video 元素audio elementtrack elementTrackEvent interfacevideo 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
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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.)