon </label></p>
<p><label> <input type=checkbox name="topping" value="cheese"> Extra Cheese </label></p>
<p><label> <input type=checkbox name="topping" value="onion"> Onion </label></p>
<p><label> <input type=checkbox name="topping" value="mushroom"> Mushroom </label></p>
</fieldset>
<p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900" name="delivery" required></label></p>
<p><label>Delivery instructions: <textarea name="comments" maxlength=1000></textarea></label></p>
<p><button>Submit order</button></p>
</form>
提交表单时，每个不合法的表单控件上，以及 form 元素上都会触发
invalid 事件，
这个事件可以用来显示表单存在哪些问题的一个描述，因为浏览器自己只会逐个报告问题。
This section is non-normative.
有些浏览器尝试帮助用户自动填充表单控件，避免让用户每次重新输入。例如询问用户电话号码的字段就可以自动填充为用户的手机号码。
为了帮助用户代理做自动填充，可以使用 autocomplete
属性来描述字段的用途。下面这个表单的例子，Pizza 配送的用户信息中三个字段都可以这样自动填充：
添加相应的信息后看起来是这样的：
<form method="post"
enctype="application/x-www-form-urlencoded"
action="https://pizza.example.com/order.cgi">
<p><label>Customer name: <input name="custname" required autocomplete="shipping name"></label></p>
<p><label>Telephone: <input type=tel name="custtel" autocomplete="shipping tel"></label></p>
<p><label>E-mail address: <input type=email name="custemail" autocomplete="shipping email"></label></p>
<fieldset>
<legend> Pizza Size </legend>
<p><label> <input type=radio name=size required value="small"> Small </label></p>
<p><label> <input type=radio name=size required value="medium"> Medium </label></p>
<p><label> <input type=radio name=size required value="large"> Large </label></p>
</fieldset>
<fieldset>
<legend> Pizza Toppings </legend>
<p><label> <input type=checkbox name="topping" value="bacon"> Bacon </label></p>
<p><label> <input type=checkbox name="topping" value="cheese"> Extra Cheese </label></p>
<p><label> <input type=checkbox name="topping" value="onion"> Onion </label></p>
<p><label> <input type=checkbox name="topping" value="mushroom"> Mushroom </label></p>
</fieldset>
<p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900" name="delivery" required></label></p>
<p><label>Delivery instructions: <textarea name="comments" maxlength=1000></textarea></label></p>
<p><button>Submit order</button></p>
</form>
This section is non-normative.
Some devices, in particular those with virtual keyboards can provide the user with multiple input modalities. For example, when typing in a credit card number the user may wish to only see keys for digits 0-9, while when typing in their name they may wish to see a form field that by default capitalizes each word.
Using the inputmode attribute we can select appropriate
input modalities:
<form method="post"
enctype="application/x-www-form-urlencoded"
action="https://pizza.example.com/order.cgi">
<p><label>Customer name: <input name="custname" required autocomplete="shipping name"></label></p>
<p><label>Telephone: <input type=tel name="custtel" autocomplete="shipping tel"></label></p>
<p><label>Buzzer code: <input name="custbuzz" inputmode="numeric"></label></p>
<p><label>Email address: <input type=email name="custemail" autocomplete="shipping email"></label></p>
<fieldset>
<legend> Pizza Size </legend>
<p><label> <input type=radio name=size required value="small"> Small </label></p>
<p><label> <input type=radio name=size required value="medium"> Medium </label></p>
<p><label> <input type=radio name=size required value="large"> Large </label></p>
</fieldset>
<fieldset>
<legend> Pizza Toppings </legend>
<p><label> <input type=checkbox name="topping" value="bacon"> Bacon </label></p>
<p><label> <input type=checkbox name="topping" value="cheese"> Extra Cheese </label></p>
<p><label> <input type=checkbox name="topping" value="onion"> Onion </label></p>
<p><label> <input type=checkbox name="topping" value="mushroom"> Mushroom </label></p>
</fieldset>
<p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900" name="delivery" required></label></p>
<p><label>Delivery instructions: <textarea name="comments" maxlength=1000></textarea></label></p>
<p><button>Submit order</button></p>
</form>This section is non-normative.
The type, autocomplete, and inputmode attributes can seem confusingly similar. For instance,
in all three cases, the string "email" is a valid value. This section
attempts to illustrate the difference between the three attributes and provides advice suggesting
how to use them.
The type attribute on input elements decides
what kind of control the user agent will use to expose the field. Choosing between different
values of this attribute is the same choice as choosing whether to use an input
element, a textarea element, a select element, etc.
The autocomplete attribute, in contrast, describes
what the value that the user will enter actually represents. Choosing between different values of
this attribute is the same choice as choosing what the label for the element will be.
First, consider telephone numbers. If a page is asking for a telephone number from the user,
the right form control to use is <input type=tel>.
However, which autocomplete value to use depends on
which phone number the page is asking for, whether they expect a telephone number in the
international format or just the local format, and so forth.
For example, a page that forms part of a checkout process on an e-commerce site for a customer buying a gift to be shipped to a friend might need both the buyer's telephone number (in case of payment issues) and the friend's telephone number (in case of delivery issues). If the site expects international phone numbers (with the country code prefix), this could thus look like this:
<p><label>Your phone number: <input type=tel name=custtel autocomplete="billing tel"></label>
<p><label>Recipient's phone number: <input type=tel name=shiptel autocomplete="shipping tel"></label>
<p>Please enter complete phone numbers including the country code prefix, as in "+1 555 123 4567".But if the site only supports British customers and recipients, it might instead look like this
(notice the use of tel-national rather than
tel):
<p><label>Your phone number: <input type=tel name=custtel autocomplete="billing tel-national"></label>
<p><label>Recipient's phone number: <input type=tel name=shiptel autocomplete="shipping tel-national"></label>
<p>Please enter complete UK phone numbers, as in "(01632) 960 123".Now, consider a person's preferred languages. The right autocomplete value is language. However, there could be a number of
different form controls used for the purpose: a text control (<input type=text>), a drop-down list (<select>), radio buttons (<input
type=radio>), etc. It only depends on what kind of interface is desired.
Finally, consider names. If a page just wants one name from the user, then the relevant control
is <input type=text>. If the page is asking for the
user's full name, then the relevant autocomplete value
is name.
<p><label>Japanese name: <input name="j" type="text" autocomplete="section-jp name"></label>
<label>Romanized name: <input name="e" type="text" autocomplete="section-en name"></label>In this example, the "section-*" keywords in
the autocomplete attributes' values tell the user agent
that the two fields expect different names. Without them, the user agent could
automatically fill the second field with the value given in the first field when the user gave a
value to the first field.
The "-jp" and "-en" parts of the
keywords are opaque to the user agent; the user agent cannot guess, from those, that the two names
are expected to be in Japanese and English respectively.
Separate from the choices regarding type and autocomplete, the inputmode attribute decides what kind of input modality (e.g.,
virtual keyboard) to use, when the control is a text control.
Consider credit card numbers. The appropriate input type is not <input type=number>, as explained below; it is instead <input type=text>. To encourage the user agent to use a
numeric input modality anyway (e.g., a virtual keyboard displaying only digits), the page would
use
<p><label>Credit card number:
<input name="cc" type="text" inputmode="numeric" pattern="[0-9]{8,19}" autocomplete="cc-number">
</label></p>This section is non-normative.
这个 Pizza 配送的例子中，时间的格式为 "HH:MM": 两位数字的小时（24小时格式），以及两位数字的时间。 （还可以指定秒，虽然这个例子中没有必要）
但是在某些时区里展示给用户的时间是不同的。例如在美国仍然经常使用 12 小时的表加一个 am/pm 指示， 比如 "2pm"。在法国通常使用一个 "h" 字符来分隔小时和分钟，例如 "14h00"。
日期也存在类似的问题，更严重的是日期各部分的顺序也总是不一致的。 — 例如在 Cyprus 2003 年 2 月 1 日会写成 "1/2/03"， 同样的日期在日本会写成 "2003年02月01日" — 数字也是一样， 在不同的时区小数点和千分点使用的标点也不一样。
因此把 HTML 和表单提交中的时间、日期和数字的格式（即本规范中定义的格式，以及 ISO 8601 标准建立的对计算机可读的时间和日期格式），和 浏览器显示给用户的或浏览器接受用户输入的时间、日期和数字格式区分开来非常重要。
协议中（即 HTML 标记和表单提交）使用的格式是为了使得计算机可读和一致，且与用户的区域设置无关。 例如日期总是表示为 "YYYY-MM-DD"，比如 "2003-02-01"。用户永远不应该看到这个格式。
页面给出的时间、日期或数字的按照协议的格式会在显示给用户之前翻译成用户偏好的显示方式 （基于用户偏好或页面自己所在的区域设置）。类似地，在用户使用他们偏好的格式输入时间、日期或数字时， 用户代理把它放回 DOM 或提交之前，要把它转换回协议格式。
这使得页面脚本和服务器可以使用一致的方式来处理时间、日期和数字，不需要支持几十种不同的格式， 同时仍然能够支持用户的需要。
关于表单控件的本地化请参考 实现纪要。
Mostly for historical reasons, elements in this section fall into several overlapping (but subtly different) categories in addition to the usual ones like flow content, phrasing content, and interactive content.
A number of the elements are form-associated elements, which means they can have a form owner.
The form-associated elements fall into several subcategories:
Denotes elements that are listed in the form.elements and fieldset.elements APIs. These elements also
have a form content attribute, and a matching form IDL attribute, that allow authors to specify an explicit
form owner.
Denotes elements that can be used for constructing the entry list when a
form element is submitted.
Some submittable elements can be, depending on their attributes, buttons. The prose below defines when an element is a button. Some buttons are specifically submit buttons.
Denotes elements that can be affected when a form element is reset.
Denotes elements that inherit the autocapitalize
attribute from their form owner.
Some elements, not all of them form-associated,
are categorized as labelable elements. These are elements that
can be associated with a label element.
buttoninput (if the type attribute is not in the state)meteroutputprogressselecttextareaform elementSupport in all current engines.
Support in all current engines.
form element descendants.accept-charset — Character encodings to use for form submissionaction — URL to use for form submissionautocomplete — Default setting for autofill feature for controls in the formenctype — Entry list encoding type to use for form submissionmethod — Variant to use for form submissionname — Name of form to use in the document.forms APInovalidate — Bypass form control validation for form submissiontarget — Browsing context for form submissionrel[Exposed=Window,
LegacyOverrideBuiltIns,
LegacyUnenumerableNamedProperties]
interface HTMLFormElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString acceptCharset;
[CEReactions] attribute USVString action;
[CEReactions] attribute DOMString autocomplete;
[CEReactions] attribute DOMString enctype;
[CEReactions] attribute DOMString encoding;
[CEReactions] attribute DOMString method;
[CEReactions] attribute DOMString name;
[CEReactions] attribute boolean noValidate;
[CEReactions] attribute DOMString target;
[CEReactions] attribute DOMString rel;
[SameObject, PutForwards=value] readonly attribute DOMTokenList relList;
[SameObject] readonly attribute HTMLFormControlsCollection elements;
readonly attribute unsigned long length;
getter Element (unsigned long index);
getter (RadioNodeList or Element) (DOMString name);
undefined submit();
undefined requestSubmit(optional HTMLElement? submitter = null);
[CEReactions] undefined reset();
boolean checkValidity();
boolean reportValidity();
};The form element represents a hyperlink that can be
manipulated through a collection of form-associated
elements, some of which can represent editable values that can be submitted to a server for
processing.
The accept-charset attribute gives the character
encodings that are to be used for the submission. If specified, the value must be an ASCII
case-insensitive match for "UTF-8". [ENCODING]
The name attribute
represents the form's name within the forms
collection. The value must not be the empty string, and the value must be unique amongst the
form elements in the forms collection that
it is in, if any.
The autocomplete attribute is an enumerated
attribute. The attribute has two states. The on keyword maps to the on state, and the off keyword maps to the off state. The attribute may also be omitted. The
missing value default and the invalid value default are the on state. The off state indicates that by default, form
controls in the form will have their autofill field name set to "off"; the on state indicates that by default, form controls
in the form will have their autofill field name set to "on".
The action, enctype,
method, novalidate,
and target attributes are attributes for form
submission.
The rel attribute on
form elements controls what kinds of links the elements create. The attribute's value
must be a unordered set of unique space-separated tokens. The allowed keywords and their meanings are defined in an earlier section.
rel's supported
tokens are the keywords defined in HTML link types which are
allowed on form elements, impact the processing model, and are supported by the user
agent. The possible supported tokens are noreferrer, noopener, and opener. rel's supported tokens must only include the tokens from this
list that the user agent implements the processing model for.
elementsSupport in all current engines.
Returns an HTMLFormControlsCollection of the form controls in the form (excluding image
buttons for historical reasons).
lengthSupport in all current engines.
Returns the number of form controls in the form (excluding image buttons for historical reasons).
Returns the indexth element in the form (excluding image buttons for historical reasons).
Returns the form control (or, if there are several, a RadioNodeList of the form
controls) in the form with the given ID or name (excluding image buttons for historical reasons); or, if there
are none, returns the img element with the given ID.
Once an element has been referenced using a particular name, that name will continue being
available as a way to reference that element in this method, even if the element's actual ID or name changes, for as long as
the element remains in the tree.
If there are multiple matching items, then a RadioNodeList object containing all
those elements is returned.
submit()Support in all current engines.
Submits the form, bypassing interactive
constraint validation and without firing a submit
event.
requestSubmit( [ submitter ] )Requests to submit the form. Unlike submit(), this
method includes interactive constraint
validation and firing a submit event, either of which
can cancel submission.
The submitter argument can be used to point to a specific submit button, whose formaction, formenctype, formmethod, formnovalidate, and formtarget attributes can impact submission. Additionally,
the submitter will be included when constructing the entry list for submission;
normally, buttons are excluded.
reset()Support in all current engines.
Resets the form.
checkValidity()Returns true if the form's controls are all valid; otherwise, returns false.
reportValidity()Returns true if the form's controls are all valid; otherwise, returns false and informs the user.
The autocomplete IDL attribute must reflect
the content attribute of the same name, limited to only known values.
Support in all current engines.
The name and
rel IDL attributes must reflect the
content attribute of the same name.
Support in all current engines.
The acceptCharset IDL attribute must
reflect the accept-charset content
attribute.
The relList IDL attribute must
reflect the rel content attribute.
The elements
IDL attribute must return an HTMLFormControlsCollection rooted at the
form element's root, whose filter matches listed elements whose form owner is the
form element, with the exception of input elements whose type attribute is in the Image Button state, which must, for historical reasons, be
excluded from this particular collection.
The length IDL
attribute must return the number of nodes represented by the elements
collection.
The supported property indices at any instant are the indices supported by the
object returned by the elements attribute at that
instant.
To determine the value of an indexed property for a
form element, the user agent must return the value returned by the item method on the elements collection, when invoked with the given index as its
argument.
Each form element has a mapping of names to elements called the past names
map. It is used to persist names of controls even when they change names.
The supported property names consist of the names obtained from the following algorithm, in the order obtained from this algorithm:
Let sourced names be an initially empty ordered list of tuples consisting of a string, an element, a source, where the source is either id, name, or past, and, if the source is past, an age.
For each listed element candidate
whose form owner is the form element, with the exception of any
input elements whose type attribute is in the
Image Button state:
If candidate has an id attribute, add
an entry to sourced names with that id
attribute's value as the string, candidate as the element, and id as
the source.
If candidate has a name attribute,
add an entry to sourced names with that name attribute's value as the string, candidate
as the element, and name as the source.
For each img element candidate whose form owner is the
form element:
If candidate has an id attribute, add
an entry to sourced names with that id
attribute's value as the string, candidate as the element, and id as
the source.
If candidate has a name attribute,
add an entry to sourced names with that name attribute's value as the string, candidate
as the element, and name as the source.
For each entry past entry in the past names map add an entry to sourced names with the past entry's name as the string, past entry's element as the element, past as the source, and the length of time past entry has been in the past names map as the age.
Sort sourced names by tree order of the element entry of each tuple, sorting entries with the same element by putting entries whose source is id first, then entries whose source is name, and finally entries whose source is past, and sorting entries with the same element and source by their age, oldest first.
Remove any entries in sourced names that have the empty string as their name.
Remove any entries in sourced names that have the same name as an earlier entry in the map.
Return the list of names from sourced names, maintaining their relative order.
To determine the value of a named property name
for a form element, the user agent must run the following steps:
Let candidates be a live RadioNodeList object
containing all the listed elements, whose form
owner is the form element, that have either an id attribute or a name attribute equal
to name, with the exception of input elements whose type attribute is in the Image Button state, in tree order.
If candidates is empty, let candidates be a live
RadioNodeList object containing all the img elements, whose form
owner is the form element, that have either an id attribute or a name attribute
equal to name, in tree order.
If candidates is empty, name is the name of one of
the entries in the form element's past names map: return the object
associated with name in that map.
If candidates contains more than one node, return candidates.
Otherwise, candidates contains exactly one node. Add a mapping from
name to the node in candidates in the form
element's past names map, replacing the previous entry with the same name, if
any.
Return the node in candidates.
If an element listed in a form element's past names map changes
form owner, then its entries must be removed from that map.
The submit()
method, when invoked, must submit the form
element from the form element itself, with the submitted from submit() method flag set.
The requestSubmit(submitter)
method, when invoked, must run the following steps:
If submitter is not null, then:
If submitter is not a submit
button, then throw a TypeError.
If submitter's form owner is not this form element,
then throw a "NotFoundError" DOMException.
Otherwise, set submitter to this form element.
The reset()
method, when invoked, must run the following steps:
If the form element is marked as locked for reset, then return.
Mark the form element as locked for reset.
Unmark the form element as locked for reset.
If the checkValidity() method is
invoked, the user agent must statically validate the constraints of the
form element, and return true if the constraint validation return a positive
result, and false if it returned a negative result.
If the reportValidity() method is
invoked, the user agent must interactively validate the constraints of the
form element, and return true if the constraint validation return a positive
result, and false if it returned a negative result.
This example shows two search forms:
<form action="https://www.google.com/search" method="get">
<label>Google: <input type="search" name="q"></label> <input type="submit" value="Search...">
</form>
<form action="https://www.bing.com/search" method="get">
<label>Bing: <input type="search" name="q"></label> <input type="submit" value="Search...">
</form>label elementSupport in all current engines.
Support in all current engines.
label elements.for — Associate the label with form control[Exposed=Window]
interface HTMLLabelElement : HTMLElement {
[HTMLConstructor] constructor();
readonly attribute HTMLFormElement? form;
[CEReactions] attribute DOMString htmlFor;
readonly attribute HTMLElement? control;
};The label element represents a caption in a user interface. The
caption can be associated with a specific form control, known as the
label element's labeled control, either using the for attribute, or by putting the form control inside the
label element itself.
Except where otherwise specified by the following rules, a label element has no
labeled control.
The for attribute may
be specified to indicate a form control with which the caption is to be associated. If the
attribute is specified, the attribute's value must be the ID of a
labelable element in the same tree as the
label element. If the attribute is specified and there is an element in
the tree whose ID is equal to the value of the for attribute, and the first such element in tree
order is a labelable element, then that element is the
label element's labeled control.
If the for attribute is not specified, but the
label element has a labelable element descendant,
then the first such descendant in tree order is the label element's
labeled control.
The label element's exact default presentation and behavior, in particular what
its activation behavior might be, if anything, should match the platform's label
behavior. The activation behavior of a label element for events targeted
at interactive content descendants of a label element, and any
descendants of those interactive content descendants, must be to do nothing.
Form-associated custom
elements are labelable elements, so for user agents
where the label element's activation behavior impacts the labeled
control, both built-in and custom elements will be impacted.
For example, on platforms where clicking a label activates the form control, clicking the
label in the following snippet could trigger the user agent to fire a click event at the input element, as if the
element itself had been triggered by the user:
<label><input type=checkbox name=lost> Lost</label>Similarly, assuming my-checkbox was declared as as a
form-associated custom element (like in this
example), then the code
<label><my-checkbox name=lost></my-checkbox> Lost</label>would have the same behavior, firing a click event at the my-checkbox
element.
On other platforms, the behavior in both cases might be just to focus the control, or to do nothing.
The following example shows three form controls each with a label, two of which have small text showing the right format for users to use.
<p><label>Full name: <input name=fn> <small>Format: First Last</small></label></p>
<p><label>Age: <input name=age type=number min=0></label></p>
<p><label>Post code: <input name=pc> <small>Format: AB12 3CD</small></label></p>controlSupport in all current engines.
Returns the form control that is associated with this element.
formSupport in all current engines.
Returns the form owner of the form control that is associated with this element.
Returns null if there isn't one.
Support in all current engines.
The htmlFor IDL attribute must
reflect the for content attribute.
The control
IDL attribute must return the label element's labeled control, if any,
or null if there isn't one.
The form IDL
attribute must run the following steps:
If the label element has no labeled control, then return
null.
If the label element's labeled control is not a
form-associated element, then return null.
Return the label element's labeled control's form
owner (which can still be null).
The form IDL attribute on the
label element is different from the form IDL
attribute on listed form-associated elements, and the label element does not have a form content attribute.
labelsSupport in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Returns a NodeList of all the label elements that the form control
is associated with.
Labelable elements and all input elements
have a live NodeList object associated with them that represents the
list of label elements, in tree order, whose labeled
control is the element in question. The labels IDL attribute of labelable elements that are not form-associated custom elements, and the labels IDL attribute of input elements, on getting,
must return that NodeList object, and that same value must always be returned, unless
this element is an input element whose type
attribute is in the state, in which case it
must instead return null.
Form-associated custom elements don't have
a labels IDL attribute. Instead, their
ElementInternals object has a labels IDL attribute. On getting, it must throw
a "NotSupportedError" DOMException if the target element is not a form-associated custom
element. Otherwise, it must return that NodeList object, and that same value
must always be returned.
This (non-conforming) example shows what happens to the NodeList and what labels returns when an input element has its type attribute changed.
<!doctype html>
<p><label><input></label></p>
<script>
const input = document.querySelector('input');
const labels = input.labels;
console.assert(labels.length === 1);
input.type = 'hidden';
console.assert(labels.length === 0); // the input is no longer the label's labeled control
console.assert(input.labels === null);
input.type = 'checkbox';
console.assert(labels.length === 1); // the input is once again the label's labeled control
console.assert(input.labels === labels); // same value as returned originally
</script>input elementSupport in all current engines.
Support in all current engines.
type attribute is not in the Hidden state: Interactive content.type attribute is not in the Hidden state: Listed, labelable, submittable, resettable, and autocapitalize-inheriting form-associated element.type attribute is in the Hidden state: Listed, submittable, resettable, and autocapitalize-inheriting form-associated element.type attribute is not in the Hidden state: Palpable content.accept — Hint for expected file type in file upload controlsalt — Replacement text for use when images are not availableautocomplete — Hint for form autofill featurechecked — Whether the control is checkeddirname — Name of form control to use for sending the element's directionality in form submissiondisabled — Whether the form control is disabledform — Associates the element with a form elementformaction — URL to use for form submissionformenctype — Entry list encoding type to use for form submissionformmethod — Variant to use for form submissionformnovalidate — Bypass form control validation for form submissionformtarget — Browsing context for form submissionheight — Vertical dimensionlist — List of autocomplete optionsmax — Maximum valuemaxlength — Maximum length of valuemin — Minimum valueminlength — Minimum length of valuemultiple — Whether to allow multiple valuesname — Name of the element to use for form submission and in the form.elements API pattern — Pattern to be matched by the form control's valueplaceholder — User-visible label to be placed within the form controlreadonly — Whether to allow the value to be edited by the userrequired — Whether the control is required for form submissionsize — Size of the controlsrc — Address of the resourcestep — Granularity to be matched by the form control's valuetype — Type of form controlvalue — Value of the form controlwidth — Horizontal dimensiontitle attribute has special semantics on this element: Description of pattern (when used with pattern attribute).type attribute in the Hidden state: for authors; for implementers.type attribute in the Text state: for authors; for implementers.type attribute in the Search state: for authors; for implementers.type attribute in the Telephone state: for authors; for implementers.type attribute in the URL state: for authors; for implementers.type attribute in the Email state: for authors; for implementers.type attribute in the Password state: for authors; for implementers.type attribute in the Date state: for authors; for implementers.type attribute in the Month state: for authors; for implementers.type attribute in the Week state: for authors; for implementers.type attribute in the Time state: for authors; for implementers.type attribute in the Local Date and Time state: for authors; for implementers.type attribute in the Number state: for authors; for implementers.type attribute in the Range state: for authors; for implementers.type attribute in the Color state: for authors; for implementers.type attribute in the Checkbox state: for authors; for implementers.type attribute in the Radio Button state: for authors; for implementers.type attribute in the File Upload state: for authors; for implementers.type attribute in the Submit Button state: for authors; for implementers.type attribute in the Image Button state: for authors; for implementers.type attribute in the Reset Button state: for authors; for implementers.type attribute in the Button state: for authors; for implementers.[Exposed=Window]
interface HTMLInputElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString accept;
[CEReactions] attribute DOMString alt;
[CEReactions] attribute DOMString autocomplete;
[CEReactions] attribute boolean defaultChecked;
attribute boolean checked;
[CEReactions] attribute DOMString dirName;
[CEReactions] attribute boolean disabled;
readonly attribute HTMLFormElement? form;
attribute FileList? files;
[CEReactions] attribute USVString formAction;
[CEReactions] attribute DOMString formEnctype;
[CEReactions] attribute DOMString formMethod;
[CEReactions] attribute boolean formNoValidate;
[CEReactions] attribute DOMString formTarget;
[CEReactions] attribute unsigned long height;
attribute boolean indeterminate;
readonly attribute HTMLElement? list;
[CEReactions] attribute DOMString max;
[CEReactions] attribute long maxLength;
[CEReactions] attribute DOMString min;
[CEReactions] attribute long minLength;
[CEReactions] attribute boolean multiple;
[CEReactions] attribute DOMString name;
[CEReactions] attribute DOMString pattern;
[CEReactions] attribute DOMString placeholder;
[CEReactions] attribute boolean readOnly;
[CEReactions] attribute boolean required;
[CEReactions] attribute unsigned long size;
[CEReactions] attribute USVString src;
[CEReactions] attribute DOMString step;
[CEReactions] attribute DOMString type;
[CEReactions] attribute DOMString defaultValue;
[CEReactions] attribute [LegacyNullToEmptyString] DOMString value;
attribute object? valueAsDate;
attribute unrestricted double valueAsNumber;
[CEReactions] attribute unsigned long width;
undefined stepUp(optional long n = 1);
undefined stepDown(optional long n = 1);
readonly attribute boolean willValidate;
readonly attribute ValidityState validity;
readonly attribute DOMString validationMessage;
boolean checkValidity();
boolean reportValidity();
undefined setCustomValidity(DOMString error);
readonly attribute NodeList? labels;
undefined select();
attribute unsigned long? selectionStart;
attribute unsigned long? selectionEnd;
attribute DOMString? selectionDirection;
undefined setRangeText(DOMString replacement);
undefined setRangeText(DOMString replacement, unsigned long start, unsigned long end, optional SelectionMode selectionMode = "preserve");
undefined setSelectionRange(unsigned long start, unsigned long end, optional DOMString direction);
// also has obsolete members
};The input element represents a typed data field, usually with a form
control to allow the user to edit the data.
The type attribute
controls the data type (and associated control) of the element. It is an enumerated
attribute. The following table lists the keywords and states for the attribute — the
keywords in the left column map to the states in the cell in the second column on the same row as
the keyword.
| Keyword | State | Data type | Control type |
|---|---|---|---|
hidden
| An arbitrary string | n/a | |
text
| Text | Text with no line breaks | A text control |
search
| Search | Text with no line breaks | Search control |
tel
| Telephone | Text with no line breaks | A text control |
url
| URL | An absolute URL | A text control |
email
| An email address or list of email addresses | A text control | |
password
| Password | Text with no line breaks (sensitive information) | A text control that obscures data entry |
date
| Date | A date (year, month, day) with no time zone | A date control |
month
| Month | A date consisting of a year and a month with no time zone | A month control |
week
| Week | A date consisting of a week-year number and a week number with no time zone | A week control |
time
| Time | A time (hour, minute, seconds, fractional seconds) with no time zone | A time control |
datetime-local
| Local Date and Time | A date and time (year, month, day, hour, minute, second, fraction of a second) with no time zone | A date and time control |
number
| Number | A numerical value | A text control or spinner control |
range
| Range | A numerical value, with the extra semantic that the exact value is not important | A slider control or similar |
color
| Color | An sRGB color with 8-bit red, green, and blue components | A color picker |
checkbox
| Checkbox | A set of zero or more values from a predefined list | A checkbox |
radio
| Radio Button | An enumerated value | A radio button |
file
| File Upload | Zero or more files each with a MIME type and optionally a file name | A label and a button |
submit
| Submit Button | An enumerated value, with the extra semantic that it must be the last value selected and initiates form submission | A button |
image
| Image Button | A coordinate, relative to a particular image's size, with the extra semantic that it must be the last value selected and initiates form submission | Either a clickable image, or a button |
reset
| Reset Button | n/a | A button |
button
| Button | n/a | A button |
The missing value default and the invalid value default are the Text state.
Which of the
accept,
alt,
autocomplete,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
list,
max,
maxlength,
min,
minlength,
multiple,
pattern,
placeholder,
readonly,
required,
size,
src,
step, and
width content attributes, the
checked,
files,
valueAsDate,
valueAsNumber, and
list IDL attributes, the
select() method, the
selectionStart,
selectionEnd, and
selectionDirection, IDL attributes, the
setRangeText() and
setSelectionRange() methods, the
stepUp() and
stepDown() methods, and the
input and
change events apply to an
input element depends on the state of its
type attribute.
The subsections that define each type also clearly define in normative "bookkeeping" sections
which of these feature apply, and which do not apply, to each type. The behavior of
these features depends on whether they apply or not, as defined in their various sections (q.v.
for content attributes, for APIs, for events).
The following table is non-normative and summarizes which of those content attributes, IDL attributes, methods, and events apply to each state:
| Text, Search | URL, Telephone | Password | Date, Month, Week, Time | Local Date and Time | Number | Range | Color | Checkbox, Radio Button | File Upload | Submit Button | Image Button | Reset Button, Button | |||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Content attributes | |||||||||||||||
accept
| · | · | · | · | · | · | · | · | · | · | · | Yes | · | · | · |
alt
| · | · | · | · | · | · | · | · | · | · | · | · | · | Yes | · |
autocomplete
| Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | · | · | · | · | · |
checked
| · | · | · | · | · | · | · | · | · | · | Yes | · | · | · | · |
dirname
| · | Yes | · | · | · | · | · | · | · | · | · | · | · | · | · |
formaction
| · | · | · | · | · | · | · | · | · | · | · | · | Yes | Yes | · |
formenctype
| · | · | · | · | · | · | · | · | · | · | · | · | Yes | Yes | · |
formmethod
| · | · | · | · | · | · | · | · | · | · | · | · | Yes | Yes | · |
formnovalidate
| · | · | · | · | · | · | · | · | · | · | · | · | Yes | Yes | · |
formtarget
| · | · | · | · | · | · | · | · | · | · | · | · | Yes | Yes | · |
height
| · | · | · | · | · | · | · | · | · | · | · | · | · | Yes | · |
list
| · | Yes | Yes | Yes | · | Yes | Yes | Yes | Yes | Yes | · | · | · | · | · |
max
| · | · | · | · | · | Yes | Yes | Yes | Yes | · | · | · | · | · | · |
maxlength
| · | Yes | Yes | Yes | Yes | · | · | · | · | · | · | · | · | · | · |
min
| · | · | · | · | · | Yes | Yes | Yes | Yes | · | · | · | · | · | · |
minlength
| · | Yes | Yes | Yes | Yes | · | · | · | · | · | · | · | · | · | · |
multiple
| · | · | · | Yes | · | · | · | · | · | · | · | Yes | · | · | · |
pattern
| · | Yes | Yes | Yes | Yes | · | · | · | · | · | · | · | · | · | · |
placeholder
| · | Yes | Yes | Yes | Yes | · | · | Yes | · | · | · | · | · | · | · |
readonly
| · | Yes | Yes | Yes | Yes | Yes | Yes | Yes | · | · | · | · | · | · | · |
required
| · | Yes | Yes | Yes | Yes | Yes | Yes | Yes | · | · | Yes | Yes | · | · | · |
size
| · | Yes | Yes | Yes | Yes | · | · | · | · | · | · | · | · | · | · |
src
| · | · | · | · | · | · | · | · | · | · | · | · | · | Yes | · |
step
| · | · | · | · | · | Yes | Yes | Yes | Yes | · | · | · | · | · | · |
width
| · | · | · | · | · | · | · | · | · | · | · | · | · | Yes | · |
| IDL attributes and methods | |||||||||||||||
checked
| · | · | · | · | · | · | · | · | · | · | Yes | · | · | · | · |
files
| · | · | · | · | · | · | · | · | · | · | · | Yes | · | · | · |
value
| default | value | value | value | value | value | value | value | value | value | default/on | filename | default | default | default |
valueAsDate
| · | · | · | · | · | Yes | · | · | · | · | · | · | · | · | · |
valueAsNumber
| · | · | · | · | · | Yes | Yes | Yes | Yes | · | · | · | · | · | · |
list
| · | Yes | Yes | Yes | · | Yes | Yes | Yes | Yes | Yes | · | · | · | · | · |
select()
| · | Yes | Yes | Yes† | Yes | Yes† | Yes† | Yes† | · | Yes† | · | Yes† | · | · | · |
selectionStart
| · | Yes | Yes | · | Yes | · | · | · | · | · | · | · | · | · | · |
selectionEnd
| · | Yes | Yes | · | Yes | · | · | · | · | · | · | · | · | · | · |
selectionDirection
| · | Yes | Yes | · | Yes | · | · | · | · | · | · | · | · | · | · |
setRangeText()
| · | Yes | Yes | · | Yes | · | · | · | · | · | · | · | · | · | · |
setSelectionRange()
| · | Yes | Yes | · | Yes | · | · | · | · | · | · | · | · | · | · |
stepDown()
| · | · | · | · | · | Yes | Yes | Yes | Yes | · | · | · | · | · | · |
stepUp()
| · | · | · | · | · | Yes | Yes | Yes | Yes | · | · | · | · | · | · |
| Events | |||||||||||||||
input event
| · | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | · | · | · |
change event
| · | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | · | · | · |
† If the control has no selectable text, the select() method results in a no-op, with no
"InvalidStateError" DOMException.
Some states of the type attribute define a value
sanitization algorithm.
Each input element has a value, which is
exposed by the value IDL attribute. Some states define an
algorithm to convert a string to a number,
an algorithm to convert a number to a
string, an algorithm to convert a string to a
Date object, and an algorithm to
convert a Date object to a string, which are used by max, min, step, valueAsDate,
valueAsNumber, and stepUp().
An input element's dirty value flag must be
set to true whenever the user interacts with the control in a way that changes the value. (It is also set to true when the value is programmatically
changed, as described in the definition of the value IDL
attribute.)
The value content
attribute gives the default value of the input
element. When the value content attribute is
added, set, or removed, if the control's dirty value flag
is false, the user agent must set the value of the element
to the value of the value content attribute, if there is
one, or the empty string otherwise, and then run the current value sanitization
algorithm, if one is defined.
Each input element has a checkedness,
which is exposed by the checked IDL attribute.
Each input element has a boolean dirty checkedness flag. When it is true, the
element is said to have a dirty checkedness.
The dirty checkedness flag must be initially
set to false when the element is created, and must be set to true whenever the user interacts with
the control in a way that changes the checkedness.
The checked
content attribute is a boolean attribute that gives the default checkedness of the input element. When the checked content attribute is added, if
the control does not have dirty checkedness, the user
agent must set the checkedness of the element to true;
when the checked content attribute is removed, if the
control does not have dirty checkedness, the user
agent must set the checkedness of the element to
false.
The reset algorithm for input
elements is to set the dirty value flag and dirty checkedness flag back to false, set the
value of the element to the value of the value content attribute, if there is one, or the empty string
otherwise, set the checkedness of the element to true if
the element has a checked content attribute and false if
it does not, empty the list of selected
files, and then invoke the value sanitization algorithm, if the type attribute's current state defines one.
Each input element can be mutable. Except where
otherwise specified, an input element is always mutable. Similarly, except where otherwise specified, the user
agent should not allow the user to modify the element's value or checkedness.
When an input element is disabled, it is not mutable.
The readonly attribute can also in some
cases (e.g. for the Date state, but not the Checkbox state) stop an input element from
being mutable.
The cloning steps for input elements
must propagate the value, dirty value flag, checkedness, and dirty checkedness flag from the node being cloned
to the copy.
The activation behavior for input elements are these steps:
If this element is not mutable and is not in the Checkbox state and is not in the Radio state, then return.
Run this element's input activation behavior, if any, and do nothing otherwise.
The legacy-pre-activation behavior for input elements are these
steps:
If this element's type attribute is in the Checkbox state, then set this element's checkedness to its opposite value (i.e. true if it is false,
false if it is true) and set this element's indeterminate IDL attribute to false.
If this element's type attribute is in the Radio Button state, then get a reference to the element in
this element's radio button group that has its checkedness set to true, if any, and then set this element's
checkedness to true.
The legacy-canceled-activation behavior for input elements are these
steps:
If the element's type attribute is in the Checkbox state, then set the element's checkedness and the element's indeterminate IDL attribute back to the values they had
before the legacy-pre-activation behavior was run.
If this element's type attribute is in the Radio Button state, then if the element to which a
reference was obtained in the legacy-pre-activation behavior, if any, is still in
what is now this element's radio button group, if it still has one, and if so,
setting that element's checkedness to true; or else, if
there was no such element, or that element is no longer in this element's radio button
group, or if this element no longer has a radio button group, setting this
element's checkedness to false.
When an input element is first created, the element's rendering and behavior must
be set to the rendering and behavior defined for the type
attribute's state, and the value sanitization algorithm, if one is defined for the
type attribute's state, must be invoked.
When an input element's type attribute
changes state, the user agent must run the following steps:
If the previous state of the element's type attribute
put the value IDL attribute in the value mode, and the element's value is not the empty string, and the new state of the element's
type attribute puts the value IDL attribute in either the default mode or the default/on mode, then set the element's value content attribute to the element's value.
Otherwise, if the previous state of the element's type attribute put the value
IDL attribute in any mode other than the value mode, and the
new state of the element's type attribute puts the value IDL attribute in the value mode, then set the value of the element to the value of the value content attribute, if there is one, or the empty string
otherwise, and then set the control's dirty value flag to
false.
Otherwise, if the previous state of the element's type attribute put the value IDL attribute in any mode other than the filename mode, and the new state of the element's type attribute puts the value IDL attribute in the filename mode, then set the value of the element to the empty string.
Update the element's rendering and behavior to the new state's.
Signal a type change for the element. (The Radio Button state uses this, in particular.)
Invoke the value sanitization algorithm, if one is defined for the type attribute's new state.
Let previouslySelectable be true if setRangeText() previously applied to the element, and false otherwise.
Let nowSelectable be true if setRangeText() now applies to the element, and false otherwise.
If previouslySelectable is false and nowSelectable is true, set the
element's text entry cursor position to the
beginning of the text control, and set its selection
direction to "none".
The name attribute represents the element's name.
The dirname attribute controls how the element's directionality is submitted.
The disabled attribute is used to make the control non-interactive and to prevent its value from being submitted.
The form attribute is used to explicitly associate the input element with its form owner.
The autocomplete attribute controls how the user agent provides autofill behavior.
HTMLInputElement#indeterminate
Support in all current engines.
The indeterminate IDL attribute must initially be set to
false. On getting, it must return the last value it was set to. On setting, it must be set to the
new value. It has no effect except for changing the appearance of checkbox controls.
Support in all current engines.
The accept,
alt, max, min, multiple, pattern, placeholder, required, size, src, and step IDL attributes must
reflect the respective content attributes of the same name. The dirName IDL attribute must reflect the dirname content attribute. The readOnly IDL attribute must reflect the
readonly content attribute. The defaultChecked IDL attribute must
reflect the checked content attribute. The
defaultValue IDL attribute must
reflect the value content attribute.
The type IDL
attribute must reflect the respective content attribute of the same name,
limited to only known values. The maxLength IDL attribute must reflect the
maxlength content attribute, limited to only
non-negative numbers. The minLength
IDL attribute must reflect the minlength
content attribute, limited to only non-negative numbers.
The IDL attributes width and height must return the rendered width and height of the
image, in CSS pixels, if an image is being rendered, and
is being rendered to a visual medium; or else the intrinsic
width and height of the image, in CSS pixels, if an image is available but not being rendered to a visual medium; or else 0,
if no image is available. When the input
element's type attribute is not in the Image Button state, then no image is available. [CSS]
On setting, they must act as if they reflected the respective content attributes of the same name.
The willValidate, validity, and validationMessage IDL attributes, and the checkValidity(), reportValidity(), and setCustomValidity() methods, are part of the
constraint validation API. The labels IDL
attribute provides a list of the element's labels. The select(), selectionStart, selectionEnd, selectionDirection, setRangeText(), and setSelectionRange() methods and IDL attributes
expose the element's text selection. The disabled,
form, and name IDL attributes
are part of the element's forms API.
type 属性的状态type=hidden)Support in all current engines.
When an element's attribute is in the state, the rules in this section apply.
The element a value that is not intended to be examined or manipulated by the user.
Constraint validation: If an element's attribute is in the state, it is .
If the attribute is present and has a value that is an match for "", then the element's attribute must be omitted.
The content attribute to this element.
The IDL attribute to this element and is in mode .
The following content attributes must not be specified and to the element: , , , , , , , , , , , , , , , , , , , , , , , and .
The following IDL attributes and methods to the element: , , , , , , , and IDL attributes; , , , , and methods.
The and events .
type=text) state and Search state (type=search)Support in all current engines.
Support in all current engines.
When an input element's type attribute is in
the Text state or the Search state, the rules in this section apply.
The input element represents a one line plain text edit control for
the element's value.
The difference between the Text state and the Search state is primarily stylistic: on platforms where search controls are distinguished from regular text controls, the Search state might result in an appearance consistent with the platform's search controls rather than appearing like a regular text control.
If the element is mutable, its value should be editable by the user. User agents must not allow users to insert U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters into the element's value.
If the element is mutable, the user agent should allow the user to change the writing direction of the element, setting it either to a left-to-right writing direction or a right-to-left writing direction. If the user does so, the user agent must then run the following steps:
Set the element's dir attribute to "ltr" if the user selected a left-to-right writing direction, and
"rtl" if the user selected a right-to-left writing
direction.
Queue an element task on the user interaction task source given
the element to fire an event named input at the element, with the bubbles and composed
attributes initialized to true.
The value attribute, if specified, must have a value that
contains no U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters.
The value sanitization algorithm is as follows: Strip newlines from the value.
The following common input element content
attributes, IDL attributes, and methods apply to the element:
autocomplete,
dirname,
list,
maxlength,
minlength,
pattern,
placeholder,
readonly,
required, and
size content attributes;
list,
selectionStart,
selectionEnd,
selectionDirection, and
value IDL attributes;
select(),
setRangeText(), and
setSelectionRange() methods.
The value IDL attribute is
in mode value.
The input and change events apply.
The following content attributes must not be specified and do not
apply to the element:
accept,
alt,
checked,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
max,
min,
multiple,
src,
step, and
width.
The following IDL attributes and methods do not apply to the
element:
checked,
files,
valueAsDate, and
valueAsNumber IDL attributes;
stepDown() and
stepUp() methods.
type=tel)Support in all current engines.
When an input element's type attribute is in
the Telephone state, the rules in this section apply.
The input element represents a control for editing a telephone number
given in the element's value.
If the element is mutable, its value should be editable by the user. User agents may change the spacing and, with care, the punctuation of values that the user enters. User agents must not allow users to insert U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters into the element's value.
The value attribute, if specified, must have a value that
contains no U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters.
The value sanitization algorithm is as follows: Strip newlines from the value.
Unlike the URL and Email types, the Telephone type does not enforce a particular syntax. This is
intentional; in practice, telephone number fields tend to be free-form fields, because there are a
wide variety of valid phone numbers. Systems that need to enforce a particular format are
encouraged to use the pattern attribute or the setCustomValidity() method to hook into the client-side
validation mechanism.
The following common input element content
attributes, IDL attributes, and methods apply to the element:
autocomplete,
list,
maxlength,
minlength,
pattern,
placeholder,
readonly,
required, and
size content attributes;
list,
selectionStart,
selectionEnd,
selectionDirection, and
value IDL attributes;
select(),
setRangeText(), and
setSelectionRange() methods.
The value IDL attribute is
in mode value.
The input and change events apply.
The following content attributes must not be specified and do not
apply to the element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
max,
min,
multiple,
src,
step, and
width.
The following IDL attributes and methods do not apply to the
element:
checked,
files,
valueAsDate, and
valueAsNumber IDL attributes;
stepDown() and
stepUp() methods.
type=url)Support in all current engines.
When an input element's type attribute is in
the URL state, the rules in this section apply.
The input element represents a control for editing a single
absolute URL given in the element's value.
If the element is mutable, the user agent should allow the user to change the URL represented by its value. User agents may allow the user to set the value to a string that is not a valid absolute URL, but may also or instead automatically escape characters entered by the user so that the value is always a valid absolute URL (even if that isn't the actual value seen and edited by the user in the interface). User agents should allow the user to set the value to the empty string. User agents must not allow users to insert U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters into the value.
The value attribute, if specified and not empty, must
have a value that is a valid URL potentially surrounded by spaces that is also an
absolute URL.
The value sanitization algorithm is as follows: Strip newlines from the value, then strip leading and trailing ASCII whitespace from the value.
Constraint validation: While the value of the element is neither the empty string nor a valid absolute URL, the element is suffering from a type mismatch.
The following common input element content
attributes, IDL attributes, and methods apply to the element:
autocomplete,
list,
maxlength,
minlength,
pattern,
placeholder,
readonly,
required, and
size content attributes;
list,
selectionStart,
selectionEnd,
selectionDirection, and
value IDL attributes;
select(),
setRangeText(), and
setSelectionRange() methods.
The value IDL attribute is
in mode value.
The input and change events apply.
The following content attributes must not be specified and do not
apply to the element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
max,
min,
multiple,
src,
step, and
width.
The following IDL attributes and methods do not apply to the
element:
checked,
files,
valueAsDate, and
valueAsNumber IDL attributes;
stepDown() and
stepUp() methods.
If a document contained the following markup:
<input type="url" name="location" list="urls">
<datalist id="urls">
<option label="MIME: Format of Internet Message Bodies" value="https://tools.ietf.org/html/rfc2045">
<option label="HTML" value="https://html.spec.whatwg.org/">
<option label="DOM" value="https://dom.spec.whatwg.org/">
<option label="Fullscreen" value="https://fullscreen.spec.whatwg.org/">
<option label="Media Session" value="https://mediasession.spec.whatwg.org/">
<option label="The Single UNIX Specification, Version 3" value="http://www.unix.org/version3/">
</datalist>...and the user had typed "spec.w", and the user agent had also found that the user
had visited https://url.spec.whatwg.org/#url-parsing and https://streams.spec.whatwg.org/ in the recent past, then the rendering might look
like this:
The first four URLs in this sample consist of the four URLs in the author-specified list that match the text the user has entered, sorted in some implementation-defined manner (maybe by how frequently the user refers to those URLs). Note how the UA is using the knowledge that the values are URLs to allow the user to omit the scheme part and perform intelligent matching on the domain name.
The last two URLs (and probably many more, given the scrollbar's indications of more values being available) are the matches from the user agent's session history data. This data is not made available to the page DOM. In this particular case, the UA has no titles to provide for those values.
type=email)Support in all current engines.
When an input element's type attribute is in
the Email state, the rules in this section apply.
How the Email state operates depends on whether the
multiple attribute is specified or not.
multiple attribute is not specified on the
elementThe input element represents a control for editing an email
address given in the element's value.
If the element is mutable, the user agent should allow the user to change the email address represented by its value. User agents may allow the user to set the value to a string that is not a valid email address. The user agent should act in a manner consistent with expecting the user to provide a single email address. User agents should allow the user to set the value to the empty string. User agents must not allow users to insert U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters into the value. User agents may transform the value for display and editing; in particular, user agents should convert punycode in the domain labels of the value to IDN in the display and vice versa.
Constraint validation: While the user interface is representing input that the user agent cannot convert to punycode, the control is suffering from bad input.
The value attribute, if specified and not empty, must
have a value that is a single valid email address.
The value sanitization algorithm is as follows: Strip newlines from the value, then strip leading and trailing ASCII whitespace from the value.
Constraint validation: While the value of the element is neither the empty string nor a single valid email address, the element is suffering from a type mismatch.
multiple attribute is specified on
the elementThe input element represents a control for adding, removing, and
editing the email addresses given in the element's values.
If the element is mutable, the user agent should allow the user to add, remove, and edit the email addresses represented by its values. User agents may allow the user to set any individual value in the list of values to a string that is not a valid email address, but must not allow users to set any individual value to a string containing U+002C COMMA (,), U+000A LINE FEED (LF), or U+000D CARRIAGE RETURN (CR) characters. User agents should allow the user to remove all the addresses in the element's values. User agents may transform the values for display and editing; in particular, user agents should convert punycode in the domain labels of the value to IDN in the display and vice versa.
Constraint validation: While the user interface describes a situation where an individual value contains a U+002C COMMA (,) or is representing input that the user agent cannot convert to punycode, the control is suffering from bad input.
Whenever the user changes the element's values, the user agent must run the following steps:
Let latest values be a copy of the element's values.
Strip leading and trailing ASCII whitespace from each value in latest values.
Let the element's value be the result of concatenating all the values in latest values, separating each value from the next by a single U+002C COMMA character (,), maintaining the list's order.
The value attribute, if specified, must have a value
that is a valid email address list.
The value sanitization algorithm is as follows:
Split on commas the element's value, strip leading and trailing ASCII whitespace from each resulting token, if any, and let the element's values be the (possibly empty) resulting list of (possibly empty) tokens, maintaining the original order.
Let the element's value be the result of concatenating the element's values, separating each value from the next by a single U+002C COMMA character (,), maintaining the list's order.
Constraint validation: While the value of the element is not a valid email address list, the element is suffering from a type mismatch.
When the multiple attribute is set or removed, the
user agent must run the value sanitization algorithm.
A valid email address is a string that matches the email production of the following ABNF, the character set for which is Unicode.
This ABNF implements the extensions described in RFC 1123. [ABNF] [RFC5322] [RFC1034] [RFC1123]
email = 1*( atext / "." ) "@" label *( "." label )
label = let-dig [ [ ldh-str ] let-dig ] ; limited to a length of 63 characters by RFC 1034 section 3.5
atext = < as defined in RFC 5322 section 3.2.3 >
let-dig = < as defined in RFC 1034 section 3.5 >
ldh-str = < as defined in RFC 1034 section 3.5 >This requirement is a willful violation of RFC 5322, which defines a syntax for email addresses that is simultaneously too strict (before the "@" character), too vague (after the "@" character), and too lax (allowing comments, whitespace characters, and quoted strings in manners unfamiliar to most users) to be of practical use here.
The following JavaScript- and Perl-compatible regular expression is an implementation of the above definition.
/^[a-zA-Z0-9.!#$%&'*+\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/
A valid email address list is a set of comma-separated tokens, where each token is itself a valid email address. To obtain the list of tokens from a valid email address list, an implementation must split the string on commas.
The following common input element content
attributes, IDL attributes, and methods apply to the element:
autocomplete,
list,
maxlength,
minlength,
multiple,
pattern,
placeholder,
readonly,
required, and
size content attributes;
list and
value IDL attributes;
select() method.
The value IDL attribute is
in mode value.
The input and change events apply.
The following content attributes must not be specified and do not
apply to the element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
max,
min,
src,
step, and
width.
The following IDL attributes and methods do not apply to the
element:
checked,
files,
selectionStart,
selectionEnd,
selectionDirection,
valueAsDate, and
valueAsNumber IDL attributes;
setRangeText(),
setSelectionRange(),
stepDown() and
stepUp() methods.
type=password)Support in all current engines.
When an input element's type attribute is in
the Password state, the rules in this section
apply.
The input element represents a one line plain text edit control for
the element's value. The user agent should obscure the value
so that people other than the user cannot see it.
If the element is mutable, its value should be editable by the user. User agents must not allow users to insert U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters into the value.
The value attribute, if specified, must have a value that
contains no U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters.
The value sanitization algorithm is as follows: Strip newlines from the value.
The following common input element content
attributes, IDL attributes, and methods apply to the element:
autocomplete,
maxlength,
minlength,
pattern,
placeholder,
readonly,
required, and
size content attributes;
selectionStart,
selectionEnd,
selectionDirection, and
value IDL attributes;
select(),
setRangeText(), and
setSelectionRange() methods.
The value IDL attribute is
in mode value.
The input and change events apply.
The following content attributes must not be specified and do not
apply to the element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
list,
max,
min,
multiple,
src,
step, and
width.
The following IDL attributes and methods do not apply to the
element:
checked,
files,
list,
valueAsDate, and
valueAsNumber IDL attributes;
stepDown() and
stepUp() methods.
type=date)Support in all current engines.
When an input element's type attribute is in
the Date state, the rules in this section apply.
The input element represents a control for setting the element's
value to a string representing a specific date.
If the element is mutable, the user agent should allow the user to change the date represented by its value, as obtained by parsing a date from it. User agents must not allow the user to set the value to a non-empty string that is not a valid date string. If the user agent provides a user interface for selecting a date, then the value must be set to a valid date string representing the user's selection. User agents should allow the user to set the value to the empty string.
Constraint validation: While the user interface describes input that the user agent cannot convert to a valid date string, the control is suffering from bad input.
See the introduction section for a discussion of the difference between the input format and submission format for date, time, and number form controls, and the implementation notes regarding localization of form controls.
The value attribute, if specified and not empty, must
have a value that is a valid date string.
The value sanitization algorithm is as follows: If the value of the element is not a valid date string, then set it to the empty string instead.
The min attribute, if specified, must have a value that is
a valid date string. The max attribute, if
specified, must have a value that is a valid date string.
The step attribute is expressed in days. The step scale factor is 86,400,000
(which converts the days to milliseconds, as used in the other algorithms). The default step is 1 day.
When the element is suffering from a step mismatch, the user agent may round the element's value to the nearest date for which the element would not suffer from a step mismatch.
The algorithm to convert a string to a
number, given a string input, is as follows: If parsing a date from input results in an
error, then return an error; otherwise, return the number of milliseconds elapsed from midnight
UTC on the morning of 1970-01-01 (the time represented by the value "1970-01-01T00:00:00.0Z") to midnight UTC on the morning of the parsed date, ignoring leap seconds.
The algorithm to convert a number to a
string, given a number input, is as follows: Return a
valid date string that represents the date that, in
UTC, is current input milliseconds after midnight UTC on the morning of
1970-01-01 (the time represented by the value "1970-01-01T00:00:00.0Z").
The algorithm to convert a string to a
Date object, given a string input, is as follows:
If parsing a date from input results
in an error, then return an error; otherwise, return a new
Date object representing midnight UTC on the morning of the parsed date.
The algorithm to convert a
Date object to a string, given a Date object input, is
as follows: Return a valid date string that represents the date current at the time represented by input in the UTC
time zone.
The Date state (and other date- and time-related states described in subsequent sections) is not intended for the entry of values for which a precise date and time relative to the contemporary calendar cannot be established. For example, it would be inappropriate for the entry of times like "one millisecond after the big bang", "the early part of the Jurassic period", or "a winter around 250 BCE".
For the input of dates before the introduction of the Gregorian calendar, authors are
encouraged to not use the Date state (and the other
date- and time-related states described in subsequent sections), as user agents are not required
to support converting dates and times from earlier periods to the Gregorian calendar, and asking
users to do so manually puts an undue burden on users. (This is complicated by the manner in
which the Gregorian calendar was phased in, which occurred at different times in different
countries, ranging from partway through the 16th century all the way to early in the 20th.)
Instead, authors are encouraged to provide fine-grained input controls using the
select element and input elements with the Number state.
The following common input element content
attributes, IDL attributes, and methods apply to the element:
autocomplete,
list,
max,
min,
readonly,
required, and
step content attributes;
list,
value,
valueAsDate, and
valueAsNumber IDL attributes;
select(),
stepDown(), and
stepUp() methods.
The value IDL attribute is
in mode value.
The input and change events apply.
The following content attributes must not be specified and do not
apply to the element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
maxlength,
minlength,
multiple,
pattern,
placeholder,
size,
src, and
width.
The following IDL attributes and methods do not apply to the
element:
checked,
selectionStart,
selectionEnd, and
selectionDirection IDL attributes;
setRangeText(), and
setSelectionRange() methods.
type=month)Support in all current engines.
When an input element's type attribute is in
the Month state, the rules in this section apply.
The input element represents a control for setting the element's
value to a string representing a specific month.
If the element is mutable, the user agent should allow the user to change the month represented by its value, as obtained by parsing a month from it. User agents must not allow the user to set the value to a non-empty string that is not a valid month string. If the user agent provides a user interface for selecting a month, then the value must be set to a valid month string representing the user's selection. User agents should allow the user to set the value to the empty string.
Constraint validation: While the user interface describes input that the user agent cannot convert to a valid month string, the control is suffering from bad input.
See the introduction section for a discussion of the difference between the input format and submission format for date, time, and number form controls, and the implementation notes regarding localization of form controls.
The value attribute, if specified and not empty, must
have a value that is a valid month string.
The value sanitization algorithm is as follows: If the value of the element is not a valid month string, then set it to the empty string instead.
The min attribute, if specified, must have a value that is
a valid month string. The max attribute, if
specified, must have a value that is a valid month string.
The step attribute is expressed in months. The step scale factor is 1 (there is no
conversion needed as the algorithms use months). The default step is 1 month.
When the element is suffering from a step mismatch, the user agent may round the element's value to the nearest month for which the element would not suffer from a step mismatch.
The algorithm to convert a string to a number, given a string input, is as follows: If parsing a month from input results in an error, then return an error; otherwise, return the number of months between January 1970 and the parsed month.
The algorithm to convert a number to a string, given a number input, is as follows: Return a valid month string that represents the month that has input months between it and January 1970.
The algorithm to convert a string to a
Date object, given a string input, is as follows:
If parsing a month from input
results in an error, then return an error; otherwise, return a
new Date object representing midnight UTC on the morning of the first day of
the parsed month.
The algorithm to convert a
Date object to a string, given a Date object input, is
as follows: Return a valid month string that represents the month current at the time represented by input in the UTC
time zone.
The following common input element content
attributes, IDL attributes, and methods apply to the element:
autocomplete,
list,
max,
min,
readonly,
required, and
step content attributes;
list,
value,
valueAsDate, and
valueAsNumber IDL attributes;
select(),
stepDown(), and
stepUp() methods.
The value IDL attribute is
in mode value.
The input and change events apply.
The following content attributes must not be specified and do not
apply to the element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
maxlength,
minlength,
multiple,
pattern,
placeholder,
size,
src, and
width.
The following IDL attributes and methods do not apply to the
element:
checked,
files,
selectionStart,
selectionEnd, and
selectionDirection IDL attributes;
setRangeText(), and
setSelectionRange() methods.
type=week)When an input element's type attribute is in
the Week state, the rules in this section apply.
The input element represents a control for setting the element's
value to a string representing a specific week.
If the element is mutable, the user agent should allow the user to change the week represented by its value, as obtained by parsing a week from it. User agents must not allow the user to set the value to a non-empty string that is not a valid week string. If the user agent provides a user interface for selecting a week, then the value must be set to a valid week string representing the user's selection. User agents should allow the user to set the value to the empty string.
Constraint validation: While the user interface describes input that the user agent cannot convert to a valid week string, the control is suffering from bad input.
See the introduction section for a discussion of the difference between the input format and submission format for date, time, and number form controls, and the implementation notes regarding localization of form controls.
The value attribute, if specified and not empty, must
have a value that is a valid week string.
The value sanitization algorithm is as follows: If the value of the element is not a valid week string, then set it to the empty string instead.
The min attribute, if specified, must have a value that is
a valid week string. The max attribute, if
specified, must have a value that is a valid week string.
The step attribute is expressed in weeks. The step scale factor is 604,800,000
(which converts the weeks to milliseconds, as used in the other algorithms). The default step is 1 week. The default step base is −259,200,000 (the start
of week 1970-W01).
When the element is suffering from a step mismatch, the user agent may round the element's value to the nearest week for which the element would not suffer from a step mismatch.
The algorithm to convert a string to a
number, given a string input, is as follows: If parsing a week string from input results in
an error, then return an error; otherwise, return the number of milliseconds elapsed from midnight
UTC on the morning of 1970-01-01 (the time represented by the value "1970-01-01T00:00:00.0Z") to midnight UTC on the morning of the Monday of the
parsed week, ignoring leap seconds.
The algorithm to convert a number to a
string, given a number input, is as follows: Return a
valid week string that represents the week that, in
UTC, is current input milliseconds after midnight UTC on the morning of
1970-01-01 (the time represented by the value "1970-01-01T00:00:00.0Z").
The algorithm to convert a string to a
Date object, given a string input, is as follows: If parsing a week from input results in an error, then
return an error; otherwise, return a new Date
object representing midnight UTC on the morning of the Monday of the parsed week.
The algorithm to convert a
Date object to a string, given a Date object input, is
as follows: Return a valid week string that represents the week current at the time represented by input in the UTC
time zone.
The following common input element content attributes, IDL attributes, and
methods apply to the element:
autocomplete,
list,
max,
min,
readonly,
required, and
step content attributes;
list,
value,
valueAsDate, and
valueAsNumber IDL attributes;
select(),
stepDown(), and
stepUp() methods.
The value IDL attribute is in mode value.
The input and change events apply.
The following content attributes must not be specified and do not apply to the
element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
maxlength,
minlength,
multiple,
pattern,
placeholder,
size,
src, and
width.
The following IDL attributes and methods do not apply to the element:
checked,
files,
selectionStart,
selectionEnd, and
selectionDirection IDL attributes;
setRangeText(), and
setSelectionRange() methods.
type=time)Support in all current engines.
When an input element's type attribute is in
the Time state, the rules in this section apply.
The input element represents a control for setting the element's
value to a string representing a specific time.
If the element is mutable, the user agent should allow the user to change the time represented by its value, as obtained by parsing a time from it. User agents must not allow the user to set the value to a non-empty string that is not a valid time string. If the user agent provides a user interface for selecting a time, then the value must be set to a valid time string representing the user's selection. User agents should allow the user to set the value to the empty string.
Constraint validation: While the user interface describes input that the user agent cannot convert to a valid time string, the control is suffering from bad input.
See the introduction section for a discussion of the difference between the input format and submission format for date, time, and number form controls, and the implementation notes regarding localization of form controls.
The value attribute, if specified and not empty, must
have a value that is a valid time string.
The value sanitization algorithm is as follows: If the value of the element is not a valid time string, then set it to the empty string instead.
The form control has a periodic domain.
The min attribute, if specified, must have a value that is
a valid time string. The max attribute, if
specified, must have a value that is a valid time string.
The step attribute is expressed in seconds. The step scale factor is 1000 (which
converts the seconds to milliseconds, as used in the other algorithms). The default step is 60 seconds.
When the element is suffering from a step mismatch, the user agent may round the element's value to the nearest time for which the element would not suffer from a step mismatch.
The algorithm to convert a string to a number, given a string input, is as follows: If parsing a time from input results in an error, then return an error; otherwise, return the number of milliseconds elapsed from midnight to the parsed time on a day with no time changes.
The algorithm to convert a number to a string, given a number input, is as follows: Return a valid time string that represents the time that is input milliseconds after midnight on a day with no time changes.
The algorithm to convert a string to a
Date object, given a string input, is as
follows: If parsing a time from
input results
in an error, then return an error; otherwise, return a new
Date object representing the parsed time in
UTC on 1970-01-01.
The algorithm to convert a
Date object to a string, given a Date object input, is
as follows: Return a valid time string that represents the UTC time component that is represented by input.
The following common input element content attributes, IDL attributes, and
methods apply to the element:
autocomplete,
list,
max,
min,
readonly,
required, and
step content attributes;
list,
value,
valueAsDate, and
valueAsNumber IDL attributes;
select(),
stepDown(), and
stepUp() methods.
The value IDL attribute is in mode value.
The input and change events apply.
The following content attributes must not be specified and do not apply to the
element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
maxlength,
minlength,
multiple,
pattern,
placeholder,
size,
src, and
width.
The following IDL attributes and methods do not apply to the element:
checked,
files,
selectionStart,
selectionEnd, and
selectionDirection IDL attributes;
setRangeText(), and
setSelectionRange() methods.
type=datetime-local)Support in all current engines.
When an input element's type attribute is in
the Local Date and Time state, the rules in
this section apply.
The input element represents a control for setting the element's
value to a string representing a local date and time, with no time-zone offset
information.
If the element is mutable, the user agent should allow the user to change the date and time represented by its value, as obtained by parsing a date and time from it. User agents must not allow the user to set the value to a non-empty string that is not a valid normalized local date and time string. If the user agent provides a user interface for selecting a local date and time, then the value must be set to a valid normalized local date and time string representing the user's selection. User agents should allow the user to set the value to the empty string.
Constraint validation: While the user interface describes input that the user agent cannot convert to a valid normalized local date and time string, the control is suffering from bad input.
See the introduction section for a discussion of the difference between the input format and submission format for date, time, and number form controls, and the implementation notes regarding localization of form controls.
The value attribute, if specified and not empty, must
have a value that is a valid local date and time string.
The value sanitization algorithm is as follows: If the value of the element is a valid local date and time string, then set it to a valid normalized local date and time string representing the same date and time; otherwise, set it to the empty string instead.
The min attribute, if specified, must have a value that is
a valid local date and time string. The max
attribute, if specified, must have a value that is a valid local date and time
string.
The step attribute is expressed in seconds. The step scale factor is 1000 (which
converts the seconds to milliseconds, as used in the other algorithms). The default step is 60 seconds.
When the element is suffering from a step mismatch, the user agent may round the element's value to the nearest local date and time for which the element would not suffer from a step mismatch.
The algorithm to convert a string to a
number, given a string input, is as follows: If parsing a date and time from input results in an error, then return an error; otherwise, return the number of
milliseconds elapsed from midnight on the morning of 1970-01-01 (the time represented by the value
"1970-01-01T00:00:00.0") to the parsed local date and time, ignoring leap seconds.
The algorithm to convert a number to a
string, given a number input, is as follows: Return a
valid normalized local date and time string that represents the date and time that is
input milliseconds after midnight on the morning of 1970-01-01 (the time
represented by the value "1970-01-01T00:00:00.0").
See the note on historical dates in the Date state section.
The following common input element content
attributes, IDL attributes, and methods apply to the element:
autocomplete,
list,
max,
min,
readonly,
required, and
step content attributes;
list,
value, and
valueAsNumber IDL attributes;
select(),
stepDown(), and
stepUp() methods.
The value IDL attribute is
in mode value.
The input and change events apply.
The following content attributes must not be specified and do not
apply to the element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
maxlength,
minlength,
multiple,
pattern,
placeholder,
size,
src, and
width.
The following IDL attributes and methods do not apply to the
element:
checked,
files,
selectionStart,
selectionEnd,
selectionDirection, and
valueAsDate IDL attributes;
setRangeText(), and
setSelectionRange() methods.
The following example shows part of a flight booking application. The application uses an
input element with its type attribute set to
datetime-local, and it then interprets the
given date and time in the time zone of the selected airport.
<fieldset>
<legend>Destination</legend>
<p><label>Airport: <input type=text name=to list=airports></label></p>
<p><label>Departure time: <input type=datetime-local name=totime step=3600></label></p>
</fieldset>
<datalist id=airports>
<option value=ATL label="Atlanta">
<option value=MEM label="Memphis">
<option value=LHR label="London Heathrow">
<option value=LAX label="Los Angeles">
<option value=FRA label="Frankfurt">
</datalist>type=number)Support in all current engines.
When an input element's type attribute is in
the Number state, the rules in this section apply.
The input element represents a control for setting the element's
value to a string representing a number.
If the element is mutable, the user agent should allow the user to change the number represented by its value, as obtained from applying the rules for parsing floating-point number values to it. User agents must not allow the user to set the value to a non-empty string that is not a valid floating-point number. If the user agent provides a user interface for selecting a number, then the value must be set to the best representation of the number representing the user's selection as a floating-point number. User agents should allow the user to set the value to the empty string.
Constraint validation: While the user interface describes input that the user agent cannot convert to a valid floating-point number, the control is suffering from bad input.
This specification does not define what user interface user agents are to use; user agent vendors are encouraged to consider what would best serve their users' needs. For example, a user agent in Persian or Arabic markets might support Persian and Arabic numeric input (converting it to the format required for submission as described above). Similarly, a user agent designed for Romans might display the value in Roman numerals rather than in decimal; or (more realistically) a user agent designed for the French market might display the value with apostrophes between thousands and commas before the decimals, and allow the user to enter a value in that manner, internally converting it to the submission format described above.
The value attribute, if specified and not empty, must
have a value that is a valid floating-point number.
The value sanitization algorithm is as follows: If the value of the element is not a valid floating-point number, then set it to the empty string instead.
The min attribute, if specified, must have a value that is
a valid floating-point number. The max attribute,
if specified, must have a value that is a valid floating-point number.
The step scale factor is 1. The default step is 1 (allowing only integers to be selected by the user, unless the step base has a non-integer value).
When the element is suffering from a step mismatch, the user agent may round the element's value to the nearest number for which the element would not suffer from a step mismatch. If there are two such numbers, user agents are encouraged to pick the one nearest positive infinity.
The algorithm to convert a string to a number, given a string input, is as follows: If applying the rules for parsing floating-point number values to input results in an error, then return an error; otherwise, return the resulting number.
The algorithm to convert a number to a string, given a number input, is as follows: Return a valid floating-point number that represents input.
The following common input element content attributes, IDL attributes, and
methods apply to the element:
autocomplete,
list,
max,
min,
placeholder,
readonly,
required, and
step content attributes;
list,
value, and
valueAsNumber IDL attributes;
select(),
stepDown(), and
stepUp() methods.
The value IDL attribute is in mode value.
The input and change events apply.
The following content attributes must not be specified and do not apply to the
element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
maxlength,
minlength,
multiple,
pattern,
size,
src, and
width.
The following IDL attributes and methods do not apply to the element:
checked,
files,
selectionStart,
selectionEnd,
selectionDirection, and
valueAsDate IDL attributes;
setRangeText(), and
setSelectionRange() methods.
Here is an example of using a numeric input control:
<label>How much do you want to charge? $<input type=number min=0 step=0.01 name=price></label>As described above, a user agent might support numeric input in the user's local format, converting it to the format required for submission as described above. This might include handling grouping separators (as in "872,000,000,000") and various decimal separators (such as "3,99" vs "3.99") or using local digits (such as those in Arabic, Devanagari, Persian, and Thai).
The type=number state
is not appropriate for input that happens to only consist of numbers but isn't strictly speaking a
number. For example, it would be inappropriate for credit card numbers or US postal codes. A
simple way of determining whether to use type=number is to consider whether
it would make sense for the input control to have a spinbox interface (e.g. with "up" and "down"
arrows). Getting a credit card number wrong by 1 in the last digit isn't a minor mistake, it's as
wrong as getting every digit incorrect. So it would not make sense for the user to select a credit
card number using "up" and "down" buttons. When a spinbox interface is not appropriate, type=text is probably the right choice (possibly with an inputmode or pattern
attribute).
type=range)Support in all current engines.
When an input element's type attribute is in
the Range state, the rules in this section apply.
The input element represents a control for setting the element's
value to a string representing a number, but with the
caveat that the exact value is not important, letting UAs provide a simpler interface than they
do for the Number state.
If the element is mutable, the user agent should allow the user to change the number represented by its value, as obtained from applying the rules for parsing floating-point number values to it. User agents must not allow the user to set the value to a string that is not a valid floating-point number. If the user agent provides a user interface for selecting a number, then the value must be set to a best representation of the number representing the user's selection as a floating-point number. User agents must not allow the user to set the value to the empty string.
Constraint validation: While the user interface describes input that the user agent cannot convert to a valid floating-point number, the control is suffering from bad input.
The value attribute, if specified, must have a value
that is a valid floating-point number.
The value sanitization algorithm is as follows: If the value of the element is not a valid floating-point number, then set it to the best representation, as a floating-point number, of the default value.
The default value is the minimum plus half the difference between the minimum and the maximum, unless the maximum is less than the minimum, in which case the default value is the minimum.
When the element is suffering from an underflow, the user agent must set the element's value to the best representation, as a floating-point number, of the minimum.
When the element is suffering from an overflow, if the maximum is not less than the minimum, the user agent must set the element's value to a valid floating-point number that represents the maximum.
When the element is suffering from a step mismatch, the user agent must round the element's value to the nearest number for which the element would not suffer from a step mismatch, and which is greater than or equal to the minimum, and, if the maximum is not less than the minimum, which is less than or equal to the maximum, if there is a number that matches these constraints. If two numbers match these constraints, then user agents must use the one nearest to positive infinity.
For example, the markup
<input type="range" min=0 max=100 step=20 value=50>
results in a range control whose initial value is 60.
Here is an example of a range control using an autocomplete list with the list attribute. This could be useful if there are values along
the full range of the control that are especially important, such as preconfigured light levels
or typical speed limits in a range control used as a speed control. The following markup
fragment:
<input type="range" min="-100" max="100" value="0" step="10" name="power" list="powers">
<datalist id="powers">
<option value="0">
<option value="-30">
<option value="30">
<option value="++50">
</datalist>...with the following style sheet applied:
input { height: 75px; width: 49px; background: #D5CCBB; color: black; }...might render as:
Note how the UA determined the orientation of the control from the ratio of the
style-sheet-specified height and width properties. The colors were similarly derived from the
style sheet. The tick marks, however, were derived from the markup. In particular, the step attribute has not affected the placement of tick marks,
the UA deciding to only use the author-specified completion values and then adding longer tick
marks at the extremes.
Note also how the invalid value ++50 was completely ignored.
For another example, consider the following markup fragment:
<input name=x type=range min=100 max=700 step=9.09090909 value=509.090909>A user agent could display in a variety of ways, for instance:
Or, alternatively, for instance:
The user agent could pick which one to display based on the dimensions given in the style sheet. This would allow it to maintain the same resolution for the tick marks, despite the differences in width.
Finally, here is an example of a range control with two labeled values:
<input type="range" name="a" list="a-values">
<datalist id="a-values">
<option value="10" label="Low">
<option value="90" label="High">
</datalist>With styles that make the control draw vertically, it might look as follows:
In this state, the range and step constraints are enforced even during user input, and there is no way to set the value to the empty string.
The min attribute, if specified, must have a value that is
a valid floating-point number. The default
minimum is 0. The max attribute, if specified, must
have a value that is a valid floating-point number. The default maximum is 100.
The step scale factor is
1. The default step is 1 (allowing only
integers, unless the min attribute has a non-integer
value).
The algorithm to convert a string to a number, given a string input, is as follows: If applying the rules for parsing floating-point number values to input results in an error, then return an error; otherwise, return the resulting number.
The algorithm to convert a number to a string, given a number input, is as follows: Return the best representation, as a floating-point number, of input.
The following common input element content attributes, IDL attributes, and
methods apply to the element:
autocomplete,
list,
max,
min, and
step content attributes;
list,
value, and
valueAsNumber IDL attributes;
stepDown() and
stepUp() methods.
The value IDL attribute is in mode value.
The input and change events apply.
The following content attributes must not be specified and do not apply to the
element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
maxlength,
minlength,
multiple,
pattern,
placeholder,
readonly,
required,
size,
src, and
width.
The following IDL attributes and methods do not apply to the element:
checked,
files,
selectionStart,
selectionEnd,
selectionDirection, and
valueAsDate IDL attributes;
select(),
setRangeText(), and
setSelectionRange() methods.
type=color)Support in all current engines.
When an input element's type attribute is in
the Color state, the rules in this section apply.
The input element represents a color well control, for setting the
element's value to a string representing a simple
color.
In this state, there is always a color picked, and there is no way to set the value to the empty string.
If the element is mutable, the user agent should allow the user to change the color represented by its value, as obtained from applying the rules for parsing simple color values to it. User agents must not allow the user to set the value to a string that is not a valid lowercase simple color. If the user agent provides a user interface for selecting a color, then the value must be set to the result of using the rules for serializing simple color values to the user's selection. User agents must not allow the user to set the value to the empty string.
Constraint validation: While the user interface describes input that the user agent cannot convert to a valid lowercase simple color, the control is suffering from bad input.
The value attribute, if specified and not empty, must
have a value that is a valid simple color.
The value sanitization algorithm is as follows: If the value of the element is a valid simple color, then
set it to the value of the element converted to ASCII
lowercase; otherwise, set it to the string "#000000".
The following common input element content attributes and IDL attributes apply to the element:
autocomplete and
list content attributes;
list and
value IDL attributes;
select() method.
The value IDL attribute is in mode value.
The input and change events apply.
The following content attributes must not be specified and do not apply to the
element:
accept,
alt,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
max,
maxlength,
min,
minlength,
multiple,
pattern,
placeholder,
readonly,
required,
size,
src,
step, and
width.
The following IDL attributes and methods do not apply to the element:
checked,
files,
selectionStart,
selectionEnd,
selectionDirection,
valueAsDate and,
valueAsNumber IDL attributes;
setRangeText(),
setSelectionRange(),
stepDown(), and
stepUp() methods.
type=checkbox)Support in all current engines.
When an input element's type attribute is in
the Checkbox state, the rules in this section
apply.
The input element represents a two-state control that represents the
element's checkedness state. If the element's checkedness state is true, the control represents a positive
selection, and if it is false, a negative selection. If the element's indeterminate IDL attribute is set to true, then the
control's selection should be obscured as if the control was in a third, indeterminate, state.
The control is never a true tri-state control, even if the element's indeterminate IDL attribute is set to true. The indeterminate IDL attribute only gives the appearance of a
third state.
The input activation behavior is to run the following steps:
If the element is not connected, then return.
Fire an event named input at the element with the bubbles and composed
attributes initialized to true.
Fire an event named change at the element with the bubbles attribute initialized to true.
Constraint validation: If the element is required and its checkedness is false, then the element is suffering from being missing.
indeterminate [ = value ]When set, overrides the rendering of checkbox controls so that the current value is not visible.
The following common input element content attributes and IDL attributes apply to the element:
checked, and
required content attributes;
checked and
value IDL attributes.
The value IDL attribute is in mode default/on.
The input and change events apply.
The following content attributes must not be specified and do not apply to the
element:
accept,
alt,
autocomplete,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
list,
max,
maxlength,
min,
minlength,
multiple,
pattern,
placeholder,
readonly,
size,
src,
step, and
width.
The following IDL attributes and methods do not apply to the element:
files,
list,
selectionStart,
selectionEnd,
selectionDirection,
valueAsDate, and
valueAsNumber IDL attributes;
select(),
setRangeText(),
setSelectionRange(),
stepDown(), and
stepUp() methods.
type=radio)Support in all current engines.
When an input element's type attribute is in
the Radio Button state, the rules in this section
apply.
The input element represents a control that, when used in conjunction
with other input elements, forms a radio button group in which only one
control can have its checkedness state set to true. If
the element's checkedness state is true, the control
represents the selected control in the group, and if it is false, it indicates a control in the
group that is not selected.
The radio button group that contains an input element
a also contains all the other input elements b that fulfill all
of the following conditions:
input element b's type attribute is in the Radio
Button state.name attribute, their name attributes are not empty, and the value of a's name attribute equals the value of b's name attribute.A tree must not contain an input element whose radio button group contains only that element.
When any of the following phenomena occur, if the element's checkedness state is true after the occurrence, the checkedness state of all the other elements in the same radio button group must be set to false:
name attribute is set, changed, or
removed.The input activation behavior is to run the following steps:
If the element is not connected, then return.
Fire an event named input at the element with the bubbles and composed
attributes initialized to true.
Fire an event named change at the element with the bubbles attribute initialized to true.
Constraint validation: If an element in the radio button group is required, and all of the
input elements in the radio button group have a
checkedness that is false, then the element is
suffering from being missing.
The following example, for some reason, has specified that puppers are both required and disabled:
<form>
<p><label><input type="radio" name="dog-type" value="pupper" required disabled> Pupper</label>
<p><label><input type="radio" name="dog-type" value="doggo"> Doggo</label>
<p><button>Make your choice</button>
</form>If the user tries to submit this form without first selecting "Doggo", then both
input elements will be suffering from being missing, since an element
in the radio button group is required
(viz. the first element), and both of the elements in the radio button group have a false checkedness.
On the other hand, if the user selects "Doggo" and then submits the form, then neither
input element will be suffering from being missing, since while one of
them is required, not all of them have a false checkedness.
If none of the radio buttons in a radio button group are checked, then they will all be initially unchecked in the interface, until such time as one of them is checked (either by the user or by script).
The following common input element content attributes and IDL attributes apply to the element:
checked and
required content attributes;
checked and
value IDL attributes.
The value IDL attribute is in mode default/on.
The input and change events apply.
The following content attributes must not be specified and do not apply to the
element:
accept,
alt,
autocomplete,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
list,
max,
maxlength,
min,
minlength,
multiple,
pattern,
placeholder,
readonly,
size,
src,
step, and
width.
The following IDL attributes and methods do not apply to the element:
files,
list,
selectionStart,
selectionEnd,
selectionDirection,
valueAsDate, and
valueAsNumber IDL attributes;
select(),
setRangeText(),
setSelectionRange(),
stepDown(), and
stepUp() methods.
type=file)Support in all current engines.
When an input element's type attribute is in
the File Upload state, the rules in this section
apply.
The input element represents a list of selected files, each file consisting of a file
name, a file type, and a file body (the contents of the file).
File names must not contain path components, even in the case that a user has selected an entire directory hierarchy or multiple files with the same name from different directories. Path components, for the purposes of the File Upload state, are those parts of file names that are separated by U+005C REVERSE SOLIDUS character (\) characters.
Unless the multiple attribute is set, there must be
no more than one file in the list of selected
files.
The element's input activation behavior is to run the following steps:
If the algorithm is invoked when the element's Window object does not have
transient activation, then return without doing anything else.
Run these steps in parallel:
Optionally, wait until any prior execution of this algorithm has terminated.
Display a prompt to the user requesting that the user specify some files. If the multiple attribute is not set, there must be no more than
one file selected; otherwise, any number may be selected. Files can be from the filesystem or
created on the fly, e.g., a picture taken from a camera connected to the user's
device.
Wait for the user to have made their selection.
Update the file selection for the input element.
If the element is mutable, the user agent should allow the user to change the files on the list in other ways also, e.g., adding or removing files by drag-and-drop. When the user does so, the user agent must update the file selection for the element.
If the element is not mutable, the user agent must not allow the user to change the element's selection.
To update the file selection for an element element:
Queue an element task on the user interaction task source given element and the following steps:
Update element's selected files so that it represents the user's selection.
Fire an event named input at the input element, with the bubbles and composed
attributes initialized to true.
Fire an event named change at the input element, with the bubbles attribute initialized to true.
Constraint validation: If the element is required and the list of selected files is empty, then the element is suffering from being missing.
Support in all current engines.
The accept
attribute may be specified to provide user agents with a hint of what file types will be
accepted.
If specified, the attribute must consist of a set of comma-separated tokens, each of which must be an ASCII case-insensitive match for one of the following:
audio/*"video/*"image/*"The tokens must not be ASCII case-insensitive matches for any of the other tokens (i.e. duplicates are not allowed). To obtain the list of tokens from the attribute, the user agent must split the attribute value on commas.
User agents may use the value of this attribute to display a more appropriate user interface
than a generic file picker. For instance, given the value image/*, a user
agent could offer the user the option of using a local camera or selecting a photograph from their
photo collection; given the value audio/*, a user agent could offer the user
the option of recording a clip using a headset microphone.
User agents should prevent the user from selecting files that are not accepted by one (or more) of these tokens.
Authors are encouraged to specify both any MIME types and any corresponding extensions when looking for data in a specific format.
For example, consider an application that converts Microsoft Word documents to Open Document Format files. Since Microsoft Word documents are described with a wide variety of MIME types and extensions, the site can list several, as follows:
<input type="file" accept=".doc,.docx,.xml,application/msword,application/vnd.openxmlformats-officedocument.wordprocessingml.document">On platforms that only use file extensions to describe file types, the extensions listed here can be used to filter the allowed documents, while the MIME types can be used with the system's type registration table (mapping MIME types to extensions used by the system), if any, to determine any other extensions to allow. Similarly, on a system that does not have file names or extensions but labels documents with MIME types internally, the MIME types can be used to pick the allowed files, while the extensions can be used if the system has an extension registration table that maps known extensions to MIME types used by the system.
Extensions tend to be ambiguous (e.g. there are an untold number of formats
that use the ".dat" extension, and users can typically quite easily rename
their files to have a ".doc" extension even if they are not Microsoft Word
documents), and MIME types tend to be unreliable (e.g. many formats have no formally registered
types, and many formats are in practice labeled using a number of different MIME types). Authors
are reminded that, as usual, data received from a client should be treated with caution, as it may
not be in an expected format even if the user is not hostile and the user agent fully obeyed the
accept attribute's requirements.
For historical reasons, the value IDL attribute prefixes
the file name with the string "C:\fakepath\". Some legacy user agents
actually included the full path (which was a security vulnerability). As a result of this,
obtaining the file name from the value IDL attribute in a
backwards-compatible way is non-trivial. The following function extracts the file name in a
suitably compatible manner:
function extractFilename(path) {
if (path.substr(0, 12) == "C:\\fakepath\\")
return path.substr(12); // modern browser
var x;
x = path.lastIndexOf('/');
if (x >= 0) // Unix-based path
return path.substr(x+1);
x = path.lastIndexOf('\\');
if (x >= 0) // Windows-based path
return path.substr(x+1);
return path; // just the file name
}This can be used as follows:
<p><input type=file name=image onchange="updateFilename(this.value)"></p>
<p>The name of the file you picked is: <span id="filename">(none)</span></p>
<script>
function updateFilename(path) {
var name = extractFilename(path);
document.getElementById('filename').textContent = name;
}
</script>The following common input element content attributes and IDL attributes apply to the element:
accept,
multiple, and
required content attributes;
files and
value IDL attributes;
select() method.
The value IDL attribute is in mode filename.
The input and change events apply.
The following content attributes must not be specified and do not apply to the
element:
alt,
autocomplete,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
list,
max,
maxlength,
min,
minlength,
pattern,
placeholder,
readonly,
size,
src,
step, and
width.
The element's value attribute must be omitted.
The following IDL attributes and methods do not apply to the element:
checked,
list,
selectionStart,
selectionEnd,
selectionDirection,
valueAsDate, and
valueAsNumber IDL attributes;
setRangeText(),
setSelectionRange(),
stepDown(), and
stepUp() methods.
type=submit)Support in all current engines.
When an input element's type attribute is in
the Submit Button state, the rules in this section
apply.
The input element represents a button that, when activated, submits the
form. If the element has a value attribute,
the button's label must be the value of that attribute; otherwise, it must be an
implementation-defined string that means "Submit" or some such. The element is
a button, specifically a submit button.
Since the default label is implementation-defined, and the width of the button typically depends on the button's label, the button's width can leak a few bits of fingerprintable information. These bits are likely to be strongly correlated to the identity of the user agent and the user's locale.
The element's input activation behavior is as follows: if the element has a
form owner, and the element's node document is fully
active, submit the form owner from
the input element; otherwise, do nothing.
The formaction, formenctype, formmethod, formnovalidate, and formtarget attributes are attributes for form
submission.
The formnovalidate attribute can be
used to make submit buttons that do not trigger the constraint validation.
The following common input element content attributes and IDL attributes apply to the element:
formaction,
formenctype,
formmethod,
formnovalidate, and
formtarget content attributes;
value IDL attribute.
The value IDL attribute is in mode default.
The following content attributes must not be specified and do not apply to the
element:
accept,
alt,
autocomplete,
checked,
dirname,
height,
list,
max,
maxlength,
min,
minlength,
multiple,
pattern,
placeholder,
readonly,
required,
size,
src,
step, and
width.
The following IDL attributes and methods do not apply to the element:
checked,
files,
list,
selectionStart,
selectionEnd,
selectionDirection,
valueAsDate, and
valueAsNumber IDL attributes;
select(),
setRangeText(),
setSelectionRange(),
stepDown(), and
stepUp() methods.
The input and change events do not apply.
type=image)Support in all current engines.
When an input element's type attribute is in
the Image Button state, the rules in this section
apply.
The input element represents either an image from which a user can
select a coordinate and submit the form, or alternatively a button from which the user can submit
the form. The element is a button, specifically a submit button.
The coordinate is sent to the server during form submission by sending two entries for the element, derived from the name
of the control but with ".x" and ".y" appended to
the name with the x and y components of the coordinate respectively.
The image is given by the src attribute. The src
attribute must be present, and must contain a valid non-empty URL potentially surrounded by
spaces referencing a non-interactive, optionally animated, image resource that is neither
paged nor scripted.
When any of the these events occur
input element's type attribute is
first set to the Image Button state (possibly when
the element is first created), and the src attribute is
presentinput element's type attribute is
changed back to the Image Button state, and the src attribute is present, and its value has changed since the last
time the type attribute was in the Image Button stateinput element's type attribute is in
the Image Button state, and the src attribute is set or changedthen unless the user agent cannot support images, or its support for images has been disabled,
or the user agent only fetches images on demand, or the src
attribute's value is the empty string, the user agent must parse the value of the src
attribute value, relative to the element's node document, and if that is successful,
then:
Let request be a new request whose
url is the resulting URL record,
client is the element's node document's
relevant settings object, destination is "image", credentials mode is "include", and whose use-URL-credentials flag is set.
Fetch request.
Fetching the image must delay the load event of the element's node document until the task that is queued by the networking task source once the resource has been fetched (defined below) has been run.
If the image was successfully obtained, with no network errors, and the image's type is a supported image type, and the image is a valid image of that type, then the image is said to be available. If this is true before the image is completely downloaded, each task that is queued by the networking task source while the image is being fetched must update the presentation of the image appropriately.
The user agent should apply the image sniffing rules to determine the type of the image, with the image's associated Content-Type headers giving the official type. If these rules are not applied, then the type of the image must be the type given by the image's associated Content-Type headers.
User agents must not support non-image resources with the input element. User
agents must not run executable code embedded in the image resource. User agents must only display
the first page of a multipage resource. User agents must not allow the resource to act in an
interactive fashion, but should honor any animation in the resource.
The task that is queued
by the networking task source once the resource has been fetched, must, if the
download was successful and the image is available,
queue an element task on the user interaction task source given the
input element to fire an event named load at the input element; and otherwise, if the fetching
process fails without a response from the remote server, or completes but the image is not a valid
or supported image, queue an element task on the user interaction task
source given the input element to fire an
event named error on the input element.
The alt attribute
provides the textual label for the button for users and user agents who cannot use the image. The
alt attribute must be present, and must contain a non-empty
string giving the label that would be appropriate for an equivalent button if the image was
unavailable.
The input element supports dimension attributes.
If the src attribute is set, and the image is available and the user agent is configured to display that image,
then: The element represents a control for selecting a coordinate from the image specified by the
src attribute; if the element is mutable, the user agent should allow the user to select this coordinate, and the element's input
activation behavior is as follows: if the element has a form owner, and the
element's node document is fully active, take the user's selected coordinate, and submit the input element's form owner
from the input element. If the user activates the control without explicitly
selecting a coordinate, then the coordinate (0,0) must be assumed.
Otherwise, the element represents a submit button whose label is given by the
value of the alt attribute; the element's input
activation behavior is as follows: if the element has a form owner, and the
element's node document is fully active, set the selected coordinate to (0,0), and submit the input element's form
owner from the input element.
In either case, if the element has no form owner or the element's node document is not fully active, then its input activation behavior must be to do nothing..
The selected coordinate must consist of an x-component and a y-component. The coordinates represent the position relative to the edge of the image, with the coordinate space having the positive x direction to the right, and the positive y direction downwards.
The x-component must be a valid integer representing a number x in the range −(borderleft+paddingleft) ≤ x ≤ width+borderright+paddingright, where width is the rendered width of the image, borderleft is the width of the border on the left of the image, paddingleft is the width of the padding on the left of the image, borderright is the width of the border on the right of the image, and paddingright is the width of the padding on the right of the image, with all dimensions given in CSS pixels.
The y-component must be a valid integer representing a number y in the range −(bordertop+paddingtop) ≤ y ≤ height+borderbottom+paddingbottom, where height is the rendered height of the image, bordertop is the width of the border above the image, paddingtop is the width of the padding above the image, borderbottom is the width of the border below the image, and paddingbottom is the width of the padding below the image, with all dimensions given in CSS pixels.
Where a border or padding is missing, its width is zero CSS pixels.
The formaction, formenctype, formmethod, formnovalidate, and formtarget attributes are attributes for form
submission.
width [ = value ]height [ = value ]These attributes return the actual rendered dimensions of the image, or zero if the dimensions are not known.
They can be set, to change the corresponding content attributes.
The following common input element content attributes and IDL attributes apply to the element:
alt,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
src, and
width content attributes;
value IDL attribute.
The value IDL attribute is in mode default.
The following content attributes must not be specified and do not apply to the
element:
accept,
autocomplete,
checked,
dirname,
list,
max,
maxlength,
min,
minlength,
multiple,
pattern,
placeholder,
readonly,
required,
size, and
step.
The element's value attribute must be omitted.
The following IDL attributes and methods do not apply to the element:
checked,
files,
list,
selectionStart,
selectionEnd,
selectionDirection,
valueAsDate, and
valueAsNumber IDL attributes;
select(),
setRangeText(),
setSelectionRange(),
stepDown(), and
stepUp() methods.
The input and change events do not apply.
Many aspects of this state's behavior are similar to the behavior of the
img element. Readers are encouraged to read that section, where many of the same
requirements are described in more detail.
Take the following form:
<form action="process.cgi">
<input type=image src=map.png name=where alt="Show location list">
</form>If the user clicked on the image at coordinate (127,40) then the URL used to submit the form
would be "process.cgi?where.x=127&where.y=40".
(In this example, it's assumed that for users who don't see the map, and who instead just see a button labeled "Show location list", clicking the button will cause the server to show a list of locations to pick from instead of the map.)
type=reset)Support in all current engines.
When an input element's type attribute is in
the Reset Button state, the rules in this section
apply.
The input element represents a button that, when activated, resets the
form. If the element has a value attribute,
the button's label must be the value of that attribute; otherwise, it must be an
implementation-defined string that means "Reset" or some such. The element is
a button.
Since the default label is implementation-defined, and the width of the button typically depends on the button's label, the button's width can leak a few bits of fingerprintable information. These bits are likely to be strongly correlated to the identity of the user agent and the user's locale.
The element's input activation behavior, if the element has a form owner and the element's node document is fully active, is to reset the form owner; otherwise, it is to do nothing.
Constraint validation: The element is barred from constraint validation.
The value IDL attribute applies to this element and is in mode default.
The following content attributes must not be specified and do not apply to the
element:
accept,
alt,
autocomplete,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
list,
max,
maxlength,
min,
minlength,
multiple,
pattern,
placeholder,
readonly,
required,
size,
src,
step, and
width.
The following IDL attributes and methods do not apply to the element:
checked,
files,
list,
selectionStart,
selectionEnd,
selectionDirection,
valueAsDate, and
valueAsNumber IDL attributes;
select(),
setRangeText(),
setSelectionRange(),
stepDown(), and
stepUp() methods.
The input and change events do not apply.
type=button)Support in all current engines.
When an input element's type attribute is in
the Button state, the rules in this section apply.
The input element represents a button with no default behavior. A
label for the button must be provided in the value
attribute, though it may be the empty string. If the element has a value attribute, the button's label must be the value of that
attribute; otherwise, it must be the empty string. The element is a button.
The element has no input activation behavior.
Constraint validation: The element is barred from constraint validation.
The value IDL attribute applies to this element and is in mode default.
The following content attributes must not be specified and do not apply to the
element:
accept,
alt,
autocomplete,
checked,
dirname,
formaction,
formenctype,
formmethod,
formnovalidate,
formtarget,
height,
list,
max,
maxlength,
min,
minlength,
multiple,
pattern,
placeholder,
readonly,
required,
size,
src,
step, and
width.
The following IDL attributes and methods do not apply to the element:
checked,
files,
list,
selectionStart,
selectionEnd,
selectionDirection,
valueAsDate, and
valueAsNumber IDL attributes;
select(),
setRangeText(),
setSelectionRange(),
stepDown(), and
stepUp() methods.
The input and change events do not apply.
This section is non-normative.
日期、时间和数字控件展示给用户的格式与提交表单使用的格式相独立。
鼓励浏览器根据 input 元素的 language 或用户偏好中的语言环境暗示的惯例
来展示日期、时间和数字的 UI。使用页面的语言环境可以确保页面提供的数据一致。
例如，如果一个美式英语区域的页面说 Cirque De Soleil 节目将会在 02/03 播出，但浏览器设置了英式英语区域， 在购票日期选择中会把日期显示为 03/02，这会让用户很困惑。 使用页面的区域设置至少可以确保日期在所有地方都以同一种格式显示。 （当然用户仍然可能迟到一个月，但对于这样的文化差异只能做到这个地步了...）
input 元素属性这些属性只 适用于
input 元素的 type
属性所处的状态定义了这个属性 适用 的情况。
当一个属性 不适用于 input 元素时，
用户代理必须 忽略 这个属性，忽略下面的要求和定义。
maxlength and minlength attributesSupport in all current engines.
The maxlength
attribute, when it applies, is a
form control maxlength
attribute.
Support in all current engines.
The minlength
attribute, when it applies, is a
form control minlength
attribute.
If the input element has a maximum allowed value length, then the
length of the value of the element's value
attribute must be equal to or less than the element's maximum allowed value
length.
The following extract shows how a messaging client's text entry could be arbitrarily restricted to a fixed number of characters, thus forcing any conversation through this medium to be terse and discouraging intelligent discourse.
<label>What are you doing? <input name=status maxlength=140></label>Here, a password is given a minimum length:
<p><label>Username: <input name=u required></label>
<p><label>Password: <input name=p required minlength=12></label>size 属性size 属性给出了
在用户编辑元素的 值 的时候，用户代理允许用户看到的字符数目。
指定 size 属性时，值必须是一个大于零的
合法的非负整数。
如果指定了这个属性，就必须使用 解析非负整数的规则 解析它的值， 如果结果是一个大于零的数字，用户代理应该确保至少那么多字符可见。
size IDL 属性
仅限于大于零的非负整数，
默认值为 20。
readonly 属性Support in all current engines.
readonly 属性是一个
布尔属性 用来控制用户是否可以编辑这个表单控件。
指定后这个元素就不 可变 了。
约束验证：如果在 input 元素上指定了
readonly 属性，这个元素就
禁止约束验证。
disabled 与 readonly 之间的区别是 read-only 控件仍然运作，
但 disabled 控件通常不具有控件的功能。这在本规范的其他地方会对
disabled 的概念的形式化要求进行详细介绍。
（例如元素的 激活行为，是否是一个
可聚焦区域，或何时
构造项目列表）。
禁用控件的任何其他与用户交互相关的行为都不在本规范中定义，比如文本是否可以选择和拷贝。
只有文本控件可以做成只读，因为其他控件（比如复选框和按钮）的只读和禁用没什么区别，
所以 readonly 属性
不适用。
在下面的例子中，已经存在的产品标识不可修改，但仍然可以显示为表单的一部分， 只是为了与显示新产品（它的标识还未填写）的行保持一致。
<form action="products.cgi" method="post" enctype="multipart/form-data"> <table> <tr> <th> Product ID <th> Product name <th> Price <th> Action <tr> <td> <input readonly="readonly" name="1.pid" value="H412"> <td> <input required="required" name="1.pname" value="Floor lamp Ulke"> <td> $<input required="required" type="number" min="0" step="0.01" name="1.pprice" value="49.99"> <td> <button formnovalidate="formnovalidate" name="action" value="delete:1">Delete</button> <tr> <td> <input readonly="readonly" name="2.pid" value="FG28"> <td> <input required="required" name="2.pname" value="Table lamp Ulke"> <td> $<input required="required" type="number" min="0" step="0.01" name="2.pprice" value="24.99"> <td> <button formnovalidate="formnovalidate" name="action" value="delete:2">Delete</button> <tr> <td> <input required="required" name="3.pid" value="" pattern="[A-Z0-9]+"> <td> <input required="required" name="3.pname" value=""> <td> $<input required="required" type="number" min="0" step="0.01" name="3.pprice" value=""> <td> <button formnovalidate="formnovalidate" name="action" value="delete:3">Delete</button> </table> <p> <button formnovalidate="formnovalidate" name="action" value="add">Add</button> </p> <p> <button name="action" value="update">Save</button> </p> </form>
required 属性required 属性是一个
布尔属性。当指定时这个元素就是 必要的。
约束验证：如果这个元素是 必要的，
且它的 value IDL 属性
适用 且处于 value 模式，而且该元素是 可变的，而且该元素的 value 为空字符串，那么这个元素就
处于缺失错误状态。
下面的表单有两个必要字段，一个是邮件地址另一个是密码。 第三个字段只在用户输入与密码字段同样的密码时才有效。
<h1>Create new account</h1>
<form action="/newaccount" method=post
oninput="up2.setCustomValidity(up2.value != up.value ? 'Passwords do not match.' : '')">
<p>
<label for="username">E-mail address:</label>
<input id="username" type=email required name=un>
<p>
<label for="password1">Password:</label>
<input id="password1" type=password required name=up>
<p>
<label for="password2">Confirm password:</label>
<input id="password2" type=password name=up2>
<p>
<input type=submit value="Create account">
</form>
对于单选按钮，如果 分组 中任意一个单选按钮被选中，
required 属性就会被满足。
因此在下面的例子中，可以选中任意一个单选按钮，而不仅仅是标记为必选的那个：
<fieldset> <legend>Did the movie pass the Bechdel test?</legend> <p><label><input type="radio" name="bechdel" value="no-characters"> No, there are not even two female characters in the movie. </label> <p><label><input type="radio" name="bechdel" value="no-names"> No, the female characters never talk to each other. </label> <p><label><input type="radio" name="bechdel" value="no-topic"> No, when female characters talk to each other it's always about a male character. </label> <p><label><input type="radio" name="bechdel" value="yes" required> Yes. </label> <p><label><input type="radio" name="bechdel" value="unknown"> I don't know. </label> </fieldset>
为避免困惑 关于是否 单选按钮分组 是否是必选的， 鼓励作者在组中的所有单选按钮上都指定属性，其实通常也鼓励作者避免这种没有初始选中的单选按钮分组， 因为用户无法返回到这个状态，因此也通常被认为是糟糕的用户界面。
multiple 属性Support in all current engines.
multiple 属性是一个
布尔属性，表示是否允许用户指定多个值。
下面的片段展示了 e-mail 客户端的 "To" 字符串如何接受多个 e-mail 地址。
<label>To: <input type=email multiple name=to></label>
如果用户的联系人数据库中有这样两个朋友："Spider-Man" （地址为"spider@parker.example.net"）和 "Scarlet Witch"（地址为"scarlet@avengers.example.net"）， 在用户输入了 "s" 后，用户代理可能会给用户建议这些 e-mail 地址。
这个页面可能会链接到用户的联系人数据库：
<label>To: <input type=email multiple name=to list=contacts></label> ... <datalist id="contacts"> <option value="hedral@damowmow.com"> <option value="pillar@example.com"> <option value="astrophy@cute.example"> <option value="astronomy@science.example.org"> </datalist>
假设用户在文本控件中输入了 "bob@example.net"，然后开始键入第二个
e-mail 地址，开头是 "s"。用户代理可能会显示前面提到的两个朋友，
以及 datalist 元素给出的 "astrophy" 和 "astronomy" 两个值。
下面的片段展示了 e-mail 客户端的 "Attachments" 字段是如何接受多个文件上传的。
<label>Attachments: <input type=file multiple name=att></label>
pattern 属性pattern 属性指定了
该控件的 值 的正则表达式，
或者当设置了 multiple 属性并且输入控件 适用这个属性 时，
就要检查这个控件的值 值。
如果指定了这个属性，它的值必须匹配 JavaScript Pattern 生成式。
如果 input 元素设置了 pattern
属性，且这个属性的值可以作为 JavaScript 正则表达式（只带 "u" 标志）成功地编译，
那么结果正则表达式就是这个元素的
编译后的模式正则表达式。
如果这个元素没有这种属性，或者其属性值不能成功编译，那么这个元素就没有
编译后的模式正则表达式。[JAVASCRIPT]
如果值不能成功编译，鼓励用户代理把这个日志打到开发控制台来帮助调试。
约束验证：如果这个元素的 值
不是空字符串，或者元素没有指定 multiple 属性，
或者它 不适用 于 input 元素的 type 属性的当前状态，且这个元素有
编译后的模式正则表达式
但这个正则不匹配元素的 值，那么就说这个元素
处于模式不匹配错误状态。
约束验证：如果这个元素的 value 不是空字符串，
且这个元素指定了 multiple 属性，
且 适用于 input 元素，
且该元素有一个 编译后的模式正则表达式，
但这个正则不匹配元素的每一个值 values，
那么这个元素 处于模式不匹配错误状态。
把 编译后的模式正则表达式 与字符串进行匹配时，必须把它的开始位置对准字符串的开始位置，它的结尾对准字符串的结尾位置。
这意味着用于这个属性的正则表达式语言与用于 JavaScript 的一样，除了
pattern 属性匹配的是整个值，而不是一个子集
（有些像是在前面添加一个 ^(?:，在后面加一个 )$）
当一个 input 元素设置有 pattern 属性时，
作者应该引入一个 title 属性来描述这个模式。
如果有的话，当用户代理提示用户模式不匹配的时候，或者其他任何适当的时候，都可以使用这个属性的内容。
比如显示为一个 tooltip，或者在控件 获得焦点 时由辅助技术读出来。
例如下面的片段：
<label> Part number:
<input pattern="[0-9][A-Z]{3}" name="part"
title="A part number is a digit followed by three uppercase letters."/>
</label>
...could cause the UA to display an alert such as:
A part number is a digit followed by three uppercase letters. You cannot submit this form when the field is incorrect.
当一个控件有 pattern 属性时，如果用了 title 属性，它就必须描述这个模式。
也可以包含额外的信息，只要是帮助用户填写这个控件的。否则辅助技术将受到影响。
假如 title 属性包含控件标题， 辅助技术可能会说 你输入的文字不匹配要求的模式：生日，这根本没用。
UA 仍然可以在没错误的情况下显示 title
（例如作为悬停在空间上时的 tooltip），这样作者应该注意不要把
title 的文字写得像是已经发生了错误一样。
min 和 max 属性有些表单控件可以有显式的约束来限制用户提供的值的范围。 通常，这样的范围将是线性的并且是连续的。 表单控件可能 有周期性的范围 但最宽可能的范围是有限的， 作者可以显式地设置一个横跨边界的范围。
具体而言，最广泛的
type=time 控件最宽的范围是午夜到午夜（24小时），
作者可以设置连续线性的范围（比如9pm到11pm），也可以设置跨过午夜的不连续的范围（比如 11pm 到 1am）。
min 和
max 属性表示这个元素允许的值的范围。
它们的语法由定义 type
属性当前状态的章节定义。
如果这个元素有 min 属性，而且对
min 属性执行
字符串转换为数字算法
的结果是一个数字，那么这个数字就是该元素的 最小值；
否则，如果 type 属性的当前状态定义了 默认最小值，那么这就是 最小值；否则，这个元素就没有 最小值。
如果这个元素有 max 属性，而且对
max 属性执行
字符串转换为数字算法
的结果是一个数字，那么这个数字就是该元素的 最大值；
否则，如果 type 属性的当前状态定义了 默认最大值，那么这就是 最大值；否则，这个元素就没有 最大值。
如果这个元素没有 周期性的范围，
max 属性的值（最大值）不得小于 min 属性的值（最小值）。
如果没有 周期性的范围 的元素的 最大值 小于它的 最小值，只要这个元素有 值， 那么它或者 处于下溢出错误状态， 或者 处于上溢出错误状态。
如果元素 有周期性的范围 且它的 最大值 小于 最小值， 那么它就 有反转的范围。
如果一个元素定义了 最小值 或 最大值， 那么它就 有范围限制。
约束验证：当元素有 最小值 而且没有 反转范围，而且对元素的 value 给出的字符串执行 字符串转换为数字算法 得到的结果是一个数字， 而且得到的这个数字小于 最小值，这个元素就 处于下溢出错误状态。
约束验证：当元素有 最大值 而且没有 反转范围，而且对元素的 value 给出的字符串执行 字符串转换为数字算法 得到的结果是一个数字， 而且得到的这个数字大于 最大值，这个元素就 处于上溢出错误状态。
约束验证：当元素 有反转范围 时， 如果在该元素的 value 给出的字符串上执行 字符串转换为数字算法 得到的结果是一个数字， 而且这个数字大于 最大值 且 小于 最小值，该元素就同时 处于下溢出状态 和 上溢出状态。
下面的日期控件限制输入在 1980 年之前：
<input name=bday type=date max="1979-12-31">
下面的数字控件限制输入为大于 0 的整数：
<input name=quantity required="" type="number" min="1" value="1">
下面的时间控件限制输入为 9pm 到 6am 之间的分钟，默认为午夜：
<input name="sleepStart" type=time min="21:00" max="06:00" step="60" value="00:00">
step attributeThe step attribute
indicates the granularity that is expected (and required) of the value or values, by
limiting the allowed values. The section that defines the type attribute's current state also defines the default step, the step scale factor, and in some cases the default step base, which are used in processing the
attribute as described below.
The step attribute, if specified, must either have a
value that is a valid floating-point number that parses to a number that is greater than zero, or must have a
value that is an ASCII case-insensitive match for the string "any".
The attribute provides the allowed value step for the element, as follows:
If the attribute does not apply, then there is no allowed value step.
Otherwise, if the attribute is absent, then the allowed value step is the default step multiplied by the step scale factor.
Otherwise, if the attribute's value is an ASCII case-insensitive match for the
string "any", then there is no allowed
value step.
Otherwise, if the rules for parsing floating-point number values, when they are applied to the attribute's value, return an error, zero, or a number less than zero, then the allowed value step is the default step multiplied by the step scale factor.
Otherwise, the allowed value step is the number returned by the rules for parsing floating-point number values when they are applied to the attribute's value, multiplied by the step scale factor.
The step base is the value returned by the following algorithm:
If the element has a min content attribute, and the
result of applying the algorithm to convert a
string to a number to the value of the min content
attribute is not an error, then return that result.
If the element has a value content attribute, and
the result of applying the algorithm to convert
a string to a number to the value of the value
content attribute is not an error, then return that result.
If a default step base is defined for
this element given its type attribute's state, then return
it.
Return zero.
Constraint validation: When the element has an allowed value step, and the result of applying the algorithm to convert a string to a number to the string given by the element's value is a number, and that number subtracted from the step base is not an integral multiple of the allowed value step, the element is suffering from a step mismatch.
The following range control only accepts values in the range 0..1, and allows 256 steps in that range:
<input name=opacity type=range min=0 max=1 step=0.00392156863>The following control allows any time in the day to be selected, with any accuracy (e.g. thousandth-of-a-second accuracy or more):
<input name=favtime type=time step=any>Normally, time controls are limited to an accuracy of one minute.
list 属性list 属性用于识别
给用户列出了建议的预定义选项的元素。
如果指定了这个属性，它的值必须是同一个 树
中的 datalist 元素的 ID。
建议来源元素 是在 树 中的
按照 树序 的第一个 ID
属性等于 list 属性值的 datalist 元素。
如果没有 list 属性，或者没有这个
ID 的元素，或者第一个有这个 ID
的元素不是 datalist 元素，那就没有
建议来源元素。
如果有一个 建议来源元素，那么当用户代理
允许用户编辑这个 input 元素的 值 时，
用户代理应该把 建议来源元素
表示的建议以与控件类型相适应的方式提供给用户。
如果合适的话，用户代理应该使用建议的 label 和
value 来给用户标识建议。
如果建议很多，鼓励用户代理过滤 建议来源元素 表示的建议， （比如基于用户目前的输入）只显示最相关的那些。没有定义精确的阈值，但 4-7 个值是比较合理的。 如果是基于用户输入进行过滤，用户代理应该对建议的 label 和 value 进行子字符串匹配。
这个文本字段允许你选择一个JavaScript 函数类型。
<input type="text" list="function-types"> <datalist id="function-types"> <option value="function">function</option> <option value="async function">async function</option> <option value="function*">generator function</option> <option value="=>">arrow function</option> <option value="async =>">async arrow function</option> <option value="async function*">async generator function</option> </datalist>
对遵守以上建议的用户代理，label 和 value 可能会显示为：
然后键入 "arrow" 或 "=>" 将会把列表过滤为 "arrow function" 和 "async arrow function"。 键入 "generator" 或 "*" 将会把列表过滤为 "generator function" 和 "async generator function"。
与往常一样，用户代理可以自己决定什么样的用户界面适合特定的要求和特定的用户场景。 但这在历史上是实现方，Web 开发者，用户都容易混淆的地方， 所以我们给出了一些 "应该" 遵守的建议。
如何处理用户选择建议项取决于这个控件元素只接受单个值还是接受多个值：
multiple 属性
或者如果 multiple 属性 不适用type 属性处于 Email 状态且该元素设置了 multiple 属性当用户选择一项建议时，用户代理必须添加一个新项目到
input 元素的 值 中，
其值为选中建议项的 值，
或者把 input 元素的 值
中的既有项改为选中的建议项的 值 给出的值。
就像是用户自己添加了一项那个值，或者把既有项编辑成那个值。
应用上述哪个行为取决于用户代理定义的用户界面。
这个 URL 字段提供了一些建议。
<label>Homepage: <input name=hp type=url list=hpurls></label> <datalist id=hpurls> <option value="https://www.google.com/" label="Google"> <option value="https://www.reddit.com/" label="Reddit"> </datalist>
用户历史中的其他 URL 也可能显示；这取决于用户代理。
这个例子演示了如何使用自动完成列表功能设计一个表单，同时仍然能在旧用户代理上做有用的降级。
如果自动完成列表仅仅是一种帮助，而且对内容不重要。那么直接使用
datalist 元素和子 option 元素就足够了。
为了阻止在旧用户代理上渲染这些值，需要把它们放到
value 属性中，而不是内联。
<p> <label> Enter a breed: <input type="text" name="breed" list="breeds"> <datalist id="breeds"> <option value="Abyssinian"> <option value="Alpaca"> <!-- ... --> </datalist> </label> </p>
但如果需要在旧 UA 显示这些值，fallback 内容可以放在 datalist 元素中，像这样：
<p>
<label>
Enter a breed:
<input type="text" name="breed" list="breeds">
</label>
<datalist id="breeds">
<label>
or select one from the list:
<select name="breed">
<option value=""> (none selected)
<option>Abyssinian
<option>Alpaca
<!-- ... -->
</select>
</label>
</datalist>
</p>
fallback 内容只会在不支持 datalist 的 UA 显示。
但这些 option 会被所有 UA 检测到，即使它们不是 datalist 元素的子节点。
注意如果在 datalist 中用到的 option 元素处于
selected 状态，
它会在旧 UA 上默认选中（因为它会影响 select），
但它在支持 datalist 的 UA 上对 input 元素将不会有任何影响。
placeholder attributeElement/input#attr-placeholder
Support in all current engines.
The placeholder attribute represents a short
hint (a word or short phrase) intended to aid the user with data entry when the control has no
value. A hint could be a sample value or a brief description of the expected format. The
attribute, if specified, must have a value that contains no U+000A LINE FEED (LF) or U+000D
CARRIAGE RETURN (CR) characters.
The placeholder attribute should not be used as an
alternative to a label. For a longer hint or other advisory text, the title attribute is more appropriate.
These mechanisms are very similar but subtly different: the hint given by the
control's label is shown at all times; the short hint given in the placeholder attribute is shown before the user enters a
value; and the hint in the title attribute is shown when the user
requests further help.
User agents should present this hint to the user, after having stripped newlines from it, when the element's value is the empty string, especially if the control is not focused.
If a user agent normally doesn't show this hint to the user when the control is
focused, then the user agent should nonetheless show the hint for the control if it
was focused as a result of the autofocus attribute, since
in that case the user will not have had an opportunity to examine the control before focusing
it.
Here is an example of a mail configuration user interface that uses the placeholder attribute:
<fieldset>
<legend>Mail Account</legend>
<p><label>Name: <input type="text" name="fullname" placeholder="John Ratzenberger"></label></p>
<p><label>Address: <input type="email" name="address" placeholder="john@example.net"></label></p>
<p><label>Password: <input type="password" name="password"></label></p>
<p><label>Description: <input type="text" name="desc" placeholder="My Email Account"></label></p>
</fieldset>In situations where the control's content has one directionality but the placeholder needs to have a different directionality, Unicode's bidirectional-algorithm formatting characters can be used in the attribute value:
<input name=t1 type=tel placeholder="‫ رقم الهاتف 1 ‮">
<input name=t2 type=tel placeholder="‫ رقم الهاتف 2 ‮">For slightly more clarity, here's the same example using numeric character references instead of inline Arabic:
<input name=t1 type=tel placeholder="‫رقم الهاتف 1‮">
<input name=t2 type=tel placeholder="‫رقم الهاتف 2‮">input element APIsvalue [ = value ]Returns the current value of the form control.
Can be set, to change the value.
Throws an "InvalidStateError" DOMException if it is
set to any value other than the empty string when the control is a file upload control.
checked [ = value ]Returns the current checkedness of the form control.
Can be set, to change the checkedness.
files [ = files ]Support in all current engines.
Returns a FileList object listing the selected files of the form control.
Returns null if the control isn't a file control.
Can be set to a FileList object to change the selected files of the form control. For
instance, as the result of a drag-and-drop operation.
valueAsDate [ = value ]Returns a Date object representing the form control's value, if applicable; otherwise, returns null.
Can be set, to change the value.
Throws an "InvalidStateError" DOMException if the
control isn't date- or time-based.
valueAsNumber [ = value ]Returns a number representing the form control's value, if applicable; otherwise, returns NaN.
Can be set, to change the value. Setting this to NaN will set the underlying value to the empty string.
Throws an "InvalidStateError" DOMException if the
control is neither date- or time-based nor numeric.
stepUp( [ n ] )stepDown( [ n ] )Changes the form control's value by the value given in
the step attribute, multiplied by n.
The default value for n is 1.
Throws "InvalidStateError" DOMException if the control
is neither date- or time-based nor numeric, or if the step
attribute's value is "any".
listReturns the datalist element indicated by the list attribute.
The value IDL
attribute allows scripts to manipulate the value of an
input element. The attribute is in one of the following modes, which define its
behavior:
On getting, return the current value of the element.
On setting:
Let oldValue be the element's value.
Set the element's value to the new value.
Set the element's dirty value flag to true.
Invoke the value sanitization algorithm, if the element's type attribute's current state defines one.
If the element's value (after applying the
value sanitization algorithm) is different from oldValue, and the
element has a text entry cursor position,
move the text entry cursor position to the
end of the text control, unselecting any selected text and resetting the selection direction to "none".
On getting, if the element has a value content
attribute, return that attribute's value; otherwise, return the empty string.
On setting, set the value of the element's value
content attribute to the new value.
On getting, if the element has a value content
attribute, return that attribute's value; otherwise, return the string "on".
On setting, set the value of the element's value
content attribute to the new value.
On getting, return the string "C:\fakepath\" followed by the name of
the first file in the list of selected
files, if any, or the empty string if the list is empty.
On setting, if the new value is the empty string, empty the list of selected files; otherwise, throw an
"InvalidStateError" DOMException.
This "fakepath" requirement is a sad accident of history. See the example in the File Upload state section for more information.
Since path components are not permitted in file names in the list of selected files, the "\fakepath\" cannot be mistaken for a path component.
The checked
IDL attribute allows scripts to manipulate the checkedness of an input element. On getting, it
must return the current checkedness of the element; and
on setting, it must set the element's checkedness to the
new value and set the element's dirty checkedness
flag to true.
The files IDL
attribute allows scripts to access the element's selected files.
On getting, if the IDL attribute applies, it must
return a FileList object that represents the current selected files. The same object must be returned
until the list of selected files changes.
If the IDL attribute does not apply, then it must instead
return null. [FILEAPI]
On setting, it must run these steps:
If the IDL attribute does not apply or the given value is null, then return.
Replace the element's selected files with the given value.
The valueAsDate IDL attribute represents
the value of the element, interpreted as a date.
On getting, if the valueAsDate attribute does not apply, as defined for the input element's type attribute's current state, then return null. Otherwise, run
the algorithm to convert a string to a
Date object defined for that state to the element's value; if the algorithm returned a Date object, then
return it, otherwise, return null.
On setting, if the valueAsDate attribute does not apply, as defined for the input element's type attribute's current state, then throw an
"InvalidStateError" DOMException; otherwise, if the new
value is not null and not a Date object throw a TypeError exception;
otherwise if the new value is null or a Date object representing the NaN time value,
then set the value of the element to the empty string;
otherwise, run the algorithm to convert a
Date object to a string, as defined for that state, on the new value, and set
the value of the element to the resulting string.
The valueAsNumber IDL attribute
represents the value of the element, interpreted as a
number.
On getting, if the valueAsNumber attribute does not apply, as defined for the input element's type attribute's current state, then return a Not-a-Number (NaN)
value. Otherwise, run the algorithm to convert a
string to a number defined for that state to the element's value; if the algorithm returned a number, then return it,
otherwise, return a Not-a-Number (NaN) value.
On setting, if the new value is infinite, then throw a TypeError exception.
Otherwise, if the valueAsNumber attribute does not apply, as defined for the input element's type attribute's current state, then throw an
"InvalidStateError" DOMException. Otherwise, if the new
value is a Not-a-Number (NaN) value, then set the value of
the element to the empty string. Otherwise, run the algorithm to convert a number to a string, as
defined for that state, on the new value, and set the value
of the element to the resulting string.
The stepDown(n) and stepUp(n) methods, when invoked,
must run the following algorithm:
If the stepDown() and stepUp() methods do not apply, as defined for the
input element's type attribute's current state,
then throw an "InvalidStateError" DOMException.
If the element has no allowed value step, then
throw an "InvalidStateError" DOMException.
If the element has a minimum and a maximum and the minimum is greater than the maximum, then return.
If the element has a minimum and a maximum and there is no value greater than or equal to the element's minimum and less than or equal to the element's maximum that, when subtracted from the step base, is an integral multiple of the allowed value step, then return.
If applying the algorithm to convert a string to a number to the string given by the element's value does not result in an error, then let value be the result of that algorithm. Otherwise, let value be zero.
Let valueBeforeStepping be value.
If value subtracted from the step
base is not an integral multiple of the allowed value
step, then set value to the nearest value that, when subtracted from
the step base, is an integral multiple of the allowed value step, and that is less than value if
the method invoked was the stepDown() method, and more
than value otherwise.
Otherwise (value subtracted from the step base is an integral multiple of the allowed value step):
Let n be the argument.
Let delta be the allowed value step multiplied by n.
If the method invoked was the stepDown() method,
negate delta.
Let value be the result of adding delta to value.
If the element has a minimum, and value is less than that minimum, then set value to the smallest value that, when subtracted from the step base, is an integral multiple of the allowed value step, and that is more than or equal to minimum.
If the element has a maximum, and value is greater than that maximum, then set value to the largest value that, when subtracted from the step base, is an integral multiple of the allowed value step, and that is less than or equal to maximum.
If either the method invoked was the stepDown()
method and value is greater than valueBeforeStepping, or the method
invoked was the stepUp() method and value is
less than valueBeforeStepping, then return.
Let value as string be the result of running the algorithm to convert a number to a string, as
defined for the input element's type
attribute's current state, on value.
Set the value of the element to value as string.
The list IDL
attribute must return the current suggestions source
element, if any, or null otherwise.
当 input 和 change
事件 适用 时
（也就是除 按钮 和
type 属性处于 状态的控件之外的 input 控件），
这些事件的触发表示用户已经在与控件进行交互。
只要用户修改了控件的数据就会触发 input 事件
当编辑的值提交时，如果这个值对该控件有效或者该控件
失去了焦点
就会触发 change 事件。
所有情况下 input 事件都会在对应的
change 事件（如果有）前到达。
当 input 元素定义有 输入激活行为，
在上一章定义 type 属性的状态时，给出了
分发这些事件的规则（如果 适用 的话），
（这适用于 type 属性处于
Checkbox 状态，
Radio Button 状态，或者
File Upload 状态的所有
input 控件。）
对没有定义 输入激活行为 的 input 元素，
如果这些事件 适用，
而且涉及交互式操作和显式的提交动作的用户交互，那么当用户改变元素的
值 时，用户代理必须
入队一个任务 来在 input 元素上
触发一个 名为 input 的事件，其 bubbles
属性初始化为 true，只要用户提交了改动，用户代理就必须
入队一个任务 来在 input 元素上
触发一个 名为 change
的事件，其 bubbles 属性初始化为 true。
一个涉及交互操作和提交动作的用户界面的例子是，
使用指点设备操作一个有滑块的 Range 控件。
当用户拖动控件的滑块时，只要位置发生改变就会触发 input 事件，
而 change 事件只会在用户放开滑块时触发，
这时指定的值才会提交。
对于没有定义 输入激活行为 的
input 元素，如果这些事件 适用，
而且用户界面涉及一个显式的提交动作但没有间接的交互，那么只要用户给该元素的
value 提交了改动，用户代理就必须
入队一个任务 来首先在 input 元素上
触发一个 名为
input 的事件，其 bubbles
属性初始为 true，然后在 input 元素上 触发一个 名为
change 的事件，其 bubbles
属性初始为 true。
一个涉及提交动作的用户界面的例子是 Color 控件， 它只有一个打开色盘的按钮：如果 value 只会在对话框关闭时发生改变， 那么这就是显式的提交动作。另一方面，如果是交互式地操作控件来改变颜色，那就可能没有提交动作了。
另一个有提交动作的用户界面例子是允许用户文本输入和从下拉日历中选择的 Date 控件： 文本输入可能没有一个显式的提交步骤，但从下拉日历中选择并关闭下拉菜单就是一个提交动作。
对于没有定义 输入激活行为 的
input 元素，如果这些事件 适用，
只要用户在没有显式的提交动作的情况下改变了元素的 值，
用户代理就必须 入队一个任务 来在 input 元素上
触发一个 名为 input 的事件，
其 bubbles 属性初始为 true。
在控件 失去焦点 时，将会触发对应的
change 事件（如果有）。
用户改变元素的 值 的例子包括， 用户在文本控件总输入，粘贴新的值到控件中，或者在那个控件中撤销编辑。 有些用户交互不会改变值，例如在空的文本控件中按下删除键， 或粘贴替换的文本恰好和控件中的文本相同。
对于有滑块的 Range 控件， 如果用户已经 获得焦点 并正在用键盘与之交互， 这将是另一个不经过提交步骤改变元素 值 的例子。
在只触发一个 input 事件的
任务 中，用户代理可以在用户交互后等待适当的时间，
再 入队 任务；
例如，用户代理可以等待用户 100ms 不敲键，这样就能只在用户输入间歇时触发，而不是每次敲键都触发。
如果用户代理要为用户改变 input 元素的 值 （比如作为表单预填充特性的一部分），
用户代理必须 入队一个任务 来首先适当地更新 值，然后在这个 input 元素上
触发一个 名为 input 的事件，
其 bubbles 属性初始化为 true，然后在 input 元素上
触发一个 名为 change 的事件，
其 bubbles 属性初始化为 true。
这些事件不会因为脚本改变表单控件的值而触发。 （这是为了让用户操作下更新表单控件的值更容易，这样就不需要过滤掉脚本自行改的情况以避免死循环。）
button elementSupport in all current engines.
Support in all current engines.
tabindex attribute
specified.disabled — Whether the form control is disabledform — Associates the element with a form elementformaction — URL to use for form submissionformenctype — Entry list encoding type to use for form submissionformmethod — Variant to use for form submissionformnovalidate — Bypass form control validation for form submissionformtarget — Browsing context for form submissionname — Name of the element to use for form submission and in the form.elements API type — Type of buttonvalue — Value to be used for form submission[Exposed=Window]
interface HTMLButtonElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute boolean disabled;
readonly attribute HTMLFormElement? form;
[CEReactions] attribute USVString formAction;
[CEReactions] attribute DOMString formEnctype;
[CEReactions] attribute DOMString formMethod;
[CEReactions] attribute boolean formNoValidate;
[CEReactions] attribute DOMString formTarget;
[CEReactions] attribute DOMString name;
[CEReactions] attribute DOMString type;
[CEReactions] attribute DOMString value;
readonly attribute boolean willValidate;
readonly attribute ValidityState validity;
readonly attribute DOMString validationMessage;
boolean checkValidity();
boolean reportValidity();
undefined setCustomValidity(DOMString error);
readonly attribute NodeList labels;
};The button element represents a button labeled by its contents.
The element is a button.
The type attribute
controls the behavior of the button when it is activated. It is an enumerated
attribute. The following table lists the keywords and states for the attribute — the
keywords in the left column map to the states in the cell in the second column on the same row as
the keyword.
| Keyword | State | Brief description |
|---|---|---|
submit
| Submit Button | Submits the form. |
reset
| Reset Button | Resets the form. |
button
| Button | Does nothing. |
The missing value default and invalid value default are the Submit Button state.
If the type attribute is in the Submit Button state, the element is specifically a
submit button.
Constraint validation: If the type
attribute is in the Reset Button state or the
Button state, the element is barred from
constraint validation.
A button element's activation behavior is to run the steps defined in
the following list for the current state of this element's type attribute, if this element is not disabled, and do nothing otherwise:
If the element has a form owner and the element's node document is
fully active, the element must submit the
form owner from the button element.
If the element has a form owner and the element's node document is fully active, the element must reset the form owner.
Do nothing.
The form attribute is used to explicitly associate the
button element with its form owner. The name attribute represents the element's name. The disabled attribute is used to make the control non-interactive and
to prevent its value from being submitted. The formaction,
formenctype, formmethod, formnovalidate, and formtarget attributes are attributes for form
submission.
The formnovalidate attribute can be
used to make submit buttons that do not trigger the constraint validation.
The formaction, formenctype, formmethod, formnovalidate, and formtarget must not be specified if the element's type attribute is not in the Submit Button state.
The value
attribute gives the element's value for the purposes of form submission. The element's value is the value of the element's value attribute, if there is one, or the empty string
otherwise.
A button (and its value) is only included in the form submission if the button itself was used to initiate the form submission.
The value
IDL attribute must reflect the content attribute of the same name.
The type IDL
attribute must reflect the content attribute of the same name, limited to only
known values.
The willValidate, validity, and validationMessage IDL attributes, and the checkValidity(), reportValidity(), and setCustomValidity() methods, are part of the
constraint validation API. The labels IDL
attribute provides a list of the element's labels. The disabled, form, and
name IDL attributes are part of the element's forms API.
The following button is labeled "Show hint" and pops up a dialog box when activated:
<button type=button
onclick="alert('This 15-20 minute piece was composed by George Gershwin.')">
Show hint
</button>select elementSupport in all current engines.
Support in all current engines.
option, optgroup, and script-supporting elements.autocomplete — Hint for form autofill featuredisabled — Whether the form control is disabledform — Associates the element with a form elementmultiple — Whether to allow multiple valuesname — Name of the element to use for form submission and in the form.elements API required — Whether the control is required for form submissionsize — Size of the controlmultiple attribute or a size attribute with a value > 1: for authors; for implementers.[Exposed=Window]
interface HTMLSelectElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString autocomplete;
[CEReactions] attribute boolean disabled;
readonly attribute HTMLFormElement? form;
[CEReactions] attribute boolean multiple;
[CEReactions] attribute DOMString name;
[CEReactions] attribute boolean required;
[CEReactions] attribute unsigned long size;
readonly attribute DOMString type;
[SameObject] readonly attribute HTMLOptionsCollection options;
[CEReactions] attribute unsigned long length;
getter Element? item(unsigned long index);
HTMLOptionElement? namedItem(DOMString name);
[CEReactions] undefined add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
[CEReactions] undefined remove(); // ChildNode overload
[CEReactions] undefined remove(long index);
[CEReactions] setter undefined (unsigned long index, HTMLOptionElement? option);
[SameObject] readonly attribute HTMLCollection selectedOptions;
attribute long selectedIndex;
attribute DOMString value;
readonly attribute boolean willValidate;
readonly attribute ValidityState validity;
readonly attribute DOMString validationMessage;
boolean checkValidity();
boolean reportValidity();
undefined setCustomValidity(DOMString error);
readonly attribute NodeList labels;
};The select element represents a control for selecting amongst a set of
options.
The multiple
attribute is a boolean attribute. If the attribute is present, then the
select element represents a control for selecting zero or more options
from the list of options. If the attribute is
absent, then the select element represents a control for selecting a
single option from the list of options.
The size attribute
gives the number of options to show to the user. The size
attribute, if specified, must have a value that is a valid non-negative integer
greater than zero.
The display size of a select element is the
result of applying the rules for parsing non-negative integers to the value of
element's size attribute, if it has one and parsing it is
successful. If applying those rules to the attribute's value is not successful, or if the size attribute is absent, then the element's display size is 4 if the element's multiple content attribute is present, and 1 otherwise.
The list of options for a select
element consists of all the option element children of the select
element, and all the option element children of all the optgroup element
children of the select element, in tree order.
The required
attribute is a boolean attribute. When specified, the user will be required to select
a value before submitting the form.
If a select element has a required
attribute specified, does not have a multiple attribute
specified, and has a display size of 1; and if the value of the first option element in the
select element's list of options (if
any) is the empty string, and that option element's parent node is the
select element (and not an optgroup element), then that
option is the select element's placeholder label option.
If a select element has a required
attribute specified, does not have a multiple attribute
specified, and has a display size of 1, then the
select element must have a placeholder label option.
In practice, the requirement stated in the paragraph above can only apply when a
select element does not have a size attribute
with a value greater than 1.
Constraint validation: If the element has its required attribute specified, and either none of the
option elements in the select element's list of options have their selectedness set to true, or the only
option element in the select element's list of options with its selectedness set to true is the placeholder label
option, then the element is suffering from being missing.
If the multiple attribute is absent, and the element
is not disabled, then the user agent should allow the
user to pick an option element in its list
of options that is itself not disabled. Upon
this option element being picked (either
through a click, or through unfocusing the element after changing its value, or through a menu command, or through any other mechanism), and before the
relevant user interaction event is queued (e.g. before the
click event), the user agent must set the selectedness of the picked option element
to true, set its dirtiness to true, and then
send select update notifications.
If the multiple attribute is absent, whenever an
option element in the select element's list of options has its selectedness set to true, and whenever an
option element with its selectedness set to true is added to the
select element's list of options,
the user agent must set the selectedness of all
the other option elements in its list of
options to false.
If the multiple attribute is absent and the
element's display size is greater than 1, then the user
agent should also allow the user to request that the option whose selectedness is true, if any, be unselected. Upon this
request being conveyed to the user agent, and before the relevant user interaction event is queued (e.g. before the click event), the user agent must set the selectedness of that option element to
false, set its dirtiness to true, and then
send select update notifications.
If nodes are inserted or nodes are removed causing the list of options to gain or lose one or more
option elements, or if an option element in the list of options asks for
a reset, then, if the select element's multiple attribute is absent, the user agent must run the
first applicable set of steps from the following list:
select
element's display size is 1, and no option
elements in the select element's list of
options have their selectedness set to
trueSet the selectedness of the first
option element in the list of
options in tree order that is not disabled, if any, to true.
option elements in the select element's list of options have their selectedness set to trueSet the selectedness of all but the last
option element with its selectedness set to true in the list of options in tree order to
false.
If the multiple attribute is present, and the
element is not disabled, then the user agent should
allow the user to toggle the selectedness of the option elements in
its list of options that are themselves not disabled. Upon such an element being toggled (either through a click, or through a menu command, or any other mechanism), and before the relevant user
interaction event is queued (e.g. before a related click event), the selectedness of the option element must
be changed (from true to false or false to true), the dirtiness of the element must be set to true, and the
user agent must send select update notifications.
When the user agent is to send select update notifications, queue
an element task on the user interaction task source given the select
element to run these steps:
Fire an event named input at the select element, with the bubbles and composed
attributes initialized to true.
Fire an event named change at the select element, with the bubbles attribute initialized to true.
The reset algorithm for select
elements is to go through all the option elements in the element's list of options, set their selectedness to true if the option
element has a selected attribute, and false otherwise,
set their dirtiness to false, and then have the
option elements ask for a reset.
The form attribute is used to explicitly associate the select element with its form owner.
The name attribute represents the element's name.
The disabled attribute is used to make the control non-interactive and to prevent its value from being submitted.
The autocomplete attribute controls how the user agent provides autofill behavior.
A select element that is not disabled is
mutable.
typeSupport in all current engines.
Returns "select-multiple" if the element has a multiple attribute, and "select-one"
otherwise.
optionsSupport in all current engines.
Returns an HTMLOptionsCollection of the list of options.
length [ = value ]Returns the number of elements in the list of options.
When set to a smaller number, truncates the number of option elements in the
select.
When set to a greater number, adds new blank option elements to the
select.
item(index)Support in all current engines.
Returns the item with index index from the list of options. The items are sorted in tree order.
namedItem(name)Support in all current engines.
Returns the first item with ID or name name from the list of options.
Returns null if no element with that ID could be found.
add(element [, before ] )Support in all current engines.
Inserts element before the node given by before.
The before argument can be a number, in which case element is inserted before the item with that number, or an element from the list of options, in which case element is inserted before that element.
If before is omitted, null, or a number out of range, then element will be added at the end of the list.
This method will throw a "HierarchyRequestError" DOMException if
element is an ancestor of the element into which it is to be inserted.
selectedOptionsHTMLSelectElement/selectedOptions
Support in all current engines.
Returns an HTMLCollection of the list
of options that are selected.
selectedIndex [ = value ]HTMLSelectElement/selectedIndex
Support in all current engines.
Returns the index of the first selected item, if any, or −1 if there is no selected item.
Can be set, to change the selection.
value [ = value ]Returns the value of the first selected item, if any, or the empty string if there is no selected item.
Can be set, to change the selection.
The type IDL
attribute, on getting, must return the string "select-one" if the multiple attribute is absent, and the string "select-multiple" if the multiple
attribute is present.
The options IDL attribute must return an
HTMLOptionsCollection rooted at the select node, whose filter matches
the elements in the list of options.
The options collection is also mirrored on the
HTMLSelectElement object. The supported property indices at any instant
are the indices supported by the object returned by the options attribute at that instant.
The length
IDL attribute must return the number of nodes represented by the options collection.
On setting, it must act like the attribute of the same name on the options collection.
The item(index) method must return the value returned
by the method of the same name on the options collection, when invoked with the same argument.
The namedItem(name)
method must return the value returned by the
method of the same name on the options collection,
when invoked with the same argument.
When the user agent is to set the value of a new indexed
property or set the value of an existing indexed property for a
select element, it must instead run the
corresponding algorithm on the select element's options collection.
Similarly, the add() method must act like its namesake method on that same
options collection.
Support in all current engines.
The remove()
method must act like its namesake method on that same options collection when it has arguments, and like its namesake
method on the ChildNode interface implemented by the HTMLSelectElement
ancestor interface Element when it has no arguments.
The selectedOptions IDL attribute
must return an HTMLCollection rooted at the select node, whose filter
matches the elements in the list of options that
have their selectedness set to true.
The selectedIndex IDL attribute, on
getting, must return the index of the first
option element in the list of
options in tree order that has its selectedness set to true, if any. If there isn't one,
then it must return −1.
On setting, the selectedIndex attribute must set
the selectedness of all the option
elements in the list of options to false, and
then the option element in the list of
options whose index is the given new value, if
any, must have its selectedness set to true and
its dirtiness set to true.
This can result in no element having a selectedness set to true even in the case of the
select element having no multiple
attribute and a display size of 1.
The value
IDL attribute, on getting, must return the value of the
first option element in the list of
options in tree order that has its selectedness set to true, if any. If there isn't one,
then it must return the empty string.
On setting, the value attribute must set the selectedness of all the option elements
in the list of options to false, and then the
first option element in the list of
options, in tree order, whose value
is equal to the given new value, if any, must have its selectedness set to true and its dirtiness set to true.
This can result in no element having a selectedness set to true even in the case of the
select element having no multiple
attribute and a display size of 1.
The multiple, required, and size IDL attributes must
reflect the respective content attributes of the same name. The size IDL attribute has a default value of zero.
For historical reasons, the default value of the size IDL attribute does not return the actual size used, which, in
the absence of the size content attribute, is either 1 or 4
depending on the presence of the multiple
attribute.
The willValidate, validity, and validationMessage IDL attributes, and the checkValidity(), reportValidity(), and setCustomValidity() methods, are part of the
constraint validation API. The labels IDL
attribute provides a list of the element's labels. The disabled, form, and
name IDL attributes are part of the element's forms API.
The following example shows how a select element can be used to offer the user
with a set of options from which the user can select a single option. The default option is
preselected.
<p>
<label for="unittype">Select unit type:</label>
<select id="unittype" name="unittype">
<option value="1"> Miner </option>
<option value="2"> Puffer </option>
<option value="3" selected> Snipey </option>
<option value="4"> Max </option>
<option value="5"> Firebot </option>
</select>
</p>When there is no default option, a placeholder can be used instead:
<select name="unittype" required>
<option value=""> Select unit type </option>
<option value="1"> Miner </option>
<option value="2"> Puffer </option>
<option value="3"> Snipey </option>
<option value="4"> Max </option>
<option value="5"> Firebot </option>
</select>Here, the user is offered a set of options from which they can select any number. By default, all five options are selected.
<p>
<label for="allowedunits">Select unit types to enable on this map:</label>
<select id="allowedunits" name="allowedunits" multiple>
<option value="1" selected> Miner </option>
<option value="2" selected> Puffer </option>
<option value="3" selected> Snipey </option>
<option value="4" selected> Max </option>
<option value="5" selected> Firebot </option>
</select>
</p>Sometimes, a user has to select one or more items. This example shows such an interface.
<label>
Select the songs from that you would like on your Act II Mix Tape:
<select multiple required name="act2">
<option value="s1">It Sucks to Be Me (Reprise)
<option value="s2">There is Life Outside Your Apartment
<option value="s3">The More You Ruv Someone
<option value="s4">Schadenfreude
<option value="s5">I Wish I Could Go Back to College
<option value="s6">The Money Song
<option value="s7">School for Monsters
<option value="s8">The Money Song (Reprise)
<option value="s9">There's a Fine, Fine Line (Reprise)
<option value="s10">What Do You Do With a B.A. in English? (Reprise)
<option value="s11">For Now
</select>
</label>datalist elementSupport in all current engines.
Support in all current engines.
option and script-supporting elements.[Exposed=Window]
interface HTMLDataListElement : HTMLElement {
[HTMLConstructor] constructor();
[SameObject] readonly attribute HTMLCollection options;
};The datalist element represents a set of option elements that
represent predefined options for other controls. In the rendering, the datalist
element represents nothing and it, along with its children, should
be hidden.
The datalist element can be used in two ways. In the simplest case, the
datalist element has just option element children.
<label>
Animal:
<input name=animal list=animals>
<datalist id=animals>
<option value="Cat">
<option value="Dog">
</datalist>
</label>In the more elaborate case, the datalist element can be given contents that are to
be displayed for down-level clients that don't support datalist. In this case, the
option elements are provided inside a select element inside the
datalist element.
<label>
Animal:
<input name=animal list=animals>
</label>
<datalist id=animals>
<label>
or select from the list:
<select name=animal>
<option value="">
<option>Cat
<option>Dog
</select>
</label>
</datalist>The datalist element is hooked up to an input element using the list attribute on the input element.
Each option element that is a descendant of the datalist element,
that is not disabled, and whose value is a string that isn't the empty string, represents a
suggestion. Each suggestion has a value and a label.
optionsReturns an HTMLCollection of the option elements of the
datalist element.
The options IDL attribute must return an
HTMLCollection rooted at the datalist node, whose filter matches
option elements.
Constraint validation: If an element has a datalist element
ancestor, it is barred from constraint validation.
optgroup elementSupport in all current engines.
Support in all current engines.
select element.option and script-supporting elements.optgroup element's end tag can be omitted
if the optgroup element is
immediately followed by another optgroup element, or if there is no more content in
the parent element.disabled — Whether the form control is disabledlabel — User-visible label[Exposed=Window]
interface HTMLOptGroupElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute boolean disabled;
[CEReactions] attribute DOMString label;
};The optgroup element represents a group of option
elements with a common label.
The element's group of option elements consists of the option
elements that are children of the optgroup element.
When showing option elements in select elements, user agents should
show the option elements of such groups as being related to each other and separate
from other option elements.
The disabled attribute is a boolean
attribute and can be used to disable a group
of option elements together.
The label
attribute must be specified. Its value gives the name of the group, for the purposes of the user
interface. User agents should use this attribute's value when labeling the group of
option elements in a select element.
The disabled and label attributes must
reflect the respective content attributes of the same name.
There is no way to select an optgroup element. Only
option elements can be selected. An optgroup element merely provides a
label for a group of option elements.
The following snippet shows how a set of lessons from three courses could be offered in a
select drop-down widget:
<form action="courseselector.dll" method="get">
<p>Which course would you like to watch today?
<p><label>Course:
<select name="c">
<optgroup label="8.01 Physics I: Classical Mechanics">
<option value="8.01.1">Lecture 01: Powers of Ten
<option value="8.01.2">Lecture 02: 1D Kinematics
<option value="8.01.3">Lecture 03: Vectors
<optgroup label="8.02 Electricity and Magnestism">
<option value="8.02.1">Lecture 01: What holds our world together?
<option value="8.02.2">Lecture 02: Electric Field
<option value="8.02.3">Lecture 03: Electric Flux
<optgroup label="8.03 Physics III: Vibrations and Waves">
<option value="8.03.1">Lecture 01: Periodic Phenomenon
<option value="8.03.2">Lecture 02: Beats
<option value="8.03.3">Lecture 03: Forced Oscillations with Damping
</select>
</label>
<p><input type=submit value="▶ Play">
</form>option elementSupport in all current engines.
Support in all current engines.
select element.datalist element.optgroup element.label attribute and a value attribute: Nothing.label attribute but no value attribute: Text.label attribute and is not a
child of a datalist element: Text that is not
inter-element whitespace.label attribute and is a child
of a datalist element: Text.option element's end tag can be omitted if
the option element is immediately followed by another option element, or
if it is immediately followed by an optgroup element, or if there is no more content
in the parent element.disabled — Whether the form control is disabledlabel — User-visible labelselected — Whether the option is selected by defaultvalue — Value to be used for form submission[Exposed=Window,
LegacyFactoryFunction=Option(optional DOMString text = "", optional DOMString value, optional boolean defaultSelected = false, optional boolean selected = false)]
interface HTMLOptionElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute boolean disabled;
readonly attribute HTMLFormElement? form;
[CEReactions] attribute DOMString label;
[CEReactions] attribute boolean defaultSelected;
attribute boolean selected;
[CEReactions] attribute DOMString value;
[CEReactions] attribute DOMString text;
readonly attribute long index;
};The option element represents an option in a select
element or as part of a list of suggestions in a datalist element.
In certain circumstances described in the definition of the select element, an
option element can be a select element's placeholder label
option. A placeholder label option does not represent an actual option, but
instead represents a label for the select control.
The disabled
attribute is a boolean attribute. An option element is disabled if its disabled attribute is present or if it is a child of an
optgroup element whose disabled
attribute is present.
An option element that is disabled must
prevent any click events that are queued on the user interaction task source from being dispatched on the
element.
The label
attribute provides a label for element. The label of an
option element is the value of the label
content attribute, if there is one and its value is not the empty string, or, otherwise, the value
of the element's text IDL attribute.
The label content attribute, if specified, must not be
empty.
The value
attribute provides a value for element. The value of an
option element is the value of the value
content attribute, if there is one, or, if there is not, the value of the element's text IDL attribute.
The selected
attribute is a boolean attribute. It represents the default selectedness of the element.
The dirtiness of an option element is
a boolean state, initially false. It controls whether adding or removing the selected content attribute has any effect.
The selectedness of an option
element is a boolean state, initially false. Except where otherwise specified, when the element is
created, its selectedness must be set to true if
the element has a selected attribute. Whenever an
option element's selected attribute is
added, if its dirtiness is false, its selectedness must be set to true. Whenever an
option element's selected attribute is
removed, if its dirtiness is false, its
selectedness must be set to false.
The Option() constructor, when called with three
or fewer arguments, overrides the initial state of the selectedness state to always be false even if the third
argument is true (implying that a selected attribute is
to be set). The fourth argument can be used to explicitly set the initial selectedness state when using the constructor.
A select element whose multiple
attribute is not specified must not have more than one descendant option element with
its selected attribute set.
An option element's index is the number of
option elements that are in the same list of
options but that come before it in tree order. If the option
element is not in a list of options, then the
option element's index is zero.
selectedReturns true if the element is selected, and false otherwise.
Can be set, to override the current state of the element.
indexReturns the index of the element in its select element's options list.
formReturns the element's form element, if any, or null otherwise.
textSame as textContent, except that spaces are collapsed and script elements are skipped.
Option( [ text [, value [, defaultSelected [, selected ] ] ] ] )Support in all current engines.
Returns a new option element.
The text argument sets the contents of the element.
The value argument sets the value
attribute.
The defaultSelected argument sets the selected attribute.
The selected argument sets whether or not the element is selected. If it is omitted, even if the defaultSelected argument is true, the element is not selected.
The disabled IDL attribute must reflect the
content attribute of the same name. The defaultSelected IDL attribute must
reflect the selected content
attribute.
The label
IDL attribute, on getting, if there is a label content
attribute, must return that attribute's value; otherwise, it must return the element's label. On setting, the element's label content attribute must be set to the new value.
The value
IDL attribute, on getting, must return the element's value. On setting, the element's value content attribute must be set to the new value.
The selected IDL attribute, on getting, must return true if
the element's selectedness is true, and false
otherwise. On setting, it must set the element's selectedness to the new value, set its dirtiness to true, and then cause the element to
ask for a reset.
The index
IDL attribute must return the element's index.
The text IDL
attribute, on getting, must return the result of stripping and collapsing ASCII whitespace from the concatenation of data of all the Text node descendants of the
option element, in tree order, excluding any that are descendants of
descendants of the option element that are themselves script or
SVG script elements.
The text attribute's setter must string replace
all with the given value within this element.
The form IDL
attribute's behavior depends on whether the option element is in a
select element or not. If the option has a select element
as its parent, or has an optgroup element as its parent and that
optgroup element has a select element as its parent, then the form IDL attribute must return the same value as the form IDL attribute on that select element. Otherwise, it
must return null.
A legacy factory function is provided for creating HTMLOptionElement objects (in
addition to the factory methods from DOM such as createElement()): Option(text, value, defaultSelected,
selected). When invoked, the legacy factory function must perform the
following steps:
Let document be the current global object's associated Document.
Let option be the result of creating an
element given document, option, and the HTML
namespace.
If text is not the empty string, then append to option a new
Text node whose data is text.
If value is given, then set
an attribute value for option using "value" and value.
If defaultSelected is true, then set an attribute value for option
using "selected" and the empty string.
If selected is true, then set option's selectedness to true; otherwise set its selectedness to false (even if defaultSelected is true).
Return option.
textarea elementSupport in all current engines.
Support in all current engines.
autocomplete — Hint for form autofill featurecols — Maximum number of characters per linedirname — Name of form control to use for sending the element's directionality in form submissiondisabled — Whether the form control is disabledform — Associates the element with a form elementmaxlength — Maximum length of valueminlength — Minimum length of valuename — Name of the element to use for form submission and in the form.elements API placeholder — User-visible label to be placed within the form controlreadonly — Whether to allow the value to be edited by the userrequired — Whether the control is required for form submissionrows — Number of lines to showwrap — How the value of the form control is to be wrapped for form submission[Exposed=Window]
interface HTMLTextAreaElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString autocomplete;
[CEReactions] attribute unsigned long cols;
[CEReactions] attribute DOMString dirName;
[CEReactions] attribute boolean disabled;
readonly attribute HTMLFormElement? form;
[CEReactions] attribute long maxLength;
[CEReactions] attribute long minLength;
[CEReactions] attribute DOMString name;
[CEReactions] attribute DOMString placeholder;
[CEReactions] attribute boolean readOnly;
[CEReactions] attribute boolean required;
[CEReactions] attribute unsigned long rows;
[CEReactions] attribute DOMString wrap;
readonly attribute DOMString type;
[CEReactions] attribute DOMString defaultValue;
attribute [LegacyNullToEmptyString] DOMString value;
readonly attribute unsigned long textLength;
readonly attribute boolean willValidate;
readonly attribute ValidityState validity;
readonly attribute DOMString validationMessage;
boolean checkValidity();
boolean reportValidity();
undefined setCustomValidity(DOMString error);
readonly attribute NodeList labels;
undefined select();
attribute unsigned long selectionStart;
attribute unsigned long selectionEnd;
attribute DOMString selectionDirection;
undefined setRangeText(DOMString replacement);
undefined setRangeText(DOMString replacement, unsigned long start, unsigned long end, optional SelectionMode selectionMode = "preserve");
undefined setSelectionRange(unsigned long start, unsigned long end, optional DOMString direction);
};The textarea element represents a multiline plain text edit
control for the element's raw
value. The contents of the control represent the control's default value.
The raw value of a textarea
control must be initially the empty string.
This element has rendering requirements involving the bidirectional algorithm.
The readonly attribute is a boolean
attribute used to control whether the text can be edited by the user or not.
In this example, a text control is marked read-only because it represents a read-only file:
Filename: <code>/etc/bash.bashrc</code>
<textarea name="buffer" readonly>
# System-wide .bashrc file for interactive bash(1) shells.
# To enable the settings / commands in this file for login shells as well,
# this file has to be sourced in /etc/profile.
# If not running interactively, don't do anything
[ -z "$PS1" ] && return
...</textarea>Constraint validation: If the readonly attribute is specified on a textarea
element, the element is barred from constraint validation.
A textarea element is mutable if it is
neither disabled nor has a readonly attribute specified.
When a textarea is mutable, its raw value should be editable by the user: the user
agent should allow the user to edit, insert, and remove text, and to insert and remove line breaks
in the form of U+000A LINE FEED (LF) characters. Any time the user causes the element's raw value to change, the user agent must queue an
element task on the user interaction task source given the
textarea element to fire an event named
input at the textarea element, with the bubbles and composed
attributes initialized to true. User agents may wait for a suitable break in the user's
interaction before queuing the task; for example, a user agent could wait for the user to have not
hit a key for 100ms, so as to only fire the event when the user pauses, instead of continuously
for each keystroke.
A textarea element's dirty value flag must
be set to true whenever the user interacts with the control in a way that changes the raw value.
The cloning steps for textarea
elements must propagate the raw value and dirty value flag from the node being cloned to the copy.
The children changed steps for textarea elements must, if the
element's dirty value flag is false, set the element's
raw value to its child text
content.
The reset algorithm for textarea
elements is to set the dirty value flag back to false, and
set the raw value of element to its child
text content.
When a textarea element is popped off the stack of open elements of
an HTML parser or XML parser, then the user agent must invoke the
element's reset algorithm.
If the element is mutable, the user agent should allow the user to change the writing direction of the element, setting it either to a left-to-right writing direction or a right-to-left writing direction. If the user does so, the user agent must then run the following steps:
Set the element's dir attribute to "ltr" if the user selected a left-to-right writing direction, and
"rtl" if the user selected a right-to-left writing
direction.
Queue an element task on the user interaction task source given
the textarea element to fire an event named
input at the textarea element, with the bubbles and composed
attributes initialized to true.
The cols
attribute specifies the expected maximum number of characters per line. If the cols attribute is specified, its value must be a valid
non-negative integer greater than zero. If applying the rules for
parsing non-negative integers to the attribute's value results in a number greater than
zero, then the element's character width is that
value; otherwise, it is 20.
The user agent may use the textarea element's character width as a hint to the user as to how many
characters the server prefers per line (e.g. for visual user agents by making the width of the
control be that many characters). In visual renderings, the user agent should wrap the user's
input in the rendering so that each line is no wider than this number of characters.
The rows
attribute specifies the number of lines to show. If the rows attribute is specified, its value must be a valid
non-negative integer greater than zero. If applying the rules for
parsing non-negative integers to the attribute's value results in a number greater than
zero, then the element's character height is that
value; otherwise, it is 2.
Visual user agents should set the height of the control to the number of lines given by character height.
The wrap
attribute is an enumerated attribute with two keywords and states: the soft keyword which maps to the Soft state, and the hard keyword which maps to the Hard state. The missing value default and invalid value default
are the Soft state.
The Soft state indicates that the text in the
textarea is not to be wrapped when it is submitted (though it can still be wrapped in
the rendering).
The Hard state indicates that the text in the
textarea is to have newlines added by the user agent so that the text is wrapped when
it is submitted.
If the element's wrap attribute is in the Hard state, the cols attribute must be specified.
For historical reasons, the element's value is normalized in three different ways for three
different purposes. The raw value is the value as
it was originally set. It is not normalized. The API
value is the value used in the value IDL
attribute, textLength IDL attribute, and by the
maxlength and minlength content attributes. It is normalized so that line
breaks use U+000A LINE FEED (LF) characters. Finally, there is the value, as used in form submission and other processing models in
this specification. It is normalized so that line breaks use U+000D CARRIAGE RETURN U+000A LINE
FEED (CRLF) character pairs, and in addition, if necessary given the element's wrap attribute, additional line breaks are inserted to wrap the
text at the given width.
The algorithm for obtaining the element's API value is to return the element's raw value, with newlines normalized.
The element's value is defined to be the element's raw value with the textarea wrapping transformation applied. The textarea wrapping transformation is the following algorithm, as applied to a string:
Replace every occurrence of a U+000D CARRIAGE RETURN (CR) character not followed by a U+000A LINE FEED (LF) character, and every occurrence of a U+000A LINE FEED (LF) character not preceded by a U+000D CARRIAGE RETURN (CR) character, by a two-character string consisting of a U+000D CARRIAGE RETURN U+000A LINE FEED (CRLF) character pair.
If the element's wrap attribute is in the Hard state, insert U+000D CARRIAGE RETURN U+000A
LINE FEED (CRLF) character pairs into the string using an implementation-defined
algorithm so that each line has no more than character
width characters. For the purposes of this requirement, lines are delimited by the start
of the string, the end of the string, and U+000D CARRIAGE RETURN U+000A LINE FEED (CRLF)
character pairs.
The maxlength attribute is a form control maxlength attribute.
If the textarea element has a maximum allowed value length, then the
element's children must be such that the length of the value of the element's
descendant text content with newlines
normalized is equal to or less than the element's maximum allowed value
length.
The minlength attribute is a form control minlength attribute.
The required attribute is a boolean
attribute. When specified, the user will be required to enter a value before submitting the
form.
Constraint validation: If the element has its required attribute specified, and the element is mutable, and the element's value is the empty string, then the element is suffering
from being missing.
The placeholder attribute represents a short
hint (a word or short phrase) intended to aid the user with data entry when the control has no
value. A hint could be a sample value or a brief description of the expected format.
The placeholder attribute should not be used as
an alternative to a label. For a longer hint or other advisory text, the title attribute is more appropriate.
These mechanisms are very similar but subtly different: the hint given by the
control's label is shown at all times; the short hint given in the placeholder attribute is shown before the user enters a
value; and the hint in the title attribute is shown when the user
requests further help.
User agents should present this hint to the user when the element's value is the empty string and the control is not focused (e.g. by displaying it inside a blank unfocused control). All U+000D CARRIAGE RETURN U+000A LINE FEED character pairs (CRLF) in the hint, as well as all other U+000D CARRIAGE RETURN (CR) and U+000A LINE FEED (LF) characters in the hint, must be treated as line breaks when rendering the hint.
If a user agent normally doesn't show this hint to the user when the control is
focused, then the user agent should nonetheless show the hint for the control if it
was focused as a result of the autofocus attribute, since
in that case the user will not have had an opportunity to examine the control before focusing
it.
The name attribute represents the element's name.
The dirname attribute controls how the element's directionality is submitted.
The disabled attribute is used to make the control
non-interactive and to prevent its value from being submitted.
The form attribute is used to explicitly associate the
textarea element with its form owner.
The autocomplete attribute controls how the user agent
provides autofill behavior.
typeReturns the string "textarea".
valueReturns the current value of the element.
Can be set, to change the value.
The cols, placeholder, required, rows, and wrap IDL attributes must reflect the
respective content attributes of the same name. The cols
and rows attributes are limited to only non-negative
numbers greater than zero with fallback. The cols
IDL attribute's default value is 20. The rows IDL
attribute's default value is 2. The dirName
IDL attribute must reflect the dirname content
attribute. The maxLength IDL attribute
must reflect the maxlength content
attribute, limited to only non-negative numbers. The minLength IDL attribute must reflect the
minlength content attribute, limited to only
non-negative numbers. The readOnly
IDL attribute must reflect the readonly
content attribute.
The type
IDL attribute must return the value "textarea".
The defaultValue attribute's getter
must return the element's child text content.
The defaultValue attribute's setter must
string replace all with the given value within this element.
The value IDL attribute must, on getting, return the
element's API value. On setting, it must perform the
following steps:
Let oldAPIValue be this element's API value.
Set this element's raw value to the new value.
Set this element's dirty value flag to true.
If the new API value is different from
oldAPIValue, then move the text entry
cursor position to the end of the text control, unselecting any selected text and resetting the selection direction to "none".
The textLength IDL attribute must
return the length of the element's API
value.
The willValidate, validity, and validationMessage IDL attributes, and the checkValidity(), reportValidity(), and setCustomValidity() methods, are part of the
constraint validation API. The labels IDL
attribute provides a list of the element's labels. The select(), selectionStart, selectionEnd, selectionDirection, setRangeText(), and setSelectionRange() methods and IDL attributes
expose the element's text selection. The disabled,
form, and name IDL attributes
are part of the element's forms API.
Here is an example of a textarea being used for unrestricted free-form text input
in a form:
<p>If you have any comments, please let us know: <textarea cols=80 name=comments></textarea></p>To specify a maximum length for the comments, one can use the maxlength attribute:
<p>If you have any short comments, please let us know: <textarea cols=80 name=comments maxlength=200></textarea></p>To give a default value, text can be included inside the element:
<p>If you have any comments, please let us know: <textarea cols=80 name=comments>You rock!</textarea></p>You can also give a minimum length. Here, a letter needs to be filled out by the user; a template (which is shorter than the minimum length) is provided, but is insufficient to submit the form:
<textarea required minlength="500">Dear Madam Speaker,
Regarding your letter dated ...
...
Yours Sincerely,
...</textarea>A placeholder can be given as well, to suggest the basic form to the user, without providing an explicit template:
<textarea placeholder="Dear Francine,
They closed the parks this week, so we won't be able to
meet your there. Should we just have dinner?
Love,
Daddy"></textarea>To have the browser submit the directionality of the element along with the
value, the dirname attribute can be specified:
<p>If you have any comments, please let us know (you may use either English or Hebrew for your comments):
<textarea cols=80 name=comments dirname=comments.dir></textarea></p>output elementSupport in all current engines.
Support in all current engines.
for — Specifies controls from which the output was calculatedform — Associates the element with a form elementname — Name of the element to use in the form.elements API. [Exposed=Window]
interface HTMLOutputElement : HTMLElement {
[HTMLConstructor] constructor();
[SameObject, PutForwards=value] readonly attribute DOMTokenList htmlFor;
readonly attribute HTMLFormElement? form;
[CEReactions] attribute DOMString name;
readonly attribute DOMString type;
[CEReactions] attribute DOMString defaultValue;
[CEReactions] attribute DOMString value;
readonly attribute boolean willValidate;
readonly attribute ValidityState validity;
readonly attribute DOMString validationMessage;
boolean checkValidity();
boolean reportValidity();
undefined setCustomValidity(DOMString error);
readonly attribute NodeList labels;
};The output element represents the result of a calculation performed
by the application, or the result of a user action.
This element can be contrasted with the samp element, which is the
appropriate element for quoting the output of other programs run previously.
The for content
attribute allows an explicit relationship to be made between the result of a calculation and the
elements that represent the values that went into the calculation or that otherwise influenced the
calculation. The for attribute, if specified, must contain a
string consisting of an unordered set of unique space-separated tokens, none of which
are identical to another token and each of which must have the value of an ID of an element in the same tree.
The form attribute is used to explicitly associate the
output element with its form owner. The name attribute represents the element's name. The output
element is associated with a form so that it can be easily referenced from the event
handlers of form controls; the element's value itself is not submitted when the form is
submitted.
The element has a default value override (null or a string). Initially it must be null.
The element's default value is determined by the following steps:
If this element's default value override is non-null, then return it.
Return this element's descendant text content.
The reset algorithm for output
elements is to run these steps:
String replace all with this element's default value within this element.
Set this element's default value override to null.
value [ = value ]Returns the element's current value.
Can be set, to change the value.
defaultValue [ = value ]Returns the element's current default value.
Can be set, to change the default value.
typeReturns the string "output".
The value
attribute's getter must return this element's descendant text content.
The value attribute's setter must run these steps:
Set this element's default value override to its default value.
String replace all with the given value within this element.
The defaultValue attribute's getter
must return the result of running this element's default value.
The defaultValue attribute's setter must run
these steps:
If this element's default value override is null, then string replace all with the given value within this element and return.
Set this element's default value override to the given value.
The type
attribute's getter must return "output".
The htmlFor IDL attribute must
reflect the for content attribute.
The willValidate, validity, and validationMessage IDL attributes, and the checkValidity(), reportValidity(), and setCustomValidity() methods, are part of the
constraint validation API. The labels IDL
attribute provides a list of the element's labels. The form and name IDL attributes are
part of the element's forms API.
A simple calculator could use output for its display of calculated results:
<form onsubmit="return false" oninput="o.value = a.valueAsNumber + b.valueAsNumber">
<input id=a type=number step=any> +
<input id=b type=number step=any> =
<output id=o for="a b"></output>
</form>In this example, an output element is used to report the results of a calculation performed by a remote
server, as they come in:
<output id="result"></output>
<script>
var primeSource = new WebSocket('ws://primes.example.net/');
primeSource.onmessage = function (event) {
document.getElementById('result').value = event.data;
}
</script>progress elementSupport in all current engines.
Support in all current engines.
progress element descendants.value — Current value of the elementmax — Upper bound of range[Exposed=Window]
interface HTMLProgressElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute double value;
[CEReactions] attribute double max;
readonly attribute double position;
readonly attribute NodeList labels;
};The progress element represents the completion progress of a task.
The progress is either indeterminate, indicating that progress is being made but that it is not
clear how much more work remains to be done before the task is complete (e.g. because the task is
waiting for a remote host to respond), or the progress is a number in the range zero to a maximum,
giving the fraction of work that has so far been completed.
There are two attributes that determine the current task completion represented by the element.
The value
attribute specifies how much of the task has been completed, and the max attribute specifies how much work
the task requires in total. The units are arbitrary and not specified.
To make a determinate progress bar, add a value attribute with the current progress (either a number from
0.0 to 1.0, or, if the max attribute is specified, a number
from 0 to the value of the max attribute). To make an
indeterminate progress bar, remove the value
attribute.
Authors are encouraged to also include the current value and the maximum value inline as text inside the element, so that the progress is made available to users of legacy user agents.
Here is a snippet of a web application that shows the progress of some automated task:
<section>
<h2>Task Progress</h2>
<p>Progress: <progress id="p" max=100><span>0</span>%</progress></p>
<script>
var progressBar = document.getElementById('p');
function updateProgress(newValue) {
progressBar.value = newValue;
progressBar.getElementsByTagName('span')[0].textContent = newValue;
}
</script>
</section>(The updateProgress() method in this example would be called by some
other code on the page to update the actual progress bar as the task progressed.)
The value and max attributes, when present, must have values that are valid floating-point numbers. The value attribute, if present, must have a value equal to or
greater than zero, and less than or equal to the value of the max attribute, if present, or 1.0, otherwise. The max attribute, if present, must have a value greater than
zero.
The progress element is the wrong element to use for something that
is just a gauge, as opposed to task progress. For instance, indicating disk space usage using
progress would be inappropriate. Instead, the meter element is available
for such use cases.
User agent requirements: If the value
attribute is omitted, then the progress bar is an indeterminate progress bar. Otherwise, it is a
determinate progress bar.
If the progress bar is a determinate progress bar and the element has a max attribute, the user agent must parse the max attribute's value according to the rules for parsing
floating-point number values. If this does not result in an error, and if the parsed value
is greater than zero, then the maximum value of the
progress bar is that value. Otherwise, if the element has no max attribute, or if it has one but parsing it resulted in an
error, or if the parsed value was less than or equal to zero, then the maximum value of the progress bar is 1.0.
If the progress bar is a determinate progress bar, user agents must parse the value attribute's value according to the rules for
parsing floating-point number values. If this does not result in an error and the parsed
value is greater than zero, then the value of the
progress bar is that parsed value. Otherwise, if parsing the value attribute's value resulted in an error or a number less
than or equal to zero, then the value of the progress
bar is zero.
If the progress bar is a determinate progress bar, then the current value is the maximum value, if value is greater than the maximum value, and value otherwise.
UA requirements for showing the progress bar: When representing a
progress element to the user, the UA should indicate whether it is a determinate or
indeterminate progress bar, and in the former case, should indicate the relative position of the
current value relative to the maximum value.
positionFor a determinate progress bar (one with known current and maximum values), returns the result of dividing the current value by the maximum value.
For an indeterminate progress bar, returns −1.
If the progress bar is an indeterminate progress bar, then the position IDL attribute must return −1.
Otherwise, it must return the result of dividing the current value by the maximum value.
If the progress bar is an indeterminate progress bar, then the value IDL attribute, on getting, must return 0.
Otherwise, it must return the current value. On
setting, the given value must be converted to the best representation of the number as a
floating-point number and then the value content
attribute must be set to that string.
Setting the value IDL attribute to itself
when the corresponding content attribute is absent would change the progress bar from an
indeterminate progress bar to a determinate progress bar with no progress.
The max
IDL attribute must reflect the content attribute of the same name, limited to
numbers greater than zero. The default value for max
is 1.0.
The labels IDL attribute provides a list of the element's
labels.
meter elementSupport in all current engines.
Support in all current engines.
meter element descendants.value — Current value of the elementmin — Lower bound of rangemax — Upper bound of rangelow — High limit of low rangehigh — Low limit of high rangeoptimum — Optimum value in gauge[Exposed=Window]
interface HTMLMeterElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute double value;
[CEReactions] attribute double min;
[CEReactions] attribute double max;
[CEReactions] attribute double low;
[CEReactions] attribute double high;
[CEReactions] attribute double optimum;
readonly attribute NodeList labels;
};The meter element represents a scalar measurement within a known
range, or a fractional value; for example disk usage, the relevance of a query result, or the
fraction of a voting population to have selected a particular candidate.
This is also known as a gauge.
The meter element should not be used to indicate progress (as in a progress bar).
For that role, HTML provides a separate progress element.
The meter element also does not represent a scalar value of arbitrary
range — for example, it would be wrong to use this to report a weight, or height, unless
there is a known maximum value.
There are six attributes that determine the semantics of the gauge represented by the element.
The min attribute
specifies the lower bound of the range, and the max attribute specifies the upper bound. The value attribute specifies
the value to have the gauge indicate as the "measured" value.
The other three attributes can be used to segment the gauge's range into "low", "medium", and
"high" parts, and to indicate which part of the gauge is the "optimum" part. The low attribute specifies the range that is
considered to be the "low" part, and the high attribute specifies the range that is considered to be
the "high" part. The optimum attribute gives the position that is "optimum";
if that is higher than the "high" value then this indicates that the higher the value, the better;
if it's lower than the "low" mark then it indicates that lower values are better, and naturally if
it is in between then it indicates that neither high nor low values are good.
Authoring requirements: The value attribute must be specified. The value, min, low, high, max, and optimum attributes,
when present, must have values that are valid
floating-point numbers.
In addition, the attributes' values are further constrained:
Let value be the value attribute's
number.
If the min attribute is specified, then let minimum be that attribute's value; otherwise, let it be zero.
If the max attribute is specified, then let maximum be that attribute's value; otherwise, let it be 1.0.
The following inequalities must hold, as applicable:
low ≤ maximum (if low is specified)high ≤ maximum (if high is specified)optimum ≤ maximum (if optimum is specified)low ≤ high (if
both low and high are
specified)If no minimum or maximum is specified, then the range is assumed to be 0..1, and the value thus has to be within that range.
Authors are encouraged to include a textual representation of the gauge's state in the
element's contents, for users of user agents that do not support the meter
element.
When used with microdata, the meter element's value attribute provides the element's machine-readable value.
The following examples show three gauges that would all be three-quarters full:
Storage space usage: <meter value=6 max=8>6 blocks used (out of 8 total)</meter>
Voter turnout: <meter value=0.75><img alt="75%" src="graph75.png"></meter>
Tickets sold: <meter min="0" max="100" value="75"></meter>The following example is incorrect use of the element, because it doesn't give a range (and since the default maximum is 1, both of the gauges would end up looking maxed out):
<p>The grapefruit pie had a radius of <meter value=12>12cm</meter>
and a height of <meter value=2>2cm</meter>.</p> <!-- BAD! -->Instead, one would either not include the meter element, or use the meter element with a defined range to give the dimensions in context compared to other pies:
<p>The grapefruit pie had a radius of 12cm and a height of
2cm.</p>
<dl>
<dt>Radius: <dd> <meter min=0 max=20 value=12>12cm</meter>
<dt>Height: <dd> <meter min=0 max=10 value=2>2cm</meter>
</dl>There is no explicit way to specify units in the meter element, but the units may
be specified in the title attribute in free-form text.
The example above could be extended to mention the units:
<dl>
<dt>Radius: <dd> <meter min=0 max=20 value=12 title="centimeters">12cm</meter>
<dt>Height: <dd> <meter min=0 max=10 value=2 title="centimeters">2cm</meter>
</dl>User agent requirements: User agents must parse the min, max, value, low, high, and optimum
attributes using the rules for parsing floating-point number values.
User agents must then use all these numbers to obtain values for six points on the gauge, as follows. (The order in which these are evaluated is important, as some of the values refer to earlier ones.)
If the min attribute is specified and a value could be
parsed out of it, then the minimum value is that value. Otherwise, the minimum value is
zero.
If the max attribute is specified and a value could be
parsed out of it, then the candidate maximum value is that value. Otherwise, the candidate
maximum value is 1.0.
If the candidate maximum value is greater than or equal to the minimum value, then the maximum value is the candidate maximum value. Otherwise, the maximum value is the same as the minimum value.
If the value attribute is specified and a value could
be parsed out of it, then that value is the candidate actual value. Otherwise, the candidate
actual value is zero.
If the candidate actual value is less than the minimum value, then the actual value is the minimum value.
Otherwise, if the candidate actual value is greater than the maximum value, then the actual value is the maximum value.
Otherwise, the actual value is the candidate actual value.
If the low attribute is specified and a value could be
parsed out of it, then the candidate low boundary is that value. Otherwise, the candidate low
boundary is the same as the minimum value.
If the candidate low boundary is less than the minimum value, then the low boundary is the minimum value.
Otherwise, if the candidate low boundary is greater than the maximum value, then the low boundary is the maximum value.
Otherwise, the low boundary is the candidate low boundary.
If the high attribute is specified and a value could be
parsed out of it, then the candidate high boundary is that value. Otherwise, the candidate high
boundary is the same as the maximum value.
If the candidate high boundary is less than the low boundary, then the high boundary is the low boundary.
Otherwise, if the candidate high boundary is greater than the maximum value, then the high boundary is the maximum value.
Otherwise, the high boundary is the candidate high boundary.
If the optimum attribute is specified and a value
could be parsed out of it, then the candidate optimum point is that value. Otherwise, the
candidate optimum point is the midpoint between the minimum value and the maximum value.
If the candidate optimum point is less than the minimum value, then the optimum point is the minimum value.
Otherwise, if the candidate optimum point is greater than the maximum value, then the optimum point is the maximum value.
Otherwise, the optimum point is the candidate optimum point.
All of which will result in the following inequalities all being true:
UA requirements for regions of the gauge: If the optimum point is equal to the low boundary or the high boundary, or anywhere in between them, then the region between the low and high boundaries of the gauge must be treated as the optimum region, and the low and high parts, if any, must be treated as suboptimal. Otherwise, if the optimum point is less than the low boundary, then the region between the minimum value and the low boundary must be treated as the optimum region, the region from the low boundary up to the high boundary must be treated as a suboptimal region, and the remaining region must be treated as an even less good region. Finally, if the optimum point is higher than the high boundary, then the situation is reversed; the region between the high boundary and the maximum value must be treated as the optimum region, the region from the high boundary down to the low boundary must be treated as a suboptimal region, and the remaining region must be treated as an even less good region.
UA requirements for showing the gauge: When representing a meter
element to the user, the UA should indicate the relative position of the actual value to the
minimum and maximum values, and the relationship between the actual value and the three regions of
the gauge.
The following markup:
<h3>Suggested groups</h3>
<menu>
<li><a href="?cmd=hsg" onclick="hideSuggestedGroups()">Hide suggested groups</a></li>
</menu>
<ul>
<li>
<p><a href="/group/comp.infosystems.www.authoring.stylesheets/view">comp.infosystems.www.authoring.stylesheets</a> -
<a href="/group/comp.infosystems.www.authoring.stylesheets/subscribe">join</a></p>
<p>Group description: <strong>Layout/presentation on the WWW.</strong></p>
<p><meter value="0.5">Moderate activity,</meter> Usenet, 618 subscribers</p>
</li>
<li>
<p><a href="/group/netscape.public.mozilla.xpinstall/view">netscape.public.mozilla.xpinstall</a> -
<a href="/group/netscape.public.mozilla.xpinstall/subscribe">join</a></p>
<p>Group description: <strong>Mozilla XPInstall discussion.</strong></p>
<p><meter value="0.25">Low activity,</meter> Usenet, 22 subscribers</p>
</li>
<li>
<p><a href="/group/mozilla.dev.general/view">mozilla.dev.general</a> -
<a href="/group/mozilla.dev.general/subscribe">join</a></p>
<p><meter value="0.25">Low activity,</meter> Usenet, 66 subscribers</p>
</li>
</ul>Might be rendered as follows:
User agents may combine the value of the title attribute and the other attributes to provide context-sensitive
help or inline text detailing the actual values.
For example, the following snippet:
<meter min=0 max=60 value=23.2 title=seconds></meter>...might cause the user agent to display a gauge with a tooltip saying "Value: 23.2 out of 60." on one line and "seconds" on a second line.
The value IDL
attribute, on getting, must return the actual value. On
setting, the given value must be converted to the best representation of the number as a
floating-point number and then the value content
attribute must be set to that string.
The min IDL
attribute, on getting, must return the minimum value.
On setting, the given value must be converted to the best representation of the number as a
floating-point number and then the min content
attribute must be set to that string.
The max IDL
attribute, on getting, must return the maximum value.
On setting, the given value must be converted to the best representation of the number as a
floating-point number and then the max content
attribute must be set to that string.
The low IDL
attribute, on getting, must return the low boundary. On
setting, the given value must be converted to the best representation of the number as a
floating-point number and then the low content
attribute must be set to that string.
The high IDL
attribute, on getting, must return the high boundary. On
setting, the given value must be converted to the best representation of the number as a
floating-point number and then the high content
attribute must be set to that string.
The optimum
IDL attribute, on getting, must return the optimum
value. On setting, the given value must be converted to the best representation of
the number as a floating-point number and then the optimum content attribute must be set to that string.
The labels IDL attribute provides a list of the element's
labels.
The following example shows how a gauge could fall back to localized or pretty-printed text.
<p>Disk usage: <meter min=0 value=170261928 max=233257824>170 261 928 bytes used
out of 233 257 824 bytes available</meter></p>fieldset elementSupport in all current engines.
Support in all current engines.
legend element, followed by flow content.disabled — Whether the descendant form controls, except any inside legend, are disabledform — Associates the element with a form elementname — Name of the element to use in the form.elements API. [Exposed=Window]
interface HTMLFieldSetElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute boolean disabled;
readonly attribute HTMLFormElement? form;
[CEReactions] attribute DOMString name;
readonly attribute DOMString type;
[SameObject] readonly attribute HTMLCollection elements;
readonly attribute boolean willValidate;
[SameObject] readonly attribute ValidityState validity;
readonly attribute DOMString validationMessage;
boolean checkValidity();
boolean reportValidity();
undefined setCustomValidity(DOMString error);
};The fieldset element represents a set of form controls (or other
content) grouped together, optionally with a caption. The caption is given by the first
legend element that is a child of the fieldset element, if any. The
remainder of the descendants form the group.
Element/fieldset#attr-disabled
Support in all current engines.
The disabled attribute, when specified, causes all the
form control descendants of the fieldset element, excluding those that are
descendants of the fieldset element's first legend element child, if
any, to be disabled.
A fieldset element is a disabled
fieldset if it matches any of the following conditions:
disabled attribute is specified
fieldset element whose disabled attribute is specified, and is not a
descendant of that fieldset element's first legend element child, if
any.The form attribute is used to explicitly associate the
fieldset element with its form owner. The name attribute represents the element's name.
typeReturns the string "fieldset".
elementsReturns an HTMLCollection of the form controls in the element.
The disabled IDL attribute must reflect the
content attribute of the same name.
The type
IDL attribute must return the string "fieldset".
The elements IDL attribute must return an
HTMLCollection rooted at the fieldset element, whose filter matches
listed elements.
The willValidate, validity, and validationMessage attributes, and the checkValidity(), reportValidity(), and setCustomValidity() methods, are part of the
constraint validation API. The form and name IDL attributes are part of the element's forms API.
This example shows a fieldset element being used to group a set of related
controls:
<fieldset>
<legend>Display</legend>
<p><label><input type=radio name=c value=0 checked> Black on White</label>
<p><label><input type=radio name=c value=1> White on Black</label>
<p><label><input type=checkbox name=g> Use grayscale</label>
<p><label>Enhance contrast <input type=range name=e list=contrast min=0 max=100 value=0 step=1></label>
<datalist id=contrast>
<option label=Normal value=0>
<option label=Maximum value=100>
</datalist>
</fieldset>The following snippet shows a fieldset with a checkbox in the legend that controls whether or not the fieldset is enabled. The contents of the fieldset consist of two required text controls and an optional year/month control.
<fieldset name="clubfields" disabled>
<legend> <label>
<input type=checkbox name=club onchange="form.clubfields.disabled = !checked">
Use Club Card
</label> </legend>
<p><label>Name on card: <input name=clubname required></label></p>
<p><label>Card number: <input name=clubnum required pattern="[-0-9]+"></label></p>
<p><label>Expiry date: <input name=clubexp type=month></label></p>
</fieldset>You can also nest fieldset elements. Here is an example expanding on the previous
one that does so:
<fieldset name="clubfields" disabled>
<legend> <label>
<input type=checkbox name=club onchange="form.clubfields.disabled = !checked">
Use Club Card
</label> </legend>
<p><label>Name on card: <input name=clubname required></label></p>
<fieldset name="numfields">
<legend> <label>
<input type=radio checked name=clubtype onchange="form.numfields.disabled = !checked">
My card has numbers on it
</label> </legend>
<p><label>Card number: <input name=clubnum required pattern="[-0-9]+"></label></p>
</fieldset>
<fieldset name="letfields" disabled>
<legend> <label>
<input type=radio name=clubtype onchange="form.letfields.disabled = !checked">
My card has letters on it
</label> </legend>
<p><label>Card code: <input name=clublet required pattern="[A-Za-z]+"></label></p>
</fieldset>
</fieldset>In this example, if the outer "Use Club Card" checkbox is not checked, everything inside the
outer fieldset, including the two radio buttons in the legends of the two nested
fieldsets, will be disabled. However, if the checkbox is checked, then the radio
buttons will both be enabled and will let you select which of the two inner
fieldsets is to be enabled.
This example shows a grouping of controls where the legend element both labels
the grouping, and the nested heading element surfaces the grouping in the document outline:
<fieldset>
<legend> <h2>
How can we best reach you?
</h2> </legend>
<p> <label>
<input type=radio checked name=contact_pref>
Phone
</label> </p>
<p> <label>
<input type=radio name=contact_pref>
Text
</label> </p>
<p> <label>
<input type=radio name=contact_pref>
Email
</label> </p>
</fieldset>legend elementSupport in all current engines.
Support in all current engines.
fieldset element.[Exposed=Window]
interface HTMLLegendElement : HTMLElement {
[HTMLConstructor] constructor();
readonly attribute HTMLFormElement? form;
// also has obsolete members
};The legend element represents a caption for the rest of the contents
of the legend element's parent fieldset element, if
any.
formReturns the element's form element, if any, or null otherwise.
The form IDL
attribute's behavior depends on whether the legend element is in a
fieldset element or not. If the legend has a fieldset
element as its parent, then the form IDL attribute must
return the same value as the form IDL attribute on that
fieldset element. Otherwise, it must return null.
Most form controls have a value and a checkedness. (The latter is only used by input
elements.) These are used to describe how the user interacts with the control.
A control's value is its internal state. As such, it might not match the user's current input.
For instance, if a user enters the word "three" into a numeric field that expects digits, the user's input would
be the string "three" but the control's value would remain
unchanged. Or, if a user enters the email address " awesome@example.com"
(with leading whitespace) into an email field, the
user's input would be the string " awesome@example.com" but the browser's UI for
email fields might translate that into a value of "awesome@example.com" (without the leading whitespace).
input
and textarea elements have a dirty value flag.
This is used to track the interaction between the value and
default value. If it is false, value mirrors the default
value. If it is true, the default value is ignored.
To define the behavior of constraint validation in the face of the input
element's multiple attribute, input elements
can also have separately defined values.
To define the behavior of the maxlength and minlength attributes, as well as other APIs specific to the
textarea element, all form control with a value also have an algorithm for obtaining an API value. By
default this algorithm is to simply return the control's value.
The select element does not have a value;
the selectedness of its option
elements is what is used instead.
表单控件可以指定为 可变的。
这决定了（本规范中有些定义和要求依赖于元素是否被指定为可变的） 用户是否可以修改表单控件的 值 或 选中状态，或控件是否可以自动填充。
A form-associated element can have a relationship with a form
element, which is called the element's form owner. If a form-associated
element is not associated with a form element, its form owner is
said to be null.
A form-associated element has an associated parser inserted flag.
Support in all current engines.
A form-associated element is, by default, associated with its nearest ancestor form element (as described
below), but, if it is listed, may have a form attribute specified to override this.
This feature allows authors to work around the lack of support for nested
form elements.
If a listed form-associated element has a
form attribute specified, then that attribute's value must be
the ID of a form element in the element's
tree.
The rules in this section are complicated by the fact that although conforming
documents or trees will never contain nested form
elements, it is quite possible (e.g., using a script that performs DOM manipulation) to generate
trees that have such nested elements. They are also complicated by
rules in the HTML parser that, for historical reasons, can result in a form-associated
element being associated with a form element that is not its ancestor.
When a form-associated element is created, its form owner must be initialized to null (no owner).
When a form-associated element is to be associated with a form, its form owner must be set to that form.
When a form-associated element or one of its ancestors is inserted, then:
If the form-associated element's parser inserted flag is set, then return.
When a form-associated element or one of its ancestors is removed, then:
If the form-associated element has a form owner and the form-associated element and its form owner are no longer in the same tree, then reset the form owner of the form-associated element.
When a listed form-associated element's
form attribute is set, changed, or removed, then the user
agent must reset the form owner of that element.
When a listed form-associated element has a
form attribute and the ID of
any of the elements in the tree changes, then the user agent must reset the
form owner of that form-associated element.
When a listed form-associated element has a
form attribute and an element with an ID is inserted
into or removed from the
Document, then the user agent must reset the form owner of that
form-associated element.
When the user agent is to reset the form owner of a form-associated element element, it must run the following steps:
Unset element's parser inserted flag.
If all of the following conditions are true
form content attribute is not present
form element
ancestor after the change to the ancestor chain
then do nothing, and return.
Set element's form owner to null.
If element is listed, has a form content attribute, and is connected, then:
If the first element in element's tree, in tree
order, to have an ID that is identical to
element's form content attribute's value, is a
form element, then associate the
element with that form element.
Otherwise, if element has an ancestor form element, then associate element with the nearest such
ancestor form element.
In the following non-conforming snippet:
...
<form id="a">
<div id="b"></div>
</form>
<script>
document.getElementById('b').innerHTML =
'<table><tr><td></form><form id="c"><input id="d"></table>' +
'<input id="e">';
</script>
...The form owner of "d" would be the inner nested form "c", while the form owner of "e" would be the outer form "a".
This happens as follows: First, the "e" node gets associated with "c" in the HTML
parser. Then, the innerHTML algorithm moves the nodes
from the temporary document to the "b" element. At this point, the nodes see their ancestor chain
change, and thus all the "magic" associations done by the parser are reset to normal ancestor
associations.
This example is a non-conforming document, though, as it is a violation of the content models
to nest form elements, and there is a parse error for the </form> tag.
formSupport in all current engines.
Support in all current engines.
Returns the element's form owner.
Returns null if there isn't one.
Listed form-associated elements except for
form-associated custom elements have a
form IDL attribute, which, on getting, must return the
element's form owner, or null if there isn't one.
Form-associated custom elements don't have
form IDL attribute. Instead, their
ElementInternals object has a form IDL attribute. On getting, it must throw a
"NotSupportedError" DOMException if the target element is not a form-associated custom
element. Otherwise, it must return the element's form owner, or null if there
isn't one.
name attributeThe name content attribute gives the name of the form control, as
used in form submission and in the form element's elements object. If the attribute is specified, its value must
not be the empty string or isindex.
A number of user agents historically implemented special support for first-in-form
text controls with the name isindex, and this specification previously
defined related user agent requirements for it. However, some user agents subsequently dropped
that special support, and the related requirements were removed from this specification. So, to
avoid problematic reinterpretations in legacy user agents, the name isindex
is no longer allowed.
Other than isindex, any non-empty value for name is allowed. An ASCII case-insensitive match for
the name _charset_ is special: if used as
the name of a control with no value attribute, then during submission the value attribute is automatically given a value consisting of the
submission character encoding.
The name IDL attribute must reflect the name content attribute.
DOM clobbering is a common cause of security issues. Avoid using the names of
built-in form properties with the name content attribute.
In this example, the input element overrides the built-in method property:
let form = document.createElement("form");
let input = document.createElement("input");
form.appendChild(input);
form.method; // => "get"
input.name = "method"; // DOM clobbering occurs here
form.method === input; // => trueSince the input name takes precedence over built-in form properties, the JavaScript reference
form.method will point to the input element named "method"
instead of the built-in method property.
dirname attribute表单控件元素上的 dirname 属性
会让元素提交 文字方向，
并且给出 表单提交 时包含这个值的字段名。
如果指定了这个属性，它的值不得为空字符串。
在这个例子中，表单包含了一个文本控件和一个提交按钮：
<form action="addcomment.cgi" method=post> <p><label>Comment: <input type=text name="comment" dirname="comment.dir" required></label></p> <p><button name="mode" type=submit value="add">Post Comment</button></p> </form>
当用户提交表单时，用户代理包含了 3 个字段，一个叫 "comment", 一个叫 "comment.dir", 一个叫 "mode"；所以如果用户键入 "Hello"，提交的消息体可能像这样：
comment=Hello&comment.dir=ltr&mode=add
如果用户手动切换到右到左书写方向，然后输入 "مرحبا"，提交的消息体可能像这样：
comment=%D9%85%D8%B1%D8%AD%D8%A8%D8%A7&comment.dir=rtl&mode=add
maxlength attributeA form control maxlength
attribute, controlled by the dirty value flag,
declares a limit on the number of characters a user can input. The "number of characters" is
measured using length and, in the case of textarea elements, with all
newlines normalized to a single character (as opposed to CRLF pairs).
If an element has its form control maxlength attribute specified, the attribute's value must be a valid
non-negative integer. If the attribute is specified and applying the rules for
parsing non-negative integers to its value results in a number, then that number is the
element's maximum allowed value length. If the attribute is omitted or parsing its
value results in an error, then there is no maximum allowed value length.
Constraint validation: If an element has a maximum allowed value length, its dirty value flag is true, its value was last changed by a user edit (as opposed to a change made by a script), and the length of the element's API value is greater than the element's maximum allowed value length, then the element is suffering from being too long.
User agents may prevent the user from causing the element's API value to be set to a value whose length is greater than the element's maximum allowed value length.
In the case of textarea elements, the API value and value
differ. In particular, newline normalization is applied
before the maximum allowed value length is checked (whereas the textarea
wrapping transformation is not applied).
minlength attributeA form control minlength
attribute, controlled by the dirty value flag,
declares a lower bound on the number of characters a user can input. The "number of characters" is
measured using length and, in the case of textarea elements, with all
newlines normalized to a single character (as opposed to CRLF pairs).
The minlength attribute does not imply the
required attribute. If the form control has no required attribute, then the value can still be omitted; the minlength attribute only kicks in once the user has entered a
value at all. If the empty string is not allowed, then the required
attribute also needs to be set.
If an element has its form control minlength attribute specified, the attribute's value must be a valid
non-negative integer. If the attribute is specified and applying the rules for
parsing non-negative integers to its value results in a number, then that number is the
element's minimum allowed value length. If the attribute is omitted or parsing its
value results in an error, then there is no minimum allowed value length.
If an element has both a maximum allowed value length and a minimum allowed value length, the minimum allowed value length must be smaller than or equal to the maximum allowed value length.
Constraint validation: If an element has a minimum allowed value length, its dirty value flag is true, its value was last changed by a user edit (as opposed to a change made by a script), its value is not the empty string, and the length of the element's API value is less than the element's minimum allowed value length, then the element is suffering from being too short.
In this example, there are four text controls. The first is required, and has to be at least 5 characters long. The other three are optional, but if the user fills one in, the user has to enter at least 10 characters.
<form action="/events/menu.cgi" method="post">
<p><label>Name of Event: <input required minlength=5 maxlength=50 name=event></label></p>
<p><label>Describe what you would like for breakfast, if anything:
<textarea name="breakfast" minlength="10"></textarea></label></p>
<p><label>Describe what you would like for lunch, if anything:
<textarea name="lunch" minlength="10"></textarea></label></p>
<p><label>Describe what you would like for dinner, if anything:
<textarea name="dinner" minlength="10"></textarea></label></p>
<p><input type=submit value="Submit Request"></p>
</form>disabled attributeThe disabled content attribute is a boolean
attribute.
The disabled attribute for
option elements and the disabled
attribute for optgroup elements are defined separately.
A form control is disabled if any of the following conditions are met:
button, input, select,
textarea, or form-associated custom element, and the disabled attribute is specified on this element (regardless of
its value).fieldset element whose disabled attribute is specified, and is not a
descendant of that fieldset element's first legend element child, if
any.A form control that is disabled must prevent any click events that are queued on the
user interaction task source from being dispatched on the element.
Constraint validation: If an element is disabled, it is barred from constraint validation.
Support in all current engines.
Support in all current engines.
The disabled IDL attribute must reflect the disabled content attribute.
Element/form#Attributes_for_form_submission
Support in all current engines.
Attributes for form submission can be specified both on form elements
and on submit buttons (elements that represent buttons
that submit forms, e.g. an input element whose type attribute is in the Submit Button state).
The attributes for form submission that may be specified on form
elements are action, enctype, method, novalidate, and target.
The corresponding attributes for form submission that may be specified on submit buttons are formaction, formenctype, formmethod, formnovalidate, and formtarget. When omitted, they default to the values given on
the corresponding attributes on the form element.
The action and
formaction
content attributes, if specified, must have a value that is a valid non-empty URL
potentially surrounded by spaces.
The action of an element is the value of the element's
formaction attribute, if the element is a submit button and has such an attribute, or the value of its
form owner's action attribute, if it has
one, or else the empty string.
The method and
formmethod
content attributes are enumerated attributes with the
following keywords and states:
get, mapping to the
state GET, indicating the HTTP GET method.post, mapping to the
state POST, indicating the HTTP POST method.dialog, mapping to
the state dialog, indicating that submitting the
form is intended to close the dialog box in which the form finds
itself, if any, and otherwise not submit.The method attribute's invalid value default and missing value default
are both the GET state.
The formmethod attribute's invalid value default is the GET state. It
has no missing value default.
The method of an element is one of those states. If the
element is a submit button and has a formmethod attribute, then the element's method is that attribute's state; otherwise, it is the form
owner's method attribute's state.
Here the method attribute is used to explicitly specify
the default value, "get", so that the search
query is submitted in the URL:
<form method="get" action="/search.cgi">
<p><label>Search terms: <input type=search name=q></label></p>
<p><input type=submit></p>
</form>On the other hand, here the method attribute is used to
specify the value "post", so that the user's
message is submitted in the HTTP request's body:
<form method="post" action="/post-message.cgi">
<p><label>Message: <input type=text name=m></label></p>
<p><input type=submit value="Submit message"></p>
</form>In this example, a form is used with a dialog. The method attribute's "dialog" keyword is used to have the dialog
automatically close when the form is submitted.
<dialog id="ship">
<form method=dialog>
<p>A ship has arrived in the harbour.</p>
<button type=submit value="board">Board the ship</button>
<button type=submit value="call">Call to the captain</button>
</form>
</dialog>
<script>
var ship = document.getElementById('ship');
ship.showModal();
ship.onclose = function (event) {
if (ship.returnValue == 'board') {
// ...
} else {
// ...
}
};
</script>The enctype and
formenctype
content attributes are enumerated attributes with the
following keywords and states:
application/x-www-form-urlencoded" keyword and corresponding state.multipart/form-data" keyword and corresponding state.text/plain" keyword and corresponding state.The enctype attribute's invalid value default and missing value default
are both the application/x-www-form-urlencoded
state.
The formenctype attribute's invalid value default is the application/x-www-form-urlencoded state. It has no missing value default.
The enctype of an element is one of those three states.
If the element is a submit button and has a formenctype attribute, then the element's enctype is that attribute's state; otherwise, it is the
form owner's enctype attribute's state.
The target and
formtarget content attributes, if specified,
must have values that are valid browsing
context names or keywords.
The novalidate and formnovalidate content attributes are boolean attributes. If present, they indicate that the form is
not to be validated during submission.
The no-validate state of an element is true if the
element is a submit button and the element's formnovalidate attribute is present, or if the element's
form owner's novalidate attribute is present,
and false otherwise.
This attribute is useful to include "save" buttons on forms that have validation constraints, to allow users to save their progress even though they haven't fully entered the data in the form. The following example shows a simple form that has two required fields. There are three buttons: one to submit the form, which requires both fields to be filled in; one to save the form so that the user can come back and fill it in later; and one to cancel the form altogether.
<form action="editor.cgi" method="post">
<p><label>Name: <input required name=fn></label></p>
<p><label>Essay: <textarea required name=essay></textarea></label></p>
<p><input type=submit name=submit value="Submit essay"></p>
<p><input type=submit formnovalidate name=save value="Save essay"></p>
<p><input type=submit formnovalidate name=cancel value="Cancel"></p>
</form>Support in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
The action IDL
attribute must reflect the content attribute of the same name, except that on
getting, when the content attribute is missing or its value is the empty string, the element's
node document's URL must be returned
instead. The target
IDL attribute must reflect the content attribute of the same name. The method and enctype IDL attributes must
reflect the respective content attributes of the same name, limited to only
known values. The encoding IDL attribute must reflect the enctype content attribute, limited to only known
values. The noValidate IDL attribute
must reflect the novalidate content
attribute. The formAction IDL attribute must
reflect the formaction content attribute,
except that on getting, when the content attribute is missing or its value is the empty string,
the element's node document's URL must be
returned instead. The formEnctype IDL
attribute must reflect the formenctype
content attribute, limited to only known values. The formMethod IDL attribute must reflect the
formmethod content attribute, limited to only known
values. The formNoValidate IDL
attribute must reflect the formnovalidate content attribute. The formTarget IDL attribute must reflect the
formtarget content attribute.
autocomplete attributeUser agents sometimes have features for helping users fill forms in, for example prefilling the
user's address based on earlier user input. The autocomplete content attribute can be used to hint to
the user agent how to, or indeed whether to, provide such a feature.
There are two ways this attribute is used. When wearing the autofill expectation
mantle, the autocomplete attribute describes what
input is expected from users. When wearing the autofill anchor mantle, the autocomplete attribute describes the meaning of the given
value.
On an input element whose type attribute is
in the state, the autocomplete attribute wears the autofill anchor
mantle. In all other cases, it wears the autofill expectation mantle.
When wearing the autofill expectation mantle, the autocomplete attribute, if specified, must have a value that
is an ordered set of space-separated tokens consisting of either a single token that
is an ASCII case-insensitive match for the string "off", or a single token that is an ASCII
case-insensitive match for the string "on",
or autofill detail tokens.
When wearing the autofill anchor
mantle, the autocomplete attribute, if specified, must have a value that is an ordered set of
space-separated tokens consisting of just autofill detail tokens (i.e. the
"on" and "off" keywords are not allowed).
Autofill detail tokens are the following, in the order given below:
Optionally, a token whose first eight characters are an ASCII case-insensitive
match for the string "section-",
meaning that the field belongs to the named group.
For example, if there are two shipping addresses in the form, then they could be marked up as:
<fieldset>
<legend>Ship the blue gift to...</legend>
<p> <label> Address: <textarea name=ba autocomplete="section-blue shipping street-address"></textarea> </label>
<p> <label> City: <input name=bc autocomplete="section-blue shipping address-level2"> </label>
<p> <label> Postal Code: <input name=bp autocomplete="section-blue shipping postal-code"> </label>
</fieldset>
<fieldset>
<legend>Ship the red gift to...</legend>
<p> <label> Address: <textarea name=ra autocomplete="section-red shipping street-address"></textarea> </label>
<p> <label> City: <input name=rc autocomplete="section-red shipping address-level2"> </label>
<p> <label> Postal Code: <input name=rp autocomplete="section-red shipping postal-code"> </label>
</fieldset>Optionally, a token that is an ASCII case-insensitive match for one of the following strings:
shipping", meaning the field
is part of the shipping address or contact information
billing", meaning the field
is part of the billing address or contact information
Either of the following two options:
A token that is an ASCII case-insensitive match for one of the following autofill field names, excluding those that are inappropriate for the control:
name"
honorific-prefix"
given-name"
additional-name"
family-name"
honorific-suffix"
nickname"
username"
new-password"
current-password"
one-time-code"
organization-title"
organization"
street-address"
address-line1"
address-line2"
address-line3"
address-level4"
address-level3"
address-level2"
address-level1"
country"
country-name"
postal-code"
cc-name"
cc-given-name"
cc-additional-name"
cc-family-name"
cc-number"
cc-exp"
cc-exp-month"
cc-exp-year"
cc-csc"
cc-type"
transaction-currency"
transaction-amount"
language"
bday"
bday-day"
bday-month"
bday-year"
sex"
url"
photo"
(See the table below for descriptions of these values.)
The following, in the given order:
Optionally, a token that is an ASCII case-insensitive match for one of the following strings:
home", meaning the field is
for contacting someone at their residence
work", meaning the field is
for contacting someone at their workplace
mobile", meaning the field is for contacting someone regardless of location
fax", meaning the field
describes a fax machine's contact details
pager", meaning the field
describes a pager's or beeper's contact details
A token that is an ASCII case-insensitive match for one of the following autofill field names, excluding those that are inappropriate for the control:
tel"
tel-country-code"
tel-national"
tel-area-code"
tel-local"
tel-local-prefix"
tel-local-suffix"
tel-extension"
email"
impp"
(See the table below for descriptions of these values.)
As noted earlier, the meaning of the attribute and its keywords depends on the mantle that the attribute is wearing.
The "off" keyword indicates either
that the control's input data is particularly sensitive (for example the activation code for a
nuclear weapon); or that it is a value that will never be reused (for example a one-time-key for a
bank login) and the user will therefore have to explicitly enter the data each time, instead of
being able to rely on the UA to prefill the value for them; or that the document provides its own
autocomplete mechanism and does not want the user agent to provide autocompletion values.
The "on" keyword indicates that the
user agent is allowed to provide the user with autocompletion values, but does not provide any
further information about what kind of data the user might be expected to enter. User agents would
have to use heuristics to decide what autocompletion values to suggest.
The autofill field listed above indicate that the user agent is allowed to provide the user with autocompletion values, and specifies what kind of value is expected. The meaning of each such keyword is described in the table below.
If the autocomplete attribute is omitted, the default
value corresponding to the state of the element's form owner's autocomplete attribute is used instead (either "on" or "off"). If there is no form owner, then the
value "on" is used.
The autofill field listed above indicate that the value of the particular kind of value specified is that value provided for this element. The meaning of each such keyword is described in the table below.
In this example the page has explicitly specified the currency and amount of the transaction. The form requests a credit card and other billing details. The user agent could use this information to suggest a credit card that it knows has sufficient balance and that supports the relevant currency.
<form method=post action="step2.cgi">
<input type=hidden autocomplete=transaction-currency value="CHF">
<input type=hidden autocomplete=transaction-amount value="15.00">
<p><label>Credit card number: <input type=text inputmode=numeric autocomplete=cc-number></label>
<p><label>Expiry Date: <input type=month autocomplete=cc-exp></label>
<p><input type=submit value="Continue...">
</form>The autofill field keywords relate to each other as described in the table below. Each field name
listed on a row of this table corresponds to the meaning given in the cell for that row in the
column labeled "Meaning". Some fields correspond to subparts of other fields; for example, a
credit card expiry date can be expressed as one field giving both the month and year of expiry
("cc-exp"), or as two fields, one giving the
month ("cc-exp-month") and one the year
("cc-exp-year"). In such cases, the names of
the broader fields cover multiple rows, in which the narrower fields are defined.
Generally, authors are encouraged to use the broader fields rather than the narrower fields, as the narrower fields tend to expose Western biases. For example, while it is common in some Western cultures to have a given name and a family name, in that order (and thus often referred to as a first name and a surname), many cultures put the family name first and the given name second, and many others simply have one name (a mononym). Having a single field is therefore more flexible.
Some fields are only appropriate for certain form controls. An autofill field name is inappropriate for a control if the control does not belong to the group listed for that autofill field in the fifth column of the first row describing that autofill field in the table below. What controls fall into each group is described below the table.
| Field name | Meaning | Canonical Format | Canonical Format Example | Control group | |||
|---|---|---|---|---|---|---|---|
"name"
| Full name | Free-form text, no newlines | Sir Timothy John Berners-Lee, OM, KBE, FRS, FREng, FRSA | Text | |||
"honorific-prefix"
| Prefix or title (e.g. "Mr.", "Ms.", "Dr.", "Mlle") | Free-form text, no newlines | Sir | Text | |||
"given-name"
| Given name (in some Western cultures, also known as the first name) | Free-form text, no newlines | Timothy | Text | |||
"additional-name"
| Additional names (in some Western cultures, also known as middle names, forenames other than the first name) | Free-form text, no newlines | John | Text | |||
"family-name"
| Family name (in some Western cultures, also known as the last name or surname) | Free-form text, no newlines | Berners-Lee | Text | |||
"honorific-suffix"
| Suffix (e.g. "Jr.", "B.Sc.", "MBASW", "II") | Free-form text, no newlines | OM, KBE, FRS, FREng, FRSA | Text | |||
"nickname"
| Nickname, screen name, handle: a typically short name used instead of the full name | Free-form text, no newlines | Tim | Text | |||
"organization-title"
| Job title (e.g. "Software Engineer", "Senior Vice President", "Deputy Managing Director") | Free-form text, no newlines | Professor | Text | |||
"username"
| A username | Free-form text, no newlines | timbl | Username | |||
"new-password"
| A new password (e.g. when creating an account or changing a password) | Free-form text, no newlines | GUMFXbadyrS3 | Password | |||
"current-password"
| The current password for the account identified by the username field (e.g. when logging in)
| Free-form text, no newlines | qwerty | Password | |||
"one-time-code"
| One-time code used for verifying user identity | Free-form text, no newlines | 123456 | Password | |||
"organization"
| Company name corresponding to the person, address, or contact information in the other fields associated with this field | Free-form text, no newlines | World Wide Web Consortium | Text | |||
"street-address"
| Street address (multiple lines, newlines preserved) | Free-form text | 32 Vassar Street MIT Room 32-G524 | Multiline | |||
"address-line1"
| Street address (one line per field) | Free-form text, no newlines | 32 Vassar Street | Text | |||
"address-line2"
| Free-form text, no newlines | MIT Room 32-G524 | Text | ||||
"address-line3"
| Free-form text, no newlines | Text | |||||
"address-level4"
| The most fine-grained administrative level, in addresses with four administrative levels | Free-form text, no newlines | Text | ||||
"address-level3"
| The third administrative level, in addresses with three or more administrative levels | Free-form text, no newlines | Text | ||||
"address-level2"
| The second administrative level, in addresses with two or more administrative levels; in the countries with two administrative levels, this would typically be the city, town, village, or other locality within which the relevant street address is found | Free-form text, no newlines | Cambridge | Text | |||
"address-level1"
| The broadest administrative level in the address, i.e. the province within which the locality is found; for example, in the US, this would be the state; in Switzerland it would be the canton; in the UK, the post town | Free-form text, no newlines | MA | Text | |||
"country"
| Country code | Valid ISO 3166-1-alpha-2 country code [ISO3166] | US | Text | |||
"country-name"
| Country name | Free-form text, no newlines; derived from country in some cases
| US | Text | |||
"postal-code"
| Postal code, post code, ZIP code, CEDEX code (if CEDEX, append "CEDEX", and the arrondissement, if relevant, to the address-level2 field)
| Free-form text, no newlines | 02139 | Text | |||
"cc-name"
| Full name as given on the payment instrument | Free-form text, no newlines | Tim Berners-Lee | Text | |||
"cc-given-name"
| Given name as given on the payment instrument (in some Western cultures, also known as the first name) | Free-form text, no newlines | Tim | Text | |||
"cc-additional-name"
| Additional names given on the payment instrument (in some Western cultures, also known as middle names, forenames other than the first name) | Free-form text, no newlines | Text | ||||
"cc-family-name"
| Family name given on the payment instrument (in some Western cultures, also known as the last name or surname) | Free-form text, no newlines | Berners-Lee | Text | |||
"cc-number"
| Code identifying the payment instrument (e.g. the credit card number) | ASCII digits | 4114360123456785 | Text | |||
"cc-exp"
| Expiration date of the payment instrument | Valid month string | 2014-12 | Month | |||
"cc-exp-month"
| Month component of the expiration date of the payment instrument | Valid integer in the range 1..12 | 12 | Numeric | |||
"cc-exp-year"
| Year component of the expiration date of the payment instrument | Valid integer greater than zero | 2014 | Numeric | |||
"cc-csc"
| Security code for the payment instrument (also known as the card security code (CSC), card validation code (CVC), card verification value (CVV), signature panel code (SPC), credit card ID (CCID), etc) | ASCII digits | 419 | Text | |||
"cc-type"
| Type of payment instrument | Free-form text, no newlines | Visa | Text | |||
"transaction-currency"
| The currency that the user would prefer the transaction to use | ISO 4217 currency code [ISO4217] | GBP | Text | |||
"transaction-amount"
| The amount that the user would like for the transaction (e.g. when entering a bid or sale price) | Valid floating-point number | 401.00 | Numeric | |||
"language"
| Preferred language | Valid BCP 47 language tag [BCP47] | en | Text | |||
"bday"
| Birthday | Valid date string | 1955-06-08 | Date | |||
"bday-day"
| Day component of birthday | Valid integer in the range 1..31 | 8 | Numeric | |||
"bday-month"
| Month component of birthday | Valid integer in the range 1..12 | 6 | Numeric | |||
"bday-year"
| Year component of birthday | Valid integer greater than zero | 1955 | Numeric | |||
"sex"
| Gender identity (e.g. Female, Fa'afafine) | Free-form text, no newlines | Male | Text | |||
"url"
| Home page or other web page corresponding to the company, person, address, or contact information in the other fields associated with this field | Valid URL string | https://www.w3.org/People/Berners-Lee/ | URL | |||
"photo"
| Photograph, icon, or other image corresponding to the company, person, address, or contact information in the other fields associated with this field | Valid URL string | https://www.w3.org/Press/Stock/Berners-Lee/2001-europaeum-eighth.jpg | URL | |||
"tel"
| Full telephone number, including country code | ASCII digits and U+0020 SPACE characters, prefixed by a U+002B PLUS SIGN character (+) | +1 617 253 5702 | Tel | |||
"tel-country-code"
| Country code component of the telephone number | ASCII digits prefixed by a U+002B PLUS SIGN character (+) | +1 | Text | |||
"tel-national"
| Telephone number without the county code component, with a country-internal prefix applied if applicable | ASCII digits and U+0020 SPACE characters | 617 253 5702 | Text | |||
"tel-area-code"
| Area code component of the telephone number, with a country-internal prefix applied if applicable | ASCII digits | 617 | Text | |||
"tel-local"
| Telephone number without the country code and area code components | ASCII digits | 2535702 | Text | |||
"tel-local-prefix"
| First part of the component of the telephone number that follows the area code, when that component is split into two components | ASCII digits | 253 | Text | |||
"tel-local-suffix"
| Second part of the component of the telephone number that follows the area code, when that component is split into two components | ASCII digits | 5702 | Text | |||
"tel-extension"
| Telephone number internal extension code | ASCII digits | 1000 | Text | |||
"email"
| Email address | Valid email address | timbl@w3.org | Username | |||
"impp"
| URL representing an instant messaging protocol endpoint (for example, "aim:goim?screenname=example" or "xmpp:fred@example.net")
| Valid URL string | irc://example.org/timbl,isuser | URL | |||
The groups correspond to controls as follows:
input elements with a type attribute in the Hidden state
input elements with a type attribute in the Text state
input elements with a type attribute in the Search state
textarea elements
select elements
input elements with a type attribute in the Hidden state
textarea elements
select elements
input elements with a type attribute in the Hidden state
input elements with a type attribute in the Text state
input elements with a type attribute in the Search state
input elements with a type attribute in the Password state
textarea elements
select elements
input elements with a type attribute in the Hidden state
input elements with a type attribute in the Text state
input elements with a type attribute in the Search state
input elements with a type attribute in the URL state
textarea elements
select elements
input elements with a type attribute in the Hidden state
input elements with a type attribute in the Text state
input elements with a type attribute in the Search state
input elements with a type attribute in the Email state
textarea elements
select elements
input elements with a type attribute in the Hidden state
input elements with a type attribute in the Text state
input elements with a type attribute in the Search state
input elements with a type attribute in the Telephone state
textarea elements
select elements
input elements with a type attribute in the Hidden state
input elements with a type attribute in the Text state
input elements with a type attribute in the Search state
input elements with a type attribute in the Number state
textarea elements
select elements
input elements with a type attribute in the Hidden state
input elements with a type attribute in the Text state
input elements with a type attribute in the Search state
input elements with a type attribute in the Month state
textarea elements
select elements
input elements with a type attribute in the Hidden state
input elements with a type attribute in the Text state
input elements with a type attribute in the Search state
input elements with a type attribute in the Date state
textarea elements
select elements
Address levels: The "address-level1" – "address-level4" fields are used to describe
the locality of the street address. Different locales have different numbers of levels. For
example, the US uses two levels (state and town), the UK uses one or two depending on the address
(the post town, and in some cases the locality), and China can use three (province, city,
district). The "address-level1" field
represents the widest administrative division. Different locales order the fields in different
ways; for example, in the US the town (level 2) precedes the state (level 1); while in Japan the
prefecture (level 1) precedes the city (level 2) which precedes the district (level 3). Authors
are encouraged to provide forms that are presented in a way that matches the country's conventions
(hiding, showing, and rearranging fields accordingly as the user changes the country).
Each input element to which the autocomplete attribute applies, each select element, and each textarea element, has an
autofill hint set, an autofill scope, an autofill field name, and
an IDL-exposed autofill value.
The autofill field name specifies the specific kind of data expected in the field,
e.g. "street-address" or "cc-exp".
The autofill hint set identifies what address or contact information type the user
agent is to look at, e.g. "shipping fax" or "billing".
The autofill scope identifies the group of fields whose information concerns the
same subject, and consists of the autofill hint set with, if
applicable, the "section-*" prefix, e.g. "billing",
"section-parent shipping", or "section-child shipping
home".
These values are defined as the result of running the following algorithm:
If the element has no autocomplete attribute,
then jump to the step labeled default.
Let tokens be the result of splitting the attribute's value on ASCII whitespace.
If tokens is empty, then jump to the step labeled default.
Let index be the index of the last token in tokens.
If the indexth token in tokens is not an ASCII case-insensitive match for one of the tokens given in the first column of the following table, or if the number of tokens in tokens is greater than the maximum number given in the cell in the second column of that token's row, then jump to the step labeled default. Otherwise, let field be the string given in the cell of the first column of the matching row, and let category be the value of the cell in the third column of that same row.
| Token | Maximum number of tokens | Category |
|---|---|---|
"off"
| 1 | Off |
"on"
| 1 | Automatic |
"name"
| 3 | Normal |
"honorific-prefix"
| 3 | Normal |
"given-name"
| 3 | Normal |
"additional-name"
| 3 | Normal |
"family-name"
| 3 | Normal |
"honorific-suffix"
| 3 | Normal |
"nickname"
| 3 | Normal |
"organization-title"
| 3 | Normal |
"username"
| 3 | Normal |
"new-password"
| 3 | Normal |
"current-password"
| 3 | Normal |
"one-time-code"
| 3 | Normal |
"organization"
| 3 | Normal |
"street-address"
| 3 | Normal |
"address-line1"
| 3 | Normal |
"address-line2"
| 3 | Normal |
"address-line3"
| 3 | Normal |
"address-level4"
| 3 | Normal |
"address-level3"
| 3 | Normal |
"address-level2"
| 3 | Normal |
"address-level1"
| 3 | Normal |
"country"
| 3 | Normal |
"country-name"
| 3 | Normal |
"postal-code"
| 3 | Normal |
"cc-name"
| 3 | Normal |
"cc-given-name"
| 3 | Normal |
"cc-additional-name"
| 3 | Normal |
"cc-family-name"
| 3 | Normal |
"cc-number"
| 3 | Normal |
"cc-exp"
| 3 | Normal |
"cc-exp-month"
| 3 | Normal |
"cc-exp-year"
| 3 | Normal |
"cc-csc"
| 3 | Normal |
"cc-type"
| 3 | Normal |
"transaction-currency"
| 3 | Normal |
"transaction-amount"
| 3 | Normal |
"language"
| 3 | Normal |
"bday"
| 3 | Normal |
"bday-day"
| 3 | Normal |
"bday-month"
| 3 | Normal |
"bday-year"
| 3 | Normal |
"sex"
| 3 | Normal |
"url"
| 3 | Normal |
"photo"
| 3 | Normal |
"tel"
| 4 | Contact |
"tel-country-code"
| 4 | Contact |
"tel-national"
| 4 | Contact |
"tel-area-code"
| 4 | Contact |
"tel-local"
| 4 | Contact |
"tel-local-prefix"
| 4 | Contact |
"tel-local-suffix"
| 4 | Contact |
"tel-extension"
| 4 | Contact |
"email"
| 4 | Contact |
"impp"
| 4 | Contact |
If category is Off or Automatic but the element's autocomplete attribute is wearing the autofill anchor
mantle, then jump to the step labeled default.
If category is Off, let the element's autofill field name
be the string "off", let its autofill hint set be empty, and
let its IDL-exposed autofill value be the string "off". Then,
return.
If category is Automatic, let the element's autofill field
name be the string "on", let its autofill hint set be
empty, and let its IDL-exposed autofill value be the string "on". Then, return.
Let scope tokens be an empty list.
Let hint tokens be an empty set.
Let IDL value have the same value as field.
If the indexth token in tokens is the first entry, then skip to the step labeled done.
Decrement index by one.
If category is Contact and the indexth token in tokens is an ASCII case-insensitive match for one of the strings in the following list, then run the substeps that follow:
The substeps are:
Let contact be the matching string from the list above.
Insert contact at the start of scope tokens.
Add contact to hint tokens.
Let IDL value be the concatenation of contact, a U+0020 SPACE character, and the previous value of IDL value (which at this point will always be field).
If the indexth entry in tokens is the first entry, then skip to the step labeled done.
Decrement index by one.
If the indexth token in tokens is an ASCII case-insensitive match for one of the strings in the following list, then run the substeps that follow:
The substeps are:
Let mode be the matching string from the list above.
Insert mode at the start of scope tokens.
Add mode to hint tokens.
Let IDL value be the concatenation of mode, a U+0020 SPACE character, and the previous value of IDL value (which at this point will either be field or the concatenation of contact, a space, and field).
If the indexth entry in tokens is the first entry, then skip to the step labeled done.
Decrement index by one.
If the indexth entry in tokens is not the first entry, then jump to the step labeled default.
If the first eight characters of the indexth token in tokens are not
an ASCII case-insensitive match for the string "section-", then jump to the step labeled
default.
Let section be the indexth token in tokens, converted to ASCII lowercase.
Insert section at the start of scope tokens.
Let IDL value be the concatenation of section, a U+0020 SPACE character, and the previous value of IDL value.
Done: Let the element's autofill hint set be hint tokens.
Let the element's autofill scope be scope tokens.
Let the element's autofill field name be field.
Let the element's IDL-exposed autofill value be IDL value.
Return.
Default: Let the element's IDL-exposed autofill value be the empty string, and its autofill hint set and autofill scope be empty.
If the element's autocomplete attribute is
wearing the autofill anchor mantle, then let the element's autofill field
name be the empty string and return.
Let form be the element's form owner, if any, or null otherwise.
If form is not null and form's autocomplete attribute is in the off state, then let the element's
autofill field name be "off".
Otherwise, let the element's autofill field name be "on".
For the purposes of autofill, a control's data depends on the kind of control:
input element with its type attribute
in the Email state and with the multiple attribute specifiedinput elementtextarea elementselect element with its multiple
attribute specifiedoption elements in the select element's list of options that have their selectedness set to true.select elementoption element in the select element's list of options that has its selectedness set to true.How to process the autofill hint set, autofill scope, and
autofill field name depends on the mantle that the autocomplete attribute is wearing.
When an element's autofill field name is "off", the user agent should not remember the control's
data, and should not offer past values to the user.
In addition, when an element's autofill field name is "off", values are reset
when traversing the history.
Banks frequently do not want UAs to prefill login information:
<p><label>Account: <input type="text" name="ac" autocomplete="off"></label></p>
<p><label>PIN: <input type="password" name="pin" autocomplete="off"></label></p>When an element's autofill field name is not "off", the user agent may store the control's
data, and may offer previously stored values to the user.
For example, suppose a user visits a page with this control:
<select name="country">
<option>Afghanistan
<option>Albania
<option>Algeria
<option>Andorra
<option>Angola
<option>Antigua and Barbuda
<option>Argentina
<option>Armenia
<!-- ... -->
<option>Yemen
<option>Zambia
<option>Zimbabwe
</select>This might render as follows:
Suppose that on the first visit to this page, the user selects "Zambia". On the second visit, the user agent could duplicate the entry for Zambia at the top of the list, so that the interface instead looks like this:
When the autofill field name is "on", the user agent should attempt to use heuristics to
determine the most appropriate values to offer the user, e.g. based on the element's name value, the position of the element in its tree,
what other fields exist in the form, and so forth.
When the autofill field name is one of the names of the autofill fields described above, the user agent should provide suggestions that match the meaning of the field name as given in the table earlier in this section. The autofill hint set should be used to select amongst multiple possible suggestions.
For example, if a user once entered one address into fields that used the
"shipping" keyword, and another address into
fields that used the "billing" keyword, then in
subsequent forms only the first address would be suggested for form controls whose autofill
hint set contains the keyword "shipping". Both addresses might be suggested,
however, for address-related form controls whose autofill hint set does not contain
either keyword.
When the autofill field name is not the empty string, then the user agent must act as if the user had specified the control's data for the given autofill hint set, autofill scope, and autofill field name combination.
When the user agent autofills form controls, elements
with the same form owner and the same autofill scope must use data
relating to the same person, address, payment instrument, and contact details. When a user agent autofills "country" and "country-name" fields with the same form
owner and autofill scope, and the user agent has a value for the country" field(s), then the "country-name" field(s) must be filled using a
human-readable name for the same country. When a user agent fills in multiple fields at
once, all fields with the same autofill field name, form owner and
autofill scope must be filled with the same value.
Suppose a user agent knows of two phone numbers, +1 555 123 1234 and +1 555 666
7777. It would not be conforming for the user agent to fill a field with autocomplete="shipping tel-local-prefix" with the value "123" and another field
in the same form with autocomplete="shipping tel-local-suffix" with the
value "7777". The only valid prefilled values given the aforementioned information would be "123"
and "1234", or "666" and "7777", respectively.
Similarly, if a form for some reason contained both a "cc-exp" field and a "cc-exp-month" field, and the user agent
prefilled the form, then the month component of the former would have to match the latter.
This requirement interacts with the autofill anchor mantle also. Consider the following markup snippet:
<form>
<input type=hidden autocomplete="nickname" value="TreePlate">
<input type=text autocomplete="nickname">
</form>The only value that a conforming user agent could suggest in the text control is "TreePlate",
the value given by the hidden input element.
The "section-*" tokens in the autofill scope are opaque;
user agents must not attempt to derive meaning from the precise values of these tokens.
For example, it would not be conforming if the user agent decided that it
should offer the address it knows to be the user's daughter's address for
"section-child" and the addresses it knows to be the user's spouses'
addresses for "section-spouse".
The autocompletion mechanism must be implemented by the user agent acting as if the user had modified the control's data, and must be done at a time where the element is mutable (e.g. just after the element has been inserted into the document, or when the user agent stops parsing). User agents must only prefill controls using values that the user could have entered.
For example, if a select element only has option
elements with values "Steve" and "Rebecca", "Jay", and "Bob", and has an autofill field
name "given-name", but the user
agent's only idea for what to prefill the field with is "Evan", then the user agent cannot prefill
the field. It would not be conforming to somehow set the select element to the value
"Evan", since the user could not have done so themselves.
A user agent prefilling a form control must not discriminate between form controls that are
in a document tree and those that are connected; that is, it is not
conforming to make the decision on whether or not to autofill based on whether the element's
root is a shadow root versus a Document.
A user agent prefilling a form control's value must not cause that control to suffer from a type mismatch, suffer from being too long, suffer from being too short, suffer from an underflow, suffer from an overflow, or suffer from a step mismatch. A user agent prefilling a form control's value must not cause that control to suffer from a pattern mismatch either. Where possible given the control's constraints, user agents must use the format given as canonical in the aforementioned table. Where it's not possible for the canonical format to be used, user agents should use heuristics to attempt to convert values so that they can be used.
For example, if the user agent knows that the user's middle name is "Ines", and attempts to prefill a form control that looks like this:
<input name=middle-initial maxlength=1 autocomplete="additional-name">...then the user agent could convert "Ines" to "I" and prefill it that way.
A more elaborate example would be with month values. If the user agent knows that the user's birthday is the 27th of July 2012, then it might try to prefill all of the following controls with slightly different values, all driven from this information:
| 2012-07 |
The day is dropped since the Month state only accepts a
month/year combination. (Note that this example is non-conforming, because the autofill
field name bday is not allowed with the
Month state.)
|
| July | The user agent picks the month from the listed options, either by noticing there are twelve options and picking the 7th, or by recognizing that one of the strings (three characters "Jul" followed by a newline and a space) is a close match for the name of the month (July) in one of the user agent's supported languages, or through some other similar mechanism. |
| 7 | User agent converts "July" to a month number in the range 1..12, like the field. |
| 6 | User agent converts "July" to a month number in the range 0..11, like the field. |
| User agent doesn't fill in the field, since it can't make a good guess as to what the form expects. |
A user agent may allow the user to override an element's autofill field name, e.g.
to change it from "off" to "on" to allow values to be remembered and prefilled despite
the page author's objections, or to always "off",
never remembering values.
More specifically, user agents may in particular consider replacing the autofill field
name of form controls that match the description given in the first column of the following
table, when their autofill field name is either "on" or "off", with the value given in the second cell of that
row. If this table is used, the replacements must be done in tree order, since all
but the first row references the autofill field name of earlier elements. When the
descriptions below refer to form controls being preceded or followed by others, they mean in the
list of listed elements that share the same form
owner.
| Form control | New autofill field name |
|---|---|
an input element whose type attribute is in
the Text state that is followed by an
input element whose type attribute is in
the Password state
|
"username"
|
an input element whose type attribute is in
the Password state that is preceded by an
input element whose autofill field name is "username"
|
"current-password"
|
an input element whose type attribute is in
the Password state that is preceded by an
input element whose autofill field name is "current-password"
|
"new-password"
|
an input element whose type attribute is in
the Password state that is preceded by an
input element whose autofill field name is "new-password"
|
"new-password"
|
The autocomplete IDL attribute, on getting, must return the
element's IDL-exposed autofill value, and on setting, must reflect the
content attribute of the same name.
The input and textarea elements define several attributes and methods
for handling their selection. Their shared algorithms are defined here.
select()Selects everything in the text control.
selectionStart [ = value ]Returns the offset to the start of the selection.
Can be set, to change the start of the selection.
selectionEnd [ = value ]Returns the offset to the end of the selection.
Can be set, to change the end of the selection.
selectionDirection [ = value ]Returns the current direction of the selection.
Can be set, to change the direction of the selection.
The possible values are "forward", "backward", and "none".
setSelectionRange(start, end [, direction] )HTMLInputElement/setSelectionRange
Support in all current engines.
Changes the selection to cover the given substring in the given direction. If the direction is omitted, it will be reset to be the platform default (none or forward).
setRangeText(replacement [, start, end [, selectionMode ] ] )Support in all current engines.
Replaces a range of text with the new text. If the start and end arguments are not provided, the range is assumed to be the selection.
The final argument determines how the selection will be set after the text has been replaced. The possible values are:
select"start"end"preserve"All input elements to which these APIs apply, and all textarea elements, have either a
selection or a text entry cursor position at all times (even for
elements that are not being rendered), measured in offsets into the code units of the control's relevant value. The initial state must
consist of a text entry cursor at the
beginning of the control.
For input elements, these APIs must operate on the element's value. For textarea elements, these APIs must
operate on the element's API value. In the below
algorithms, we call the value string being operated on the relevant value.
The use of API value instead of raw value for textarea elements means
that U+000D (CR) characters are normalized away. For example,
<textarea id="demo"></textarea>
<script>
demo.value = "A\r\nB";
demo.setRangeText("replaced", 0, 2);
assert(demo.value === "replacedB");
</script>If we had operated on the raw value of "A\r\nB", then we would have replaced the characters "A\r", ending up with a result of "replaced\nB". But since
we used the API value of "A\nB", we replaced the characters "A\n", giving "replacedB".
Characters with no visible rendering, such as U+200D ZERO WIDTH JOINER, still count as characters. Thus, for instance, the selection can include just an invisible character, and the text insertion cursor can be placed to one side or another of such a character.
Whenever the relevant value changes for an element to which these APIs apply, run these steps:
If the element has a selection:
If the start of the selection is now past the end of the relevant value, set it to the end of the relevant value.
If the end of the selection is now past the end of the relevant value, set it to the end of the relevant value.
If the user agent does not support empty selection, and both the start and end of the selection are now pointing to the end of the relevant value, then instead set the element's text entry cursor position to the end of the relevant value, removing any selection.
Otherwise, the element must have a text entry cursor position position. If it is now past the end of the relevant value, set it to the end of the relevant value.
In some cases where the relevant value changes, other parts of the
specification will also modify the text entry cursor
position, beyond just the clamping steps above. For example, see the value setter for textarea.
Where possible, user interface features for changing the text selection in input and
textarea elements must be implemented using the set the selection range
algorithm so that, e.g., all the same events fire.
The selections of input and
textarea elements have a selection direction, which is either "forward", "backward", or "none".
The exact meaning of the selection direction depends on the platform. This direction is set when
the user manipulates the selection. The initial selection direction must be "none" if the platform supports that direction, or "forward" otherwise.
To set the selection direction of an element to a given direction, update the
element's selection direction to the given direction, unless the direction is "none" and the platform does not support that direction; in that case, update the
element's selection direction to "forward".
On Windows, the direction indicates the position of the caret relative to
the selection: a "forward" selection has the caret at the end of the
selection and a "backward" selection has the caret at the start of the
selection. Windows has no "none" direction.
On Mac, the direction indicates which end of the selection is affected when the user adjusts
the size of the selection using the arrow keys with the Shift modifier: the "forward" direction means the end of the selection is modified, and the "backward" direction means the start of the selection is modified. The "none" direction is the default on Mac, it indicates that no particular direction
has yet been selected. The user sets the direction implicitly when first adjusting the selection,
based on which directional arrow key was used.
Support in all current engines.
The select() method, when invoked, must run the
following steps:
If this element is an input element, and either select() does not
apply to this element or the corresponding control has no selectable text, return.
For instance, in a user agent where <input type=color> is rendered as a color well with a
picker, as opposed to a text control accepting a hexadecimal color code, there would be no
selectable text, and thus calls to the method are ignored.
Set the selection range with 0 and infinity.
The selectionStart
attribute's getter must run the following steps:
If this element is an input element, and selectionStart does
not apply to this element, return null.
If there is no selection, return the code unit offset within the relevant value to the character that immediately follows the text entry cursor.
Return the code unit offset within the relevant value to the character that immediately follows the start of the selection.
The selectionStart attribute's setter
must run the following steps:
If this element is an input element, and selectionStart does
not apply to this element, throw an "InvalidStateError"
DOMException.
Let end be the value of this element's selectionEnd attribute.
If end is less than the given value, set end to the given value.
Set the selection range with the given value, end, and the value
of this element's selectionDirection
attribute.
The selectionEnd attribute's
getter must run the following steps:
If this element is an input element, and selectionEnd does
not apply to this element, return null.
If there is no selection, return the code unit offset within the relevant value to the character that immediately follows the text entry cursor.
Return the code unit offset within the relevant value to the character that immediately follows the end of the selection.
The selectionEnd attribute's setter must
run the following steps:
If this element is an input element, and selectionEnd does not
apply to this element, throw an "InvalidStateError"
DOMException.
Set the selection range with the value of this element's selectionStart attribute, the given value, and
the value of this element's selectionDirection attribute.
The selectionDirection
attribute's getter must run the following steps:
If this element is an input element, and selectionDirection does not apply to this element, return null.
Return this element's selection direction.
The selectionDirection attribute's
setter must run the following steps:
If this element is an input element, and selectionDirection does not apply to this element, throw an
"InvalidStateError" DOMException.
Set the selection range with the value of this element's selectionStart attribute, the value of this
element's selectionEnd attribute, and the
given value.
The setSelectionRange(start, end,
direction) method, when invoked, must run the following steps:
If this element is an input element, and setSelectionRange() does not apply to this element, throw an
"InvalidStateError" DOMException.
Set the selection range with start, end, and direction.
To set the selection range with an integer or null start, an integer or null or the special value infinity end, and optionally a string direction, run the following steps:
If start is null, let start be zero.
If end is null, let end be zero.
Set the selection of the text control to the sequence of code units within the relevant value starting with the code unit at the startth position (in logical order) and ending with the code unit at the (end-1)th position. Arguments greater than the length of the relevant value of the text control (including the special value infinity) must be treated as pointing at the end of the text control. If end is less than or equal to start then the start of the selection and the end of the selection must both be placed immediately before the character with offset end. In UAs where there is no concept of an empty selection, this must set the cursor to be just before the character with offset end.
If direction is not identical to either "backward" or "forward", or if the direction
argument was not given, set direction to "none".
Set the selection direction of the text control to direction.
If the previous steps caused the selection of the text control to be modified (in
either extent or direction), then queue an
element task on the user interaction task source given the element to fire an event named select
at the element, with the bubbles attribute initialized to
true.
The setRangeText(replacement,
start, end, selectMode) method, when invoked, must
run the following steps:
If this element is an input element, and setRangeText() does
not apply to this element, throw an "InvalidStateError"
DOMException.
Set this element's dirty value flag to true.
If the method has only one argument, then let start and end have the values of the selectionStart attribute and the selectionEnd attribute respectively.
Otherwise, let start, end have the values of the second and third arguments respectively.
If start is greater than end, then throw an
"IndexSizeError" DOMException.
If start is greater than the length of the relevant value of the text control, then set it to the length of the relevant value of the text control.
If end is greater than the length of the relevant value of the text control, then set it to the length of the relevant value of the text control.
Let selection start be the current value of the selectionStart attribute.
Let selection end be the current value of the selectionEnd attribute.
If start is less than end, delete the sequence of code units within the element's relevant value starting with the code unit at the startth position and ending with the code unit at the (end-1)th position.
Insert the value of the first argument into the text of the relevant value of the text control, immediately before the startth code unit.
Let new length be the length of the value of the first argument.
Let new end be the sum of start and new length.
Run the appropriate set of substeps from the following list:
select"Let selection start be start.
Let selection end be new end.
start"Let selection start and selection end be start.
end"Let selection start and selection end be new end.
preserve"Let old length be end minus start.
Let delta be new length minus old length.
If selection start is greater than end, then increment it by delta. (If delta is negative, i.e. the new text is shorter than the old text, then this will decrease the value of selection start.)
Otherwise: if selection start is greater than start, then set it to start. (This snaps the start of the selection to the start of the new text if it was in the middle of the text that it replaced.)
If selection end is greater than end, then increment it by delta in the same way.
Otherwise: if selection end is greater than start, then set it to new end. (This snaps the end of the selection to the end of the new text if it was in the middle of the text that it replaced.)
Set the selection range with selection start and selection end.
The setRangeText() method uses the
following enumeration:
enum SelectionMode {
"select",
"start",
"end",
"preserve" // default
};To obtain the currently selected text, the following JavaScript suffices:
var selectionText = control.value.substring(control.selectionStart, control.selectionEnd);To add some text at the start of a text control, while maintaining the text selection, the three attributes must be preserved:
var oldStart = control.selectionStart;
var oldEnd = control.selectionEnd;
var oldDirection = control.selectionDirection;
var prefix = "http://";
control.value = prefix + control.value;
control.setSelectionRange(oldStart + prefix.length, oldEnd + prefix.length, oldDirection);可提交元素 都是
约束验证候选项，
除非有条件
禁止了该元素的约束验证。
（例如，object 元素就会
禁止约束验证。）
元素可以定义 自定义验证错误信息。
元素必须把它的 自定义验证错误信息
初始化为空字符串。当它的值不是空字符串时，该元素就
处于自定义错误状态。
可以使用 setCustomValidity() 方法设置它。
用户代理应该使用 自定义验证错误信息
把控件的错误提示给用户。
元素可以有很多种约束方式。下面的 合法性状态 列表给出了约束验证下表单控件可能的状态。 （下面的定义是非规范的；本规范的其他地方更精确地定义了每个状态是否适用。）
当一个控件没有 值 但设置了 required 属性时（input required, textarea required）；
另外一种情况是 select 元素和
单选按钮组 中的控件，
规则较复杂，它们的定义请参考相应章节。
当控件有 值 但长度超过了
表单控件的 maxlength 属性
（input maxlength, textarea
maxlength）。
当控件有 值 但长度小于
表单控件 minlength 属性
（input minlength, textarea
minlength）。
当控件的输入不完整，且用户代理认为用户不应该以当前状态提交表单时。
当控件的 自定义验证错误信息
（通过元素的 setCustomValidity() 方法设置）
是非空字符串时。
即使元素被 禁用，也可能处于上述状态； 因此即使表单提交时的表单验证不向用户指出问题，这些状态也可以表示在 DOM 中。
如果一个元素没有处于任何上述 合法性状态， 就说这个元素 符合它的约束。
When the user agent is required to statically validate the constraints of
form element form, it must run the following steps, which return
either a positive result (all the controls in the form are valid) or a negative
result (there are invalid controls) along with a (possibly empty) list of elements that are
invalid and for which no script has claimed responsibility:
Let controls be a list of all the submittable elements whose form owner is form, in tree order.
Let invalid controls be an initially empty list of elements.
For each element field in controls, in tree order:
If field is not a candidate for constraint validation, then move on to the next element.
Otherwise, if field satisfies its constraints, then move on to the next element.
Otherwise, add field to invalid controls.
If invalid controls is empty, then return a positive result.
Let unhandled invalid controls be an initially empty list of elements.
For each element field in invalid controls, if any, in tree order:
Let notCanceled be the result of firing an
event named invalid at field, with the
cancelable attribute initialized to true.
If notCanceled is true, then add field to unhandled invalid controls.
Return a negative result with the list of elements in the unhandled invalid controls list.
If a user agent is to interactively validate the constraints of form
element form, then the user agent must run the following steps:
Statically validate the constraints of form, and let unhandled invalid controls be the list of elements returned if the result was negative.
If the result was positive, then return that result.
Report the problems with the constraints of at least one of the elements given in unhandled invalid controls to the user.
User agents may focus one of those elements in the process, by running the focusing steps for that element, and may change the scrolling position of the document, or perform some other action that brings the element to the user's attention. For elements that are form-associated custom elements, user agents should use their validation anchor instead, for the purposes of these actions.
User agents may report more than one constraint violation.
User agents may coalesce related constraint violation reports if appropriate (e.g. if multiple radio buttons in a group are marked as required, only one error need be reported).
If one of the controls is not being rendered (e.g. it has the attribute set) then user agents may report a script error.
Return a negative result.
willValidateHTMLObjectElement/willValidate
Support in all current engines.
Returns true if the element will be validated when the form is submitted; false otherwise.
setCustomValidity(message)HTMLObjectElement/setCustomValidity
Support in all current engines.
HTMLSelectElement/setCustomValidity
Support in all current engines.
Sets a custom error, so that the element would fail to validate. The given message is the message to be shown to the user when reporting the problem to the user.
If the argument is the empty string, clears the custom error.
validity . valueMissingReturns true if the element has no value but is a required field; false otherwise.
validity . typeMismatchReturns true if the element's value is not in the correct syntax; false otherwise.
validity . patternMismatchReturns true if the element's value doesn't match the provided pattern; false otherwise.
validity . tooLongSupport in all current engines.
Returns true if the element's value is longer than the provided maximum length; false otherwise.
validity . tooShortSupport in all current engines.
Returns true if the element's value, if it is not the empty string, is shorter than the provided minimum length; false otherwise.
validity . rangeUnderflowReturns true if the element's value is lower than the provided minimum; false otherwise.
validity . rangeOverflowReturns true if the element's value is higher than the provided maximum; false otherwise.
validity . stepMismatchReturns true if the element's value doesn't fit the rules given by the step attribute; false otherwise.
validity . badInputSupport in all current engines.
Returns true if the user has provided input in the user interface that the user agent is unable to convert to a value; false otherwise.
validity . customErrorReturns true if the element has a custom error; false otherwise.
validity . validReturns true if the element's value has no validity problems; false otherwise.
checkValidity()HTMLObjectElement/checkValidity
Support in all current engines.
HTMLSelectElement/checkValidity
Support in all current engines.
Returns true if the element's value has no validity problems; false otherwise. Fires an invalid event at the element in the latter case.
reportValidity()HTMLFormElement/reportValidity
Support in all current engines.
HTMLInputElement/reportValidity
Support in all current engines.
Returns true if the element's value has no validity problems; otherwise, returns false, fires
an invalid event at the element, and (if the event isn't
canceled) reports the problem to the user.
validationMessageHTMLObjectElement/validationMessage
Support in all current engines.
Returns the error message that would be shown to the user if the element was to be checked for validity.
The willValidate attribute's getter must
return true, if this element is a candidate for constraint validation, and false
otherwise (i.e., false if any conditions are barring it from constraint validation).
The willValidate attribute
of ElementInternals interface, on getting, must throw a
"NotSupportedError" DOMException if the
target element is not a
form-associated custom element. Otherwise, it must return true if the
target element is a
candidate for constraint validation, and false otherwise.
HTMLInputElement/setCustomValidity
Support in all current engines.
The setCustomValidity(message) method, when
invoked, must set the custom validity error message to message.
In the following example, a script checks the value of a form control each time it is edited,
and whenever it is not a valid value, uses the setCustomValidity() method to set an appropriate
message.
<label>Feeling: <input name=f type="text" oninput="check(this)"></label>
<script>
function check(input) {
if (input.value == "good" ||
input.value == "fine" ||
input.value == "tired") {
input.setCustomValidity('"' + input.value + '" is not a feeling.');
} else {
// input is fine -- reset the error message
input.setCustomValidity('');
}
}
</script>Support in all current engines.
The validity attribute's getter must return a
ValidityState object that represents the validity states of this
element. This object is live.
The validity attribute of
ElementInternals interface, on getting, must throw a
"NotSupportedError" DOMException if the
target element is not a
form-associated custom element. Otherwise, it must return a
ValidityState object that represents the validity states of the
target element. This object is live.
[Exposed=Window]
interface ValidityState {
readonly attribute boolean valueMissing;
readonly attribute boolean typeMismatch;
readonly attribute boolean patternMismatch;
readonly attribute boolean tooLong;
readonly attribute boolean tooShort;
readonly attribute boolean rangeUnderflow;
readonly attribute boolean rangeOverflow;
readonly attribute boolean stepMismatch;
readonly attribute boolean badInput;
readonly attribute boolean customError;
readonly attribute boolean valid;
};A ValidityState object has the following attributes. On getting, they must return
true if the corresponding condition given in the following list is true, and false otherwise.
valueMissingThe control is suffering from being missing.
typeMismatchSupport in all current engines.
The control is suffering from a type mismatch.
patternMismatchSupport in all current engines.
The control is suffering from a pattern mismatch.
tooLongThe control is suffering from being too long.
tooShortThe control is suffering from being too short.
rangeUnderflowSupport in all current engines.
The control is suffering from an underflow.
rangeOverflowSupport in all current engines.
The control is suffering from an overflow.
stepMismatchSupport in all current engines.
The control is suffering from a step mismatch.
badInputThe control is suffering from bad input.
customErrorThe control is suffering from a custom error.
validNone of the other conditions are true.
The check validity steps for an element element are:
If element is a candidate for constraint validation and does not satisfy its constraints, then:
Fire an event named invalid at element, with the cancelable attribute initialized to true (though canceling
has no effect).
Return false.
Return true.
The checkValidity() method, when
invoked, must run the check validity steps on this element.
ElementInternals/checkValidity
The checkValidity() method
of the ElementInternals interface must run these steps:
Let element be this ElementInternals's target element.
If element is not a form-associated custom element, then throw a
"NotSupportedError" DOMException.
Run the check validity steps on element.
The report validity steps for an element element are:
If element is a candidate for constraint validation and does not satisfy its constraints, then:
Let report be the result of firing an
event named invalid at element, with the
cancelable attribute initialized to true.
If report is true, then report the problems with the constraints of this element to the user. When reporting the problem with the constraints to the user, the user agent may run the focusing steps for element, and may change the scrolling position of the document, or perform some other action that brings element to the user's attention. User agents may report more than one constraint violation, if element suffers from multiple problems at once. If element is not being rendered, then the user agent may, instead of notifying the user, report the error for the running script.
Return false.
Return true.
The reportValidity() method, when
invoked, must run the report validity steps on this element.
ElementInternals/reportValidity
The reportValidity()
method of the ElementInternals interface must run these steps:
Let element be this ElementInternals's target element.
If element is not a form-associated custom element, then throw a
"NotSupportedError" DOMException.
Run the report validity steps on element.
The validationMessage attribute's
getter must run these steps:
If this element is not a candidate for constraint validation or if this element satisfies its constraints, then return the empty string.
Return a suitably localized message that the user agent would show the user if this were the only form control with a validity constraint problem. If the user agent would not actually show a textual message in such a situation (e.g., it would show a graphical cue instead), then return a suitably localized message that expresses (one or more of) the validity constraint(s) that the control does not satisfy. If the element is a candidate for constraint validation and is suffering from a custom error, then the custom validity error message should be present in the return value.
服务器不应依赖客户端验证。恶意用户可以轻松绕过客户端验证， 没实现这些功能的旧的用户代理或自动化工具也会让用户无意地绕过客户端验证。 约束验证功能只是为了提升用户体验，并未提供任何安全机制。
This section is non-normative.
提交表单时，表单中的数据会转换为 enctype 指定的结构， 然后使用给定的 method 发送到 action 指定的目的地址。
例如下面的表单：
<form action="/find.cgi" method=get> <input type=text name=t> <input type=search name=q> <input type=submit> </form>
如果用户在第一个字段输入 "cats"，在第二个字段输入 "fur"，然后点击提交按钮，
那么用户代理就会加载 /find.cgi?t=cats&q=fur。
另一方面考虑这个表单：
<form action="/find.cgi" method=post enctype="multipart/form-data"> <input type=text name=t> <input type=search name=q> <input type=submit> </form>
给同样的用户输入，提交的结果会非常不同：用户代理会发一个 HTTP POST 请求到给定的 URL，消息体是像这样的文本：
------kYFrd4jNJEgCervE Content-Disposition: form-data; name="t" cats ------kYFrd4jNJEgCervE Content-Disposition: form-data; name="q" fur ------kYFrd4jNJEgCervE--
form 元素的 默认按钮 是
表单 owner 是这个 form 元素的
按照 树序 的第一个 提交按钮。
如果用户代理支持用户隐式地提交表单（例如在某些平台下当文本控件
获得焦点 时按下回车键就会暗示提交表单），
然后如果表单的 默认按钮 有
激活行为
且不处于 禁用状态 就必须提交表单，
而且用户代理就必须在那个 默认按钮 上
触发一个 click 事件。
Web 上有些页面只有在隐式提交表单时才可用，因此强烈建议用户代理支持此功能。
对于表单没有
提交按钮 的情况，如果多个字段 阻止了隐式提交，
那么隐式提交机制必须什么都不做；
否则用户代理必须从 form 元素自己 提交
form 元素。
上一段中，form 元素中 阻止隐式提交 的元素是指，
表单 owner 的 form 元素
的 type 属性处于以下状态的 input 元素：
Text,
Search,
URL,
Telephone,
E-mail,
Password,
Date,
Month,
Week,
Time,
Local Date and Time,
Number
Each form element has a constructing entry list boolean, initially
false.
Each form element has a firing submission events boolean, initially
false.
When a form element form is submitted from an element submitter
(typically a button), optionally with a submitted from submit() method flag set, the user agent must run the
following steps:
If form cannot navigate, then return.
If form's constructing entry list is true, then return.
Let form document be form's node document.
If form document's active sandboxing flag set has its sandboxed forms browsing context flag set, then return.
Let form browsing context be the browsing context of form document.
If the submitted from submit() method flag
is not set, then:
If form's firing submission events is true, then return.
Set form's firing submission events to true.
If the submitter element's no-validate state is false, then interactively validate the constraints of form and examine the result. If the result is negative (i.e., the constraint validation concluded that there were invalid fields and probably informed the user of this), then:
Set form's firing submission events to false.
Return.
Let submitterButton be null if submitter is form. Otherwise, let submitterButton be submitter.
Let continue be the result of firing an
event named submit at form using
SubmitEvent, with the submitter
attribute initialized to submitterButton, the bubbles attribute initialized to true, and the cancelable attribute initialized to true.
Set form's firing submission events to false.
If continue is false, then return.
If form cannot navigate, then return.
Cannot navigate is run again as dispatching the submit event could have changed the outcome.
Let encoding be the result of picking an encoding for the form.
Let entry list be the result of constructing the entry list with form, submitter, and encoding.
If form cannot navigate, then return.
Cannot navigate is run again as dispatching the formdata event in constructing the entry list
could have changed the outcome.
Let action be the submitter element's action.
If action is the empty string, let action be the URL of the form document.
Parse the URL action, relative to the submitter element's node document. If this fails, return.
Let parsed action be the resulting URL record.
Let scheme be the scheme of parsed action.
Let enctype be the submitter element's enctype.
Let method be the submitter element's method.
Let target be the submitter element's formtarget attribute value, if the element is a submit button and has such an attribute. Otherwise, let it
be the result of getting an element's target given
submitter's form owner.
Let noopener be the result of getting an element's noopener with form and targetAttributeValue.
Let target browsing context and windowType be the result of applying the rules for choosing a browsing context using target, form browsing context, and noopener.
Let historyHandling be "replace" if
windowType is either "new and unrestricted" or "new with no opener"; otherwise "default".
If target browsing context is null, then return.
If form document has not yet completely loaded and the
submitted from submit() method flag is set, then
set historyHandling to "replace".
If the value of method is dialog then jump to the submit dialog steps.
Otherwise, select the appropriate row in the table below based on the value of scheme as given by the first cell of each row. Then, select the appropriate cell on that row based on the value of method as given in the first cell of each column. Then, jump to the steps named in that cell and defined below the table.
| GET | POST | |
|---|---|---|
http
| Mutate action URL | Submit as entity body |
https
| Mutate action URL | Submit as entity body |
ftp
| Get action URL | Get action URL |
javascript
| Get action URL | Get action URL |
data
| Mutate action URL | Get action URL |
mailto
| Mail with headers | Mail as body |
If scheme is not one of those listed in this table, then the behavior is not defined by this specification. User agents should, in the absence of another specification defining this, act in a manner analogous to that defined in this specification for similar schemes.
Each form element has a planned navigation, which is either null or a
task; when the form is first created, its
planned navigation must be set to null. In the behaviors described below, when the
user agent is required to plan to navigate to a particular resource destination, it must run the following steps:
If destination is not a request, then set destination to a new request whose URL is destination.
If the form element's link types include the noreferrer keyword, then set destination's referrer to "no-referrer".
If the form has a non-null planned navigation, remove it from
its task queue.
Queue an element task on the DOM manipulation task source given
the form element and the following steps:
Set the form's planned navigation to null.
Navigate target browsing context to
destination, with historyHandling set to
historyHandling and navigationType
set to "form-submission".
Set the form's planned navigation to the just-queued task.
The behaviors are as follows:
Let query be the result of running the
application/x-www-form-urlencoded serializer with entry
list and encoding.
Set parsed action's query component to query.
Plan to navigate to parsed action.
Switch on enctype:
application/x-www-form-urlencodedLet body be the result of running the
application/x-www-form-urlencoded serializer with entry
list and encoding.
Set body to the result of encoding body.
Let MIME type be "application/x-www-form-urlencoded".
multipart/form-dataLet body be the result of running the multipart/form-data encoding algorithm with entry list
and encoding.
Let MIME type be the concatenation of the string "multipart/form-data;", a U+0020 SPACE character, the string "boundary=", and the multipart/form-data
boundary string generated by the multipart/form-data
encoding algorithm.
text/plainLet body be the result of running the text/plain
encoding algorithm with entry list.
Set body to the result of encoding body using encoding.
Let MIME type be "text/plain".
Plan to navigate to a new request whose
url is parsed action, method is method, header list consists of `Content-Type`/MIME type, and body is body.
Plan to navigate to parsed action.
entry list is discarded.
Let headers be the result of running the
application/x-www-form-urlencoded serializer with entry
list and encoding.
Replace occurrences of U+002B PLUS SIGN characters (+) in headers with
the string "%20".
Set parsed action's query to headers.
Plan to navigate to parsed action.
Switch on enctype:
text/plainLet body be the result of running the text/plain
encoding algorithm with entry list.
Set body to the result of running UTF-8 percent-encode on body using the default encode set. [URL]
Let body be the result of running the
application/x-www-form-urlencoded serializer with entry
list and encoding.
If parsed action's query is null, then set it to the empty string.
If parsed action's query is not the empty string, then append a single U+0026 AMPERSAND character (&) to it.
Append "body=" to parsed action's query.
Append body to parsed action's query.
Plan to navigate to parsed action.
Let subject be the nearest ancestor dialog element of form, if any.
If there isn't one, or if it does not have an open
attribute, do nothing. Otherwise, proceed as follows:
If submitter is an input element whose type attribute is in the Image Button state, then let result
be the string formed by concatenating the selected coordinate's x-component, expressed as a base-ten number using ASCII digits, a
U+002C COMMA character (,), and the selected
coordinate's y-component, expressed in the same way as the x-component.
Otherwise, if submitter has a value, then let result be that value.
Otherwise, there is no result.
Then, close the dialog subject. If there is a result, let that be the return value.
The algorithm to construct the entry list given a form, an optional submitter, and an optional encoding, is as follows. If not specified otherwise, submitter is null.
If form's constructing entry list is true, then return null.
Set form's constructing entry list to true.
Let controls be a list of all the submittable elements whose form owner is form, in tree order.
For each element field in controls, in tree order:
If any of the following is true:
datalist element ancestor.input element whose type attribute is in the Checkbox state and whose checkedness is false.input element whose type attribute is in the Radio Button state and whose checkedness is false.object element that is not using
a plugin.Then continue.
If the field element is an input element whose type attribute is in the Image Button state, then:
If the field element has a name
attribute specified and its value is not the empty string, let name be
that value followed by a single U+002E FULL STOP character (.). Otherwise, let name be the empty string.
Let namex be the string consisting of the concatenation of name and a single U+0078 LATIN SMALL LETTER X character (x).
Let namey be the string consisting of the concatenation of name and a single U+0079 LATIN SMALL LETTER Y character (y).
The field element is submitter, and before this algorithm was invoked the user indicated a coordinate. Let x be the x-component of the coordinate selected by the user, and let y be the y-component of the coordinate selected by the user.
Append an entry to entry list with namex and x.
Append an entry to entry list with namey and y.
Continue.
If the field is a form-associated custom element, then perform the entry construction algorithm given field and entry list, then continue.
If either the field element does not have a
name attribute specified, or its
name attribute's value is the empty string, then
continue.
Let name be the value of the field element's
name attribute.
If the field element is a select element, then for each
option element in the select element's list of options whose selectedness is true and that is not disabled, append an entry to entry
list with name and the value of the
option element.
Otherwise, if the field element is an input element whose
type attribute is in the Checkbox state or the Radio Button state, then:
If the field element has a value attribute specified, then let value
be the value of that attribute; otherwise, let value be the string "on".
Append an entry to entry list with name and value.
Otherwise, if the field element is an input element whose type attribute is in the File Upload state, then:
If there are no selected files,
then append an entry to entry list with name and a new
File object with an empty name, application/octet-stream as type,
and an empty body.
Otherwise, for each file in selected
files, append an entry to entry list with name and
a File object representing the file.
Otherwise, if the field element is an object element:
try to obtain a form submission value from the plugin, and if that is successful,
append an entry to entry list with name and the returned
form submission value.
Otherwise, if the field element is an input element whose type attribute is in the state and name is an ASCII
case-insensitive match for "_charset_":
Let charset be the name of
encoding if encoding is given, and "UTF-8"
otherwise.
Append an entry to entry list with name and charset.
Otherwise, if the field element is a textarea element, append
an entry to entry list with name and the value of the field element, and the prevent
line break normalization flag set.
In the case of the value of
textarea elements, the line break normalization is already performed during the
conversion of the control's raw value into
the control's value (which also performs any necessary
line wrapping).
Otherwise, append an entry to entry list with name and the value of the field element.
If the element has a dirname attribute, and that
attribute's value is not the empty string, then:
Let dirname be the value of the element's dirname attribute.
Let dir be the string "ltr" if the
directionality of the element is 'ltr', and "rtl" otherwise (i.e., when the directionality of the element is
'rtl').
Append an entry to entry list with dirname and dir.
An element can only have a dirname
attribute if it is a textarea element or an input element whose
type attribute is in either the Text state or the Search state.
Let form data be a new FormData object associated with
entry list.
Fire an event named
formdata at form using
FormDataEvent, with the formData
attribute initialized to form data and the
bubbles attribute initialized to true.
Set form's constructing entry list to false.
Return a clone of entry list.
To append an entry to entry list, given name, value, and optional prevent line break normalization flag, run these steps:
For name, replace every occurrence of U+000D (CR) not followed by U+000A (LF), and every occurrence of U+000A (LF) not preceded by U+000D (CR), by a string consisting of a U+000D (CR) and U+000A (LF).
Replace name with the result of converting to a sequence of Unicode scalar values.
If value is not a File object, then:
If the prevent line break normalization flag is unset, then replace every occurrence of U+000D (CR) not followed by U+000A (LF), and every occurrence of U+000A (LF) not preceded by U+000D (CR) in value, by a string consisting of a U+000D (CR) and U+000A (LF).
Replace value with the result of converting to a sequence of Unicode scalar values.
Create an entry with name and value, and append it to entry list.
If the user agent is to pick an encoding for a form, it must run the following steps:
Let encoding be the document's character encoding.
If the form element has an accept-charset attribute, set encoding to
the return value of running these substeps:
Let input be the value of the form element's accept-charset attribute.
Let candidate encoding labels be the result of splitting input on ASCII whitespace.
Let candidate encodings be an empty list of character encodings.
For each token in candidate encoding labels in turn (in the order in which they were found in input), get an encoding for the token and, if this does not result in failure, append the encoding to candidate encodings.
If candidate encodings is empty, return UTF-8.
Return the first encoding in candidate encodings.
Return the result of getting an output encoding from encoding.
application/x-www-form-urlencoded 的细节请参考 WHATWG URL 标准。[URL]
The multipart/form-data encoding algorithm, given an entry
list and encoding, is as follows:
Let result be the empty string.
For each entry in entry list:
For each character in the entry's name and value that cannot be expressed using the selected character encoding, replace the character by a string consisting of a U+0026 AMPERSAND character (&), a U+0023 NUMBER SIGN character (#), one or more ASCII digits representing the code point of the character in base ten, and finally a U+003B (;).
Encode the (now mutated) entry list using the rules described by RFC 7578,
Returning Values from Forms: multipart/form-data, and return
the resulting byte stream. [RFC7578]
Each entry in entry list is a field, the name of the entry is the field name and the value of the entry is the field value.
The order of parts must be the same as the order of fields in entry list. Multiple entries with the same name must be treated as distinct fields.
The parts of the generated multipart/form-data resource that correspond to
non-file fields must not have a `Content-Type` header specified. Their names and
values must be encoded using the character encoding selected above.
File names included in the generated multipart/form-data resource (as part of
file fields) must use the character encoding selected above, though the precise name may be
approximated if necessary (e.g. newlines could be removed from file names, quotes could be
changed to "%22", and characters not expressible in the selected character encoding could be
replaced by other characters).
The boundary used by the user agent in generating the return value of this algorithm is the
multipart/form-data boundary string. (This value is used to
generate the MIME type of the form submission payload generated by this algorithm.)
For details on how to interpret multipart/form-data payloads, see RFC 7578. [RFC7578]
The text/plain encoding algorithm, given an entry
list, is as follows:
Let result be the empty string.
For each entry in entry list:
Return result.
Payloads using the text/plain format are intended to be human readable. They are
not reliably interpretable by computer, as the format is ambiguous (for example, there is no way
to distinguish a literal newline in a value from the newline at the end of the value).
SubmitEvent interfaceSupport in all current engines.
Support in all current engines.
[Exposed=Window]
interface SubmitEvent : Event {
constructor(DOMString type, optional SubmitEventInit eventInitDict = {});
readonly attribute HTMLElement? submitter;
};
dictionary SubmitEventInit : EventInit {
HTMLElement? submitter = null;
};submitterReturns the element representing the submit button that triggered the form submission, or null if the submission was not triggered by a button.
The submitter attribute must return
the value it was initialized to.
FormDataEvent interfaceSupport in all current engines.
Support in all current engines.
[Exposed=Window]
interface FormDataEvent : Event {
constructor(DOMString type, FormDataEventInit eventInitDict);
readonly attribute FormData formData;
};
dictionary FormDataEventInit : EventInit {
required FormData formData;
};formDataReturns a FormData object representing names and values of elements associated
to the target form. Operations on the FormData object will affect
form data to be submitted.
The formData attribute must return the value it was
initialized to. It represents a FormData object associated to the entry list that is constructed when the form is submitted.
重置
form 元素 form 时，要执行这些步骤：
在 form 上
触发一个 名为 reset 的事件，
其 bubbles 和
cancelable 属性初始化为 true，
令 reset 为上述触发事件操作的结果。
每个 可重置元素 定义了它自己的 重置算法。
这些算法中对表单控件的改变，不计为用户引起的改变
（因此，不会引发 input 事件）。
details elementSupport in all current engines.
Support in all current engines.
summary element followed by flow content.open — Whether the details are visible[Exposed=Window]
interface HTMLDetailsElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute boolean open;
};The details element represents a disclosure widget from which the
user can obtain additional information or controls.
The details element is not appropriate for footnotes. Please see the section on footnotes for details on how to mark up footnotes.
The first summary element child of the element, if any,
represents the summary or legend of the details. If there is no
child summary element, the user agent should provide its own legend (e.g.
"Details").
The rest of the element's contents represents the additional information or controls.
The open content
attribute is a boolean attribute. If present, it indicates that both the summary and
the additional information is to be shown to the user. If the attribute is absent, only the
summary is to be shown.
When the element is created, if the attribute is absent, the additional information should be hidden; if the attribute is present, that information should be shown. Subsequently, if the attribute is removed, then the information should be hidden; if the attribute is added, the information should be shown.
The user agent should allow the user to request that the additional information be shown or
hidden. To honor a request for the details to be shown, the user agent must set the open attribute on the element to the empty string. To honor a
request for the information to be hidden, the user agent must remove the open attribute from the element.
This ability to request that additional information be shown or hidden
may simply be the activation behavior of the appropriate
summary element, in the case such an element exists. However, if no such element
exists, user agents can still provide this ability through some other user interface
affordance.
Whenever the open attribute is added to or removed from
a details element, the user agent must queue an element task on the
DOM manipulation task source given then details element that runs the
following steps, which are known as the details notification task steps, for this
details element:
If another task has been queued to run the details notification task steps for this
details element, then return.
When the open attribute is toggled
several times in succession, these steps essentially get coalesced so that only one event is
fired.
Fire an event named toggle at the details element.
The open
IDL attribute must reflect the open content
attribute.
The following example shows the details element being used to hide technical
details in a progress report.
<section class="progress window">
<h1>Copying "Really Achieving Your Childhood Dreams"</h1>
<details>
<summary>Copying... <progress max="375505392" value="97543282"></progress> 25%</summary>
<dl>
<dt>Transfer rate:</dt> <dd>452KB/s</dd>
<dt>Local filename:</dt> <dd>/home/rpausch/raycd.m4v</dd>
<dt>Remote filename:</dt> <dd>/var/www/lectures/raycd.m4v</dd>
<dt>Duration:</dt> <dd>01:16:27</dd>
<dt>Color profile:</dt> <dd>SD (6-1-6)</dd>
<dt>Dimensions:</dt> <dd>320×240</dd>
</dl>
</details>
</section>The following shows how a details element can be used to hide some controls by
default:
<details>
<summary><label for=fn>Name & Extension:</label></summary>
<p><input type=text id=fn name=fn value="Pillar Magazine.pdf">
<p><label><input type=checkbox name=ext checked> Hide extension</label>
</details>One could use this in conjunction with other details in a list to allow the user
to collapse a set of fields down to a small set of headings, with the ability to open each
one.
In these examples, the summary really just summarizes what the controls can change, and not the actual values, which is less than ideal.
Because the open attribute is added and removed
automatically as the user interacts with the control, it can be used in CSS to style the element
differently based on its state. Here, a style sheet is used to animate the color of the summary
when the element is opened or closed:
<style>
details > summary { transition: color 1s; color: black; }
details[open] > summary { color: red; }
</style>
<details>
<summary>Automated Status: Operational</summary>
<p>Velocity: 12m/s</p>
<p>Direction: North</p>
</details>summary elementSupport in all current engines.
details element.HTMLElement.The summary element represents a summary, caption, or legend for the
rest of the contents of the summary element's parent details
element, if any.
A summary element is a summary for its parent details if the following
algorithm returns true:
If this summary element has no parent, then return false.
Let parent be this summary element's parent.
If parent is not a details element, then return false.
If parent's first summary element child is not this
summary element, then return false.
Return true.
The activation behavior of summary elements is to run the following
steps:
If this summary element is not the summary for its parent
details, then return.
Let parent be