1. 13 HTML 语法
    1. 13.1 Writing HTML documents
      1. 13.1.1 The DOCTYPE
      2. 13.1.2 Elements
        1. 13.1.2.1 Start tags
        2. 13.1.2.2 End tags
        3. 13.1.2.3 Attributes
        4. 13.1.2.4 Optional tags
        5. 13.1.2.5 内容模型限制
        6. 13.1.2.6 对原始文本和可转义的原始文本元素的内容的限制
      3. 13.1.3 文本
        1. 13.1.3.1 换行
      4. 13.1.4 Character references
      5. 13.1.5 CDATA 部分
      6. 13.1.6 注释

13 HTML 语法

本章只描述有 HTML MIME type 标注的资源的语法规则。XML 资源的规则在下一章 " XML 语法" 中讨论。

13.1 Writing HTML documents

This section only applies to documents, authoring tools, and markup generators. In particular, it does not apply to conformance checkers; conformance checkers must use the requirements given in the next section ("parsing HTML documents").

Documents must consist of the following parts, in the given order:

  1. Optionally, a single U+FEFF BYTE ORDER MARK (BOM) character.
  2. Any number of comments and ASCII whitespace.
  3. A DOCTYPE.
  4. Any number of comments and ASCII whitespace.
  5. The document element, in the form of an html element.
  6. Any number of comments and ASCII whitespace.

The various types of content mentioned above are described in the next few sections.

In addition, there are some restrictions on how character encoding declarations are to be serialized, as discussed in the section on that topic.

ASCII whitespace before the html element, at the start of the html element and before the head element, will be dropped when the document is parsed; ASCII whitespace after the html element will be parsed as if it were at the end of the body element. Thus, ASCII whitespace around the document element does not round-trip.

It is suggested that newlines be inserted after the DOCTYPE, after any comments that are before the document element, after the html element's start tag (if it is not omitted), and after any comments that are inside the html element but before the head element.

Many strings in the HTML syntax (e.g. the names of elements and their attributes) are case-insensitive, but only for ASCII upper alphas and ASCII lower alphas. For convenience, in this section this is just referred to as "case-insensitive".

13.1.1 The DOCTYPE

A DOCTYPE is a required preamble.

DOCTYPEs are required for legacy reasons. When omitted, browsers tend to use a different rendering mode that is incompatible with some specifications. Including the DOCTYPE in a document ensures that the browser makes a best-effort attempt at following the relevant specifications.

A DOCTYPE must consist of the following components, in this order:

  1. A string that is an ASCII case-insensitive match for the string "<!DOCTYPE".
  2. One or more ASCII whitespace.
  3. A string that is an ASCII case-insensitive match for the string "html".
  4. Optionally, a DOCTYPE legacy string.
  5. Zero or more ASCII whitespace.
  6. A U+003E GREATER-THAN SIGN character (>).

In other words, <!DOCTYPE html>, case-insensitively.


For the purposes of HTML generators that cannot output HTML markup with the short DOCTYPE "<!DOCTYPE html>", a DOCTYPE legacy string may be inserted into the DOCTYPE (in the position defined above). This string must consist of:

  1. One or more ASCII whitespace.
  2. A string that is an ASCII case-insensitive match for the string "SYSTEM".
  3. One or more ASCII whitespace.
  4. A U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (the quote mark).
  5. The literal string "about:legacy-compat".
  6. A matching U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (i.e. the same character as in the earlier step labeled quote mark).

In other words, <!DOCTYPE html SYSTEM "about:legacy-compat"> or <!DOCTYPE html SYSTEM 'about:legacy-compat'>, case-insensitively except for the part in single or double quotes.

The DOCTYPE legacy string should not be used unless the document is generated from a system that cannot output the shorter string.

13.1.2 Elements

There are six different kinds of elements: void elements, the template element, raw text elements, escapable raw text elements, foreign elements, and normal elements.

Void elements
area, base, br, col, embed, hr, img, input, link, meta, param, source, track, wbr
The template element
template
Raw text elements
script, style
Escapable raw text elements
textarea, title
Foreign elements
Elements from the MathML namespace and the SVG namespace.
Normal elements
All other allowed HTML elements are normal elements.

Tags are used to delimit the start and end of elements in the markup. Raw text, escapable raw text, and normal elements have a start tag to indicate where they begin, and an end tag to indicate where they end. The start and end tags of certain normal elements can be omitted, as described below in the section on optional tags. Those that cannot be omitted must not be omitted. Void elements only have a start tag; end tags must not be specified for void elements. Foreign elements must either have a start tag and an end tag, or a start tag that is marked as self-closing, in which case they must not have an end tag.

The contents of the element must be placed between just after the start tag (which might be implied, in certain cases) and just before the end tag (which again, might be implied in certain cases). The exact allowed contents of each individual element depend on the content model of that element, as described earlier in this specification. Elements must not contain content that their content model disallows. In addition to the restrictions placed on the contents by those content models, however, the five types of elements have additional syntactic requirements.

Void elements can't have any contents (since there's no end tag, no content can be put between the start tag and the end tag).

The template element can have template contents, but such template contents are not children of the template element itself. Instead, they are stored in a DocumentFragment associated with a different Document — without a browsing context — so as to avoid the template contents interfering with the main Document. The markup for the template contents of a template element is placed just after the template element's start tag and just before template element's end tag (as with other elements), and may consist of any text, character references, elements, and comments, but the text must not contain the character U+003C LESS-THAN SIGN (<) or an ambiguous ampersand.

Raw text elements can have text, though it has restrictions described below.

Escapable raw text elements can have text and character references, but the text must not contain an ambiguous ampersand. There are also further restrictions described below.

Foreign elements whose start tag is marked as self-closing can't have any contents (since, again, as there's no end tag, no content can be put between the start tag and the end tag). Foreign elements whose start tag is not marked as self-closing can have text, character references, CDATA sections, other elements, and comments, but the text must not contain the character U+003C LESS-THAN SIGN (<) or an ambiguous ampersand.

The HTML syntax does not support namespace declarations, even in foreign elements.

For instance, consider the following HTML fragment:

<p>
 <svg>
  <metadata>
   <!-- this is invalid -->
   <cdr:license xmlns:cdr="https://www.example.com/cdr/metadata" name="MIT"/>
  </metadata>
 </svg>
</p>

The innermost element, cdr:license, is actually in the SVG namespace, as the "xmlns:cdr" attribute has no effect (unlike in XML). In fact, as the comment in the fragment above says, the fragment is actually non-conforming. This is because SVG 2 does not define any elements called "cdr:license" in the SVG namespace.

Normal elements can have text, character references, other elements, and comments, but the text must not contain the character U+003C LESS-THAN SIGN (<) or an ambiguous ampersand. Some normal elements also have yet more restrictions on what content they are allowed to hold, beyond the restrictions imposed by the content model and those described in this paragraph. Those restrictions are described below.

Tags contain a tag name, giving the element's name. HTML elements all have names that only use ASCII alphanumerics. In the HTML syntax, tag names, even those for foreign elements, may be written with any mix of lower- and uppercase letters that, when converted to all-lowercase, matches the element's tag name; tag names are case-insensitive.

13.1.2.1 Start tags

Start tags must have the following format:

  1. The first character of a start tag must be a U+003C LESS-THAN SIGN character (<).
  2. The next few characters of a start tag must be the element's tag name.
  3. If there are to be any attributes in the next step, there must first be one or more ASCII whitespace.
  4. Then, the start tag may have a number of attributes, the syntax for which is described below. Attributes must be separated from each other by one or more ASCII whitespace.
  5. After the attributes, or after the tag name if there are no attributes, there may be one or more ASCII whitespace. (Some attributes are required to be followed by a space. See the attributes section below.)
  6. Then, if the element is one of the void elements, or if the element is a foreign element, then there may be a single U+002F SOLIDUS character (/). This character has no effect on void elements, but on foreign elements it marks the start tag as self-closing.
  7. Finally, start tags must be closed by a U+003E GREATER-THAN SIGN character (>).
13.1.2.2 End tags

End tags must have the following format:

  1. The first character of an end tag must be a U+003C LESS-THAN SIGN character (<).
  2. The second character of an end tag must be a U+002F SOLIDUS character (/).
  3. The next few characters of an end tag must be the element's tag name.
  4. After the tag name, there may be one or more ASCII whitespace.
  5. Finally, end tags must be closed by a U+003E GREATER-THAN SIGN character (>).
13.1.2.3 Attributes

Attributes for an element are expressed inside the element's start tag.

Attributes have a name and a value. Attribute names must consist of one or more characters other than controls, U+0020 SPACE, U+0022 ("), U+0027 ('), U+003E (>), U+002F (/), U+003D (=), and noncharacters. In the HTML syntax, attribute names, even those for foreign elements, may be written with any mix of ASCII lower and ASCII upper alphas.

Attribute values are a mixture of text and character references, except with the additional restriction that the text cannot contain an ambiguous ampersand.

Attributes can be specified in four different ways:

Empty attribute syntax

Just the attribute name. The value is implicitly the empty string.

In the following example, the disabled attribute is given with the empty attribute syntax:

<input disabled>

If an attribute using the empty attribute syntax is to be followed by another attribute, then there must be ASCII whitespace separating the two.

Unquoted attribute value syntax

The attribute name, followed by zero or more ASCII whitespace, followed by a single U+003D EQUALS SIGN character, followed by zero or more ASCII whitespace, followed by the attribute value, which, in addition to the requirements given above for attribute values, must not contain any literal ASCII whitespace, any U+0022 QUOTATION MARK characters ("), U+0027 APOSTROPHE characters ('), U+003D EQUALS SIGN characters (=), U+003C LESS-THAN SIGN characters (<), U+003E GREATER-THAN SIGN characters (>), or U+0060 GRAVE ACCENT characters (`), and must not be the empty string.

In the following example, the value attribute is given with the unquoted attribute value syntax:

<input value=yes>

If an attribute using the unquoted attribute syntax is to be followed by another attribute or by the optional U+002F SOLIDUS character (/) allowed in step 6 of the start tag syntax above, then there must be ASCII whitespace separating the two.

Single-quoted attribute value syntax

The attribute name, followed by zero or more ASCII whitespace, followed by a single U+003D EQUALS SIGN character, followed by zero or more ASCII whitespace, followed by a single U+0027 APOSTROPHE character ('), followed by the attribute value, which, in addition to the requirements given above for attribute values, must not contain any literal U+0027 APOSTROPHE characters ('), and finally followed by a second single U+0027 APOSTROPHE character (').

In the following example, the type attribute is given with the single-quoted attribute value syntax:

<input type='checkbox'>

If an attribute using the single-quoted attribute syntax is to be followed by another attribute, then there must be ASCII whitespace separating the two.

Double-quoted attribute value syntax

The attribute name, followed by zero or more ASCII whitespace, followed by a single U+003D EQUALS SIGN character, followed by zero or more ASCII whitespace, followed by a single U+0022 QUOTATION MARK character ("), followed by the attribute value, which, in addition to the requirements given above for attribute values, must not contain any literal U+0022 QUOTATION MARK characters ("), and finally followed by a second single U+0022 QUOTATION MARK character (").

In the following example, the name attribute is given with the double-quoted attribute value syntax:

<input name="be evil">

If an attribute using the double-quoted attribute syntax is to be followed by another attribute, then there must be ASCII whitespace separating the two.

There must never be two or more attributes on the same start tag whose names are an ASCII case-insensitive match for each other.


When a foreign element has one of the namespaced attributes given by the local name and namespace of the first and second cells of a row from the following table, it must be written using the name given by the third cell from the same row.

Local name Namespace Attribute name
actuate XLink namespace xlink:actuate
arcrole XLink namespace xlink:arcrole
href XLink namespace xlink:href
role XLink namespace xlink:role
show XLink namespace xlink:show
title XLink namespace xlink:title
type XLink namespace xlink:type
lang XML namespace xml:lang
space XML namespace xml:space
xmlns XMLNS namespace xmlns
xlink XMLNS namespace xmlns:xlink

No other namespaced attribute can be expressed in the HTML syntax.

Whether the attributes in the table above are conforming or not is defined by other specifications (e.g. SVG 2 and MathML); this section only describes the syntax rules if the attributes are serialized using the HTML syntax.

13.1.2.4 Optional tags

Certain tags can be omitted.

Omitting an element's start tag in the situations described below does not mean the element is not present; it is implied, but it is still there. For example, an HTML document always has a root html element, even if the string <html> doesn't appear anywhere in the markup.

An html element's start tag may be omitted if the first thing inside the html element is not a comment.

For example, in the following case it's ok to remove the "<html>" tag:

<!DOCTYPE HTML>
<html>
  <head>
    <title>Hello</title>
  </head>
  <body>
    <p>Welcome to this example.</p>
  </body>
</html>

Doing so would make the document look like this:

<!DOCTYPE HTML>

  <head>
    <title>Hello</title>
  </head>
  <body>
    <p>Welcome to this example.</p>
  </body>
</html>

This has the exact same DOM. In particular, note that whitespace around the document element is ignored by the parser. The following example would also have the exact same DOM:

<!DOCTYPE HTML><head>
    <title>Hello</title>
  </head>
  <body>
    <p>Welcome to this example.</p>
  </body>
</html>

However, in the following example, removing the start tag moves the comment to before the html element:

<!DOCTYPE HTML>
<html>
  <!-- where is this comment in the DOM? -->
  <head>
    <title>Hello</title>
  </head>
  <body>
    <p>Welcome to this example.</p>
  </body>
</html>

With the tag removed, the document actually turns into the same as this:

<!DOCTYPE HTML>
<!-- where is this comment in the DOM? -->
<html>
  <head>
    <title>Hello</title>
  </head>
  <body>
    <p>Welcome to this example.</p>
  </body>
</html>

This is why the tag can only be removed if it is not followed by a comment: removing the tag when there is a comment there changes the document's resulting parse tree. Of course, if the position of the comment does not matter, then the tag can be omitted, as if the comment had been moved to before the start tag in the first place.

An html element's end tag may be omitted if the html element is not immediately followed by a comment.

A head element's start tag may be omitted if the element is empty, or if the first thing inside the head element is an element.

A head element's end tag may be omitted if the head element is not immediately followed by ASCII whitespace or a comment.

A body element's start tag may 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.

A body element's end tag may be omitted if the body element is not immediately followed by a comment.

Note that in the example above, the head element start and end tags, and the body element start tag, can't be omitted, because they are surrounded by whitespace:

<!DOCTYPE HTML>
<html>
  <head>
    <title>Hello</title>
  </head>
  <body>
    <p>Welcome to this example.</p>
  </body>
</html>

(The body and html element end tags could be omitted without trouble; any spaces after those get parsed into the body element anyway.)

Usually, however, whitespace isn't an issue. If we first remove the whitespace we don't care about:

<!DOCTYPE HTML><html><head><title>Hello</title></head><body><p>Welcome to this example.</p></body></html>

Then we can omit a number of tags without affecting the DOM:

<!DOCTYPE HTML><title>Hello</title><p>Welcome to this example.</p>

At that point, we can also add some whitespace back:

<!DOCTYPE HTML>
<title>Hello</title>
<p>Welcome to this example.</p>

This would be equivalent to this document, with the omitted tags shown in their parser-implied positions; the only whitespace text node that results from this is the newline at the end of the head element:

<!DOCTYPE HTML>
<html><head><title>Hello</title>
</head><body><p>Welcome to this example.</p></body></html>

An li element's end tag may be omitted if the li element is immediately followed by another li element or if there is no more content in the parent element.

A dt element's end tag may be omitted if the dt element is immediately followed by another dt element or a dd element.

A dd element's end tag may 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.

A p element's end tag may 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.

We can thus simplify the earlier example further:

<!DOCTYPE HTML><title>Hello</title><p>Welcome to this example.

An rt element's end tag may 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.

An rp element's end tag may 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.

An optgroup element's end tag may be omitted if the optgroup element is immediately followed by another optgroup element, or if there is no more content in the parent element.

An option element's end tag may 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.

A colgroup element's start tag may 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.)

A colgroup element's end tag may be omitted if the colgroup element is not immediately followed by ASCII whitespace or a comment.

A caption element's end tag may be omitted if the caption element is not immediately followed by ASCII whitespace or a comment.

A thead element's end tag may be omitted if the thead element is immediately followed by a tbody or tfoot element.

A tbody element's start tag may 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.)

A tbody element's end tag may 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.

A tfoot element's end tag may be omitted if there is no more content in the parent element.

A tr element's end tag may be omitted if the tr element is immediately followed by another tr element, or if there is no more content in the parent element.

A td element's end tag may 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.

A th element's end tag may 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.

The ability to omit all these table-related tags makes table markup much terser.

Take this example:

<table>
 <caption>37547 TEE Electric Powered Rail Car Train Functions (Abbreviated)</caption>
 <colgroup><col><col><col></colgroup>
 <thead>
  <tr>
   <th>Function</th>
   <th>Control Unit</th>
   <th>Central Station</th>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td>Headlights</td>
   <td>✔</td>
   <td>✔</td>
  </tr>
  <tr>
   <td>Interior Lights</td>
   <td>✔</td>
   <td>✔</td>
  </tr>
  <tr>
   <td>Electric locomotive operating sounds</td>
   <td>✔</td>
   <td>✔</td>
  </tr>
  <tr>
   <td>Engineer's cab lighting</td>
   <td></td>
   <td>✔</td>
  </tr>
  <tr>
   <td>Station Announcements - Swiss</td>
   <td></td>
   <td>✔</td>
  </tr>
 </tbody>
</table>

The exact same table, modulo some whitespace differences, could be marked up as follows:

<table>
 <caption>37547 TEE Electric Powered Rail Car Train Functions (Abbreviated)
 <colgroup><col><col><col>
 <thead>
  <tr>
   <th>Function
   <th>Control Unit
   <th>Central Station
 <tbody>
  <tr>
   <td>Headlights
   <td>✔
   <td>✔
  <tr>
   <td>Interior Lights
   <td>✔
   <td>✔
  <tr>
   <td>Electric locomotive operating sounds
   <td>✔
   <td>✔
  <tr>
   <td>Engineer's cab lighting
   <td>
   <td>✔
  <tr>
   <td>Station Announcements - Swiss
   <td>
   <td>✔
</table>

Since the cells take up much less room this way, this can be made even terser by having each row on one line:

<table>
 <caption>37547 TEE Electric Powered Rail Car Train Functions (Abbreviated)
 <colgroup><col><col><col>
 <thead>
  <tr> <th>Function                              <th>Control Unit     <th>Central Station
 <tbody>
  <tr> <td>Headlights                            <td>✔                <td>✔
  <tr> <td>Interior Lights                       <td>✔                <td>✔
  <tr> <td>Electric locomotive operating sounds  <td>✔                <td>✔
  <tr> <td>Engineer's cab lighting               <td>                 <td>✔
  <tr> <td>Station Announcements - Swiss         <td>                 <td>✔
</table>

The only differences between these tables, at the DOM level, is with the precise position of the (in any case semantically-neutral) whitespace.

However, a start tag must never be omitted if it has any attributes.

Returning to the earlier example with all the whitespace removed and then all the optional tags removed:

<!DOCTYPE HTML><title>Hello</title><p>Welcome to this example.

If the body element in this example had to have a class attribute and the html element had to have a lang attribute, the markup would have to become:

<!DOCTYPE HTML><html lang="en"><title>Hello</title><body class="demo"><p>Welcome to this example.

This section assumes that the document is conforming, in particular, that there are no content model violations. Omitting tags in the fashion described in this section in a document that does not conform to the content models described in this specification is likely to result in unexpected DOM differences (this is, in part, what the content models are designed to avoid).

13.1.2.5 内容模型限制

由于历史原因,某些元素在其内容模型之外还有额外的限制。

table 元素不得包含 tr 元素,虽然根据本规范描述的内容模型 这些元素技术上是允许在 table 里的。 (如果代码中在 table 里放一个 tr 元素, 实际上暗示着在它前面加一个 tbody 开始标签。)

单个 newline 可以紧挨着放在 pretextarea 元素的 开始标签 后面。 这不影响元素的处理。如果元素的内容本身以 换行 开头, 就 必须 包含这个可选的 换行 (否则内容中的前导换行符将被视为可选换行符并被忽略)。

下面两个 pre 块是等价的:

<pre>Hello</pre>
<pre>
Hello</pre>
13.1.2.6 对原始文本和可转义的原始文本元素的内容的限制

原始文本可转义的原始文本元素 中的内容不得出现任何 "</" (U+003C LESS-THAN SIGN, U+002F SOLIDUS) 加那个元素名的大小写不敏感匹配, 再加一个 U+0009 CHARACTER TABULATION (tab), U+000A LINE FEED (LF), U+000C FORM FEED (FF), U+000D CARRIAGE RETURN (CR), U+0020 SPACE, U+003E GREATER-THAN SIGN (>) 或 U+002F SOLIDUS (/) 字符。

13.1.3 文本

在元素内允许 文本,属性值和注释。 如其他部分所述,基于放置文本的位置,对文本中的内容和不允许的内容有额外的限制。

13.1.3.1 换行

HTML 中的 换行 可以表示为 U+000D CARRIAGE RETURN (CR) 字符, U+000A LINE FEED (LF) 字符,或一对 U+000D CARRIAGE RETURN (CR), U+000A LINE FEED (LF) 字符(按这个顺序)。

在允许 字符引用 的地方,一个 U+000A LINE FEED (LF) 字符(但U+000D CARRIAGE RETURN (CR) 字符不行) 的字符引用也表示一个 换行

13.1.4 Character references

In certain cases described in other sections, text may be mixed with character references. These can be used to escape characters that couldn't otherwise legally be included in text.

Character references must start with a U+0026 AMPERSAND character (&). Following this, there are three possible kinds of character references:

Named character references
The ampersand must be followed by one of the names given in the named character references section, using the same case. The name must be one that is terminated by a U+003B SEMICOLON character (;).
Decimal numeric character reference
The ampersand must be followed by a U+0023 NUMBER SIGN character (#), followed by one or more ASCII digits, representing a base-ten integer that corresponds to a code point that is allowed according to the definition below. The digits must then be followed by a U+003B SEMICOLON character (;).
Hexadecimal numeric character reference
The ampersand must be followed by a U+0023 NUMBER SIGN character (#), which must be followed by either a U+0078 LATIN SMALL LETTER X character (x) or a U+0058 LATIN CAPITAL LETTER X character (X), which must then be followed by one or more ASCII hex digits, representing a hexadecimal integer that corresponds to a code point that is allowed according to the definition below. The digits must then be followed by a U+003B SEMICOLON character (;).

The numeric character reference forms described above are allowed to reference any code point excluding U+000D CR, noncharacters, and controls other than ASCII whitespace.

An ambiguous ampersand is a U+0026 AMPERSAND character (&) that is followed by one or more ASCII alphanumerics, followed by a U+003B SEMICOLON character (;), where these characters do not match any of the names given in the named character references section.

13.1.5 CDATA 部分

CDATA 部分 必须按顺序包含以下部分:

  1. 字符串 "<![CDATA["。
  2. 可选地,不包含字符串 "]]>" 的 文本
  3. 字符串 "]]>"。

CDATA 部分职能用于外部内容(MathML 或 SVG)。在这个例子中, CDATA 部分用于转义 MathML ms 元素的内容:

<p>You can add a string to a number, but this stringifies the number:</p>
<math>
 <ms><![CDATA[x<y]]></ms>
 <mo>+</mo>
 <mn>3</mn>
 <mo>=</mo>
 <ms><![CDATA[x<y3]]></ms>
</math>

13.1.6 注释

注释 必须有如下格式:

  1. 字符串 "<!--"。
  2. 可选的 文本,额外的限制是该文本不得以字符串 ">" 或 "->" 起始。 也不的包含 "<!--", "-->" 或 "--!>",也不得以字符串 "<!-" 结尾。
  3. 字符串 "-->"。

text 允许以字符串 "<!" 结束,比如 <!--My favorite operators are > and <!-->