HTML5

If the specified resource is to be used, then, when the element is created or when the poster attribute is set, changed, or removed, the user agent must run the following steps to determine the element's poster frame:

1. If there is an existing instance of this algorithm running for this video element, abort that instance of this algorithm without changing the poster frame.

2. If the poster attribute's value is the empty string, then there is no poster frame; abort these steps.

3. Resolve the poster attribute's value relative to the element. If this fails, then there is no poster frame; abort these steps.

4. Fetch the resulting absolute URL, from the element's Document's origin. This must delay the load event of the element's document.

5. If an image is thus obtained, the poster frame is that image. Otherwise, there is no poster frame.

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When no video data is available (the element's readyState attribute is either HAVE_NOTHING, or HAVE_METADATA but no video data has yet been obtained at all), the video element represents either the poster frame, or nothing.

When a video element is paused and the current playback position is the first frame of video, the element represents either the frame of video corresponding to the current playback position or the poster frame, at the discretion of the user agent.

Notwithstanding the above, the poster frame should be preferred over nothing, but the poster frame should not be shown again after a frame of video has been shown.

When a video element is paused at any other position, the element represents the frame of video corresponding to the current playback position, or, if that is not yet available (e.g. because the video is seeking or buffering), the last frame of the video to have been rendered.

When a video element is potentially playing, it 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. Similarly, any audio associated with the video must, if played, be played synchronized with the current playback position, at the specified volume with the specified mute state.

When a video element is neither potentially playing nor paused (e.g. when seeking or stalled), the element represents the last frame of the video to have been rendered.

Which frame in a video stream corresponds to a particular playback position is defined by the video stream's format.

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 or showing by default modes.

In addition to the above, the user agent may provide messages to the user (such as "buffering", "no video loaded", "error", or more detailed information) by overlaying text or icons on the video or other areas of the element's playback area, or in another appropriate manner.

User agents that cannot render the video may instead make the element represent a link to an external video playback utility or to the video data itself.

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The intrinsic width and intrinsic height of the media resource are the dimensions of the resource in CSS pixels after taking into account the resource's dimensions, aspect ratio, clean aperture, resolution, and so forth, as defined for the format used by the resource. If an anamorphic format does not define how to apply the aspect ratio to the video data's dimensions to obtain the "correct" dimensions, then the user agent must apply the ratio by increasing one dimension and leaving the other unchanged.

The videoWidth IDL attribute must return the intrinsic width of the video in CSS pixels. The videoHeight IDL attribute must return the intrinsic height of the video in CSS pixels. If the element's readyState attribute is HAVE_NOTHING, then the attributes must return 0.

Video content should be rendered inside the element's playback area such that the video content is shown centered in the playback area at the largest possible size that fits completely within it, with the video content's aspect ratio being preserved. Thus, if the aspect ratio of the playback area does not match the aspect ratio of the video, the video will be shown letterboxed or pillarboxed. Areas of the element's playback area that do not contain the video represent nothing.

The intrinsic width of a video element's playback area is the intrinsic width of the video resource, if that is available; otherwise it is the intrinsic width of the poster frame, if that is available; otherwise it is 300 CSS pixels.

The intrinsic height of a video element's playback area is the intrinsic height of the video resource, if that is available; otherwise it is the intrinsic height of the poster frame, if that is available; otherwise it is 150 CSS pixels.

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User agents should provide controls to enable or disable the display of closed captions, audio description tracks, and other additional data associated with the video stream, though such features should, again, not interfere with the page's normal rendering.

User agents may allow users to view the video content in manners more suitable to the user (e.g. full-screen or in an independent resizable window). As for the other user interface features, controls to enable this should not interfere with the page's normal rendering unless the user agent is exposing a user interface. In such an independent context, however, user agents may make full user interfaces visible, with, e.g., play, pause, seeking, and volume controls, even if the controls attribute is absent.

User agents may allow video playback to affect system features that could interfere with the user's experience; for example, user agents could disable screensavers while video playback is in progress.

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The poster IDL attribute must reflect the poster content attribute.

The audio IDL attribute must reflect the audio content attribute.

<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>

When an audio element is potentially playing, it must have its audio data played synchronized with the current playback position, at the specified volume with the specified mute state.

When an audio element is not potentially playing, audio must not play for the element.

Two constructors are provided for creating HTMLAudioElement objects (in addition to the factory methods from DOM Core such as createElement()): Audio() and Audio(src). When invoked as constructors, these must return a new HTMLAudioElement object (a new audio element). The element must have its preload attribute set to the literal value "auto". If the src argument is present, the object created must have its src content attribute set to the provided value, and the user agent must invoke the object's resource selection algorithm before returning. The element's document must be the active document of the browsing context of the Window object on which the interface object of the invoked constructor is found.

If a source element is inserted as a child of a media element that has no src attribute and whose networkState has the value NETWORK_EMPTY, the user agent must invoke the media element's resource selection algorithm.

The IDL attributes src, type, and media must reflect the respective content attributes of the same name.

<script>
 function fallback(video) {
   // replace <video> with its contents
   while (video.hasChildNodes()) {
     if (video.firstChild instanceof HTMLSourceElement)
       video.removeChild(video.firstChild);
     else
       video.parentNode.insertBefore(video.firstChild, video);
   }
   video.parentNode.removeChild(video);
 }
</script>
<video controls autoplay>
 <source src='video.mp4' type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'>
 <source src='video.ogv' type='video/ogg; codecs="theora, vorbis"'
         onerror="fallback(parentNode)">
 ...
</video>

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 resolved relative to the element, then the element's track URL is the resulting absolute URL. Otherwise, the element's track URL is the empty string.

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.

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 a user-agent defined string (e.g. the string "untitled" in the user's locale, or a value automatically generated from the other attributes).

The track IDL attribute must, on getting, return the track element's text track's corresponding TextTrack object.

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.

<video src="brave.webm">
 <track kind=subtitles src=brave.en.vtt srclang=en label="English">
 <track kind=captions src=brave.en.vtt srclang=en label="English for the Hard of Hearing">
 <track kind=subtitles src=brave.fr.vtt srclang=fr label="Français">
 <track kind=subtitles src=brave.de.vtt srclang=de label="Deutsch">
</video>

interface HTMLMediaElement : HTMLElement {

  // error state
  readonly attribute MediaError error;

  // network state
           attribute DOMString src;
  readonly attribute DOMString currentSrc;
  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;
           attribute DOMString preload;
  readonly attribute TimeRanges buffered;
  void load();
  DOMString canPlayType(in 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;
  readonly attribute double initialTime;
  readonly attribute double duration;
  readonly attribute Date startOffsetTime;
  readonly attribute boolean paused;
           attribute double defaultPlaybackRate;
           attribute double playbackRate;
  readonly attribute TimeRanges played;
  readonly attribute TimeRanges seekable;
  readonly attribute boolean ended;
           attribute boolean autoplay;
           attribute boolean loop;
  void play();
  void pause();

  // controls
           attribute boolean controls;
           attribute double volume;
           attribute boolean muted;

  // text tracks
  readonly attribute TextTrack[] tracks;
  MutableTextTrack addTrack(in DOMString kind, in optional DOMString label, in optional DOMString language);

};

Except where otherwise specified, the task source for all the tasks queued in this section and its subsections is the media element event task source.

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.

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;
};

The code attribute of a MediaError object must return the code for the error, which must be one of the following:

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.)

The src IDL attribute on media elements must reflect the content attribute of the same name.

The currentSrc IDL attribute is initially the empty string. Its value is changed by the resource selection algorithm defined below.

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.

<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 Plug-in" 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>

As media elements interact with the network, their current network activity is represented by the networkState attribute. On getting, it must return the current network state of the element, which must be one of the following values:

The resource selection algorithm defined below describes exactly when the networkState attribute changes value and what events fire to indicate changes in this state.

All media elements have an autoplaying 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.

1. Abort any already-running instance of the resource selection algorithm for this element.

2. If there are any tasks from the media element's media element event task source in one of the task queues, then remove those tasks.

   Basically, pending events and callbacks for the media element are discarded when the media element starts loading a new resource.

3. If the media element's networkState is set to NETWORK_LOADING or NETWORK_IDLE, queue a task to fire a simple event named abort at the media element.

4. If the media element's networkState is not set to NETWORK_EMPTY, then run these substeps:

   1. If a fetching process is in progress for the media element, the user agent should stop it.

   2. Set the networkState attribute to NETWORK_EMPTY.

   3. Forget the media element's media-resource-specific text tracks.

   4. If readyState is not set to HAVE_NOTHING, then set it to that state.

   5. If the paused attribute is false, then set to true.

   6. If seeking is true, set it to false.

   7. Set the current playback position to 0.

      If this changed the current playback position, then queue a task to fire a simple event named timeupdate at the media element.

   8. Set the initial playback position to 0.

   9. Set the timeline offset to Not-a-Number (NaN).

   10. 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.

   11. Queue a task to fire a simple event named emptied at the media element.

5. Set the playbackRate attribute to the value of the defaultPlaybackRate attribute.

6. Set the error attribute to null and the autoplaying flag to true.

7. Invoke the media element's resource selection algorithm.

8. 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 synchronously, but one of the first steps in the algorithm is to return and continue running the remaining steps asynchronously, meaning that it runs in the background with scripts and other tasks running 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 ⌛.

1. Set the networkState to NETWORK_NO_SOURCE.

2. Asynchronously 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 ⌛.)

3. ⌛ If the media element has a src attribute, then let mode be attribute.

   ⌛ Otherwise, if the media element does not have a src attribute but has 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 neither a src attribute nor a source element child: set the networkState to NETWORK_EMPTY, and abort these steps; the synchronous section ends.

4. ⌛ Set the media element's delaying-the-load-event flag to true (this delays the load event), and set its networkState to NETWORK_LOADING.

5. ⌛ Queue a task to fire a simple event named loadstart at the media element.

6. If mode is attribute, then run these substeps:

   1. ⌛ Process candidate: If the src attribute's value is the empty string, then end the synchronous section, and jump down to the failed step below.

   2. ⌛ Let absolute URL be the absolute URL that would have resulted from resolving the URL specified by the src attribute's value relative to the media element when the src attribute was last changed.

   3. ⌛ If absolute URL was obtained successfully, set the currentSrc attribute to absolute URL.

   4. End the synchronous section, continuing the remaining steps asynchronously.

   5. If absolute URL was obtained successfully, run the resource fetch algorithm with absolute URL. If that algorithm returns without aborting this one, then the load failed.

   6. Failed: Reaching this step indicates that the media resource failed to load or that the given URL could not be resolved. In one atomic operation, run the following steps:

      1. Set the error attribute to a new MediaError object whose code attribute is set to MEDIA_ERR_SRC_NOT_SUPPORTED.

      2. Forget the media element's media-resource-specific text tracks.

      3. Set the element's networkState attribute to the NETWORK_NO_SOURCE value.

   7. Queue a task to fire a simple event named error at the media element.

   8. Set the element's delaying-the-load-event flag to false. This stops delaying the load event.

   9. Abort these steps. Until the load() method is invoked or the src attribute is changed, the element won't attempt to load another resource.

   Otherwise, the source elements will be used; run these substeps:

   1. ⌛ 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:

      If a new node is inserted between the two nodes that define pointer

      Let pointer be the point between the node before pointer and the new node. In other words, insertions at pointer go after pointer.

      If the node before pointer is removed

      Let pointer be the point between the node after pointer and the node before the node after pointer. In other words, pointer doesn't move relative to the remaining nodes.

      If the node after pointer is removed

      Let pointer be the point between the node before pointer and the node after the node before pointer. Just as with the previous case, pointer doesn't move relative to the remaining nodes.

      Other changes don't affect pointer.

   2. ⌛ 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 step below.

   3. ⌛ Let absolute URL be the absolute URL that would have resulted from resolving the URL specified by candidate's src attribute's value relative to the candidate when the src attribute was last changed.

   4. ⌛ If absolute URL was not obtained successfully, then end the synchronous section, and jump down to the failed step below.

   5. ⌛ 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 step below.

   6. ⌛ If candidate has a media attribute whose value does not match the environment, then end the synchronous section, and jump down to the failed step below.

   7. ⌛ Set the currentSrc attribute to absolute URL.

   8. End the synchronous section, continuing the remaining steps asynchronously.

   9. Run the resource fetch algorithm with absolute URL. If that algorithm returns without aborting this one, then the load failed.

   10. Failed: Queue a task to fire a simple event named error at the candidate element, in the context of the fetching process that was used to try to obtain candidate's corresponding media resource in the resource fetch algorithm.

   11. Asynchronously 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 ⌛.)

   12. ⌛ Forget the media element's media-resource-specific text tracks.

   13. ⌛ Find next candidate: Let candidate be null.

   14. ⌛ Search loop: If the node after pointer is the end of the list, then jump to the waiting step below.

   15. ⌛ If the node after pointer is a source element, let candidate be that element.

   16. ⌛ 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.

   17. ⌛ If candidate is null, jump back to the search loop step. Otherwise, jump back to the process candidate step.

   18. ⌛ Waiting: Set the element's networkState attribute to the NETWORK_NO_SOURCE value.

   19. ⌛ Set the element's delaying-the-load-event flag to false. This stops delaying the load event.

   20. End the synchronous section, continuing the remaining steps asynchronously.

   21. Wait until the node after pointer is a node other than the end of the list. (This step might wait forever.)

   22. Asynchronously 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 ⌛.)

   23. ⌛ 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).

   24. ⌛ Set the networkState back to NETWORK_LOADING.

   25. ⌛ Jump back to the find next candidate step above.

The resource fetch algorithm for a media element and a given absolute URL is as follows:

1. Let the current media resource be the resource given by the absolute URL passed to this algorithm. This is now the element's media resource.

2. Begin to fetch the current media resource, from the media element's Document's origin, with the force same-origin flag set.

   Every 350ms (±200ms) or for every byte received, whichever is least frequent, queue a task to fire a simple event named progress at the element.

   The stall timeout is a user-agent 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 task to fire a simple 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, or while waiting for user input in an interactive resource. When a media element's download has been suspended, the user agent must set the networkState to NETWORK_IDLE and queue a task to fire a simple event named suspend at the element. If and when downloading of the resource resumes, the user agent must set the networkState to NETWORK_LOADING.

   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 stall a download, e.g. if it is waiting until the user starts playback before downloading any further content, the element's delaying-the-load-event flag must be set 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.

   The networking task source tasks to process the data as it is being fetched must, when appropriate, include the relevant substeps from the following list:

   If the media data cannot be fetched at all, due to network errors, causing the user agent to give up trying to fetch the resource

   If the media resource is found to have Content-Type metadata that, when parsed as a MIME type (including any codecs described by the codecs parameter, if the parameter is defined for that type), represents a type that the user agent knows it cannot render (even if the actual media data is in a supported format)

   If the media data can be fetched but is found by inspection to be in an unsupported format, or can otherwise not be rendered at all

   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:

   1. The user agent should cancel the fetching process.

   2. Abort this subalgorithm, returning to the resource selection algorithm.

   Once enough of the media data has been fetched to determine the duration of the media resource, its dimensions, and other metadata, and once the text tracks are ready

   This indicates that the resource is usable. The user agent must follow these substeps:

   1. Establish the media timeline for the purposes of the current playback position, the earliest possible position, and the initial playback position, based on the media data.

   2. 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).

   3. Set the current playback position to the earliest possible position.

   4. 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 task to fire a simple event named durationchange at the element at this point.

   5. Set the readyState attribute to HAVE_METADATA.

   6. For video elements, set the videoWidth and videoHeight attributes.

   7. Queue a task to fire a simple 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.

   8. If either the media resource or the address of the current media resource indicate a particular start time, then set the initial playback position to that time and then seek seek to that time. Ignore any resulting exceptions (if the position is out of range, it is effectively ignored).

      For example, a fragment identifier could be used to indicate a start position.

   9. 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, causing the networkState attribute to switch to the NETWORK_IDLE value.

   The user agent is required to determine the duration of the media resource and go through this step before playing.

   Once the entire media resource has been fetched (but potentially before any of it has been decoded)

   Queue a task to fire a simple event named progress at the media element.

   If the connection is interrupted, causing the user agent to give up trying to fetch the resource

   Fatal network errors that occur after the user agent has established whether the current media resource is usable must cause the user agent to execute the following steps:

   1. The user agent should cancel the fetching process.

   2. Set the error attribute to a new MediaError object whose code attribute is set to MEDIA_ERR_NETWORK.

   3. Queue a task to fire a simple event named error at the media element.

   4. 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 and queue a task to fire a simple event named emptied at the element. Otherwise, set the element's networkState attribute to the NETWORK_IDLE value.

   5. Set the element's delaying-the-load-event flag to false. This stops delaying the load event.

   6. Abort the overall resource selection algorithm.

   If the media data is corrupted

   Fatal errors in decoding the media data that occur after the user agent has established whether the current media resource is usable must cause the user agent to execute the following steps:

   1. The user agent should cancel the fetching process.

   2. Set the error attribute to a new MediaError object whose code attribute is set to MEDIA_ERR_DECODE.

   3. Queue a task to fire a simple event named error at the media element.

   4. 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 and queue a task to fire a simple event named emptied at the element. Otherwise, set the element's networkState attribute to the NETWORK_IDLE value.

   5. Set the element's delaying-the-load-event flag to false. This stops delaying the load event.

   6. Abort the overall resource selection algorithm.

   If the media data fetching process is aborted by the user

   The fetching process is aborted by the user, e.g. because the user navigated the browsing context to another page, 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.

   1. The user agent should cancel the fetching process.

   2. Set the error attribute to a new MediaError object whose code attribute is set to MEDIA_ERR_ABORTED.

   3. Queue a task to fire a simple event named abort at the media element.

   4. 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 and queue a task to fire a simple event named emptied at the element. Otherwise, set the element's networkState attribute to the NETWORK_IDLE value.

   5. Set the element's delaying-the-load-event flag to false. This stops delaying the load event.

   6. Abort the overall resource selection algorithm.

   If the media data can be fetched but has non-fatal errors or uses, in part, codecs that are unsupported, preventing the user agent from rendering the content completely correctly but not preventing playback altogether

   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 resource is found to declare a media-resource-specific text track that the user agent supports

   If the media resource's origin is the same origin as the media element's Document's origin, queue a task to run the steps to expose a media-resource-specific text track with the relevant data.

   Cross-origin files do not expose their subtitles in the DOM, for security reasons. However, user agents may still provide the user with access to such data in their user interface.

   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 next step. 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 dispatching a stalled event, as described earlier.

3. 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.

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 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.

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, 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 startOffsetTime attribute.

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 absence of an explicit timeline, the zero time on the media timeline should correspond to the first frame of the media resource. For static audio and video files this is generally trivial. For streaming resources, if the user agent will 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).

Another example would be 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. 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 startOffsetTime attribute 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 startOffsetTime attribute would return a Date object with a time corresponding to 2010-03-20 23:20:00 UTC.

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.

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.

The currentTime attribute must, on getting, return the current playback position, expressed in seconds. On setting, the user agent must seek to the new value (which might raise an exception).

Media elements have an initial playback position, which must initially (i.e. in the absence of media data) be zero seconds. The initial playback position is updated when a media resource is loaded. The initial playback position is a time on the media timeline.

The initialTime attribute must, on getting, return the initial playback position, expressed 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 task to fire a simple 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.

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 known to be unbounded (e.g. a streaming radio), 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 task to fire a simple 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 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 startOffsetTime attribute must return a new Date object representing the current timeline offset.

The loop IDL attribute must reflect the content attribute of the same name.

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:

When the ready state of a media element whose networkState is not NETWORK_EMPTY changes, the user agent must follow the steps given below:

If the previous ready state was HAVE_NOTHING, and the new ready state is HAVE_METADATA

A loadedmetadata DOM event will be fired as part of the load() algorithm.

If the previous ready state was HAVE_METADATA and the new ready state is HAVE_CURRENT_DATA or greater

If this is the first time this occurs for this media element since the load() algorithm was last invoked, the user agent must queue a task to fire a simple 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.

If the previous ready state was HAVE_FUTURE_DATA or more, and the new ready state is HAVE_CURRENT_DATA or less

A waiting DOM event can be fired, depending on the current state of playback.

If the previous ready state was HAVE_CURRENT_DATA or less, and the new ready state is HAVE_FUTURE_DATA

The user agent must queue a task to fire a simple event named canplay.

If the element is potentially playing, the user agent must queue a task to fire a simple event named playing.

If the new ready state is HAVE_ENOUGH_DATA

If the previous ready state was HAVE_CURRENT_DATA or less, the user agent must queue a task to fire a simple event named canplay, and, if the element is also potentially playing, queue a task to fire a simple event named playing.

If the autoplaying flag is true, and the paused attribute is true, and the media element has an autoplay attribute specified, and the media element's Document's browsing context did not have the sandboxed automatic features browsing context flag set when the Document was created, then the user agent may also set the paused attribute to false, queue a task to fire a simple event named play, and queue a task to fire a simple event named playing.

User agents are not required to 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.

In any case, the user agent must finally queue a task to fire a simple event named canplaythrough.

The readyState IDL attribute must, on getting, return the value described above that describes the current ready state of the media element.

The autoplay IDL attribute must reflect the content attribute of the same name.

The paused attribute represents whether the media element is paused or not. The attribute must initially be true.

A media element is said to be potentially playing when its paused attribute is false, the readyState attribute is either HAVE_FUTURE_DATA or HAVE_ENOUGH_DATA, the element has not ended playback, playback has not stopped due to errors, and the element has not paused for user interaction.

A media element is said to have ended playback when the element's readyState attribute is HAVE_METADATA or greater, and either the current playback position is the end of the media resource and the direction of playback is forwards and the media element does not have a loop attribute specified, or the current playback position is the earliest possible position and the direction of playback is backwards.

The ended attribute must return true if the media element has ended playback and the direction of playback is 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 task to fire a simple event named timeupdate at the element.

When a media element that is potentially playing stops playing because its readyState attribute changes to a value lower than HAVE_FUTURE_DATA, without the element having ended playback, or playback having stopped due to errors, or playback having paused for user interaction, or the seeking algorithm being invoked, the user agent must queue a task to fire a simple event named timeupdate at the element, and queue a task to fire a simple event named waiting at the element.

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:

1. If the media element has a loop attribute specified, then seek to the earliest possible position of the media resource and abort these steps.

2. Stop playback.

   The ended attribute becomes true.

3. The user agent must queue a task to fire a simple event named timeupdate at the element.

4. The user agent must queue a task to fire a simple event named ended at the 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 follow these steps:

1. Stop playback.

2. The user agent must queue a task to fire a simple event named timeupdate at the element.

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 playbackRate attribute gives 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 attribute must be set to the new value, and the playback must change speed (if the element is potentially playing).

If the playbackRate is positive or zero, then the direction of playback is forwards. Otherwise, it is backwards.

The "play" function in a user agent's interface must set the playbackRate attribute to the value of the defaultPlaybackRate attribute before invoking the play() method's steps. Features such as fast-forward or rewind must be implemented by only changing the playbackRate attribute.

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 task to fire a simple event named ratechange at the media element.

The played attribute must return a new static normalized TimeRanges object that represents the ranges of the media resource, if any, that the user agent has so far rendered, at the time the attribute is evaluated.

---

When the play() method on a media element is invoked, the user agent must run the following steps.

1. If the media element's networkState attribute has the value NETWORK_EMPTY, invoke the media element's resource selection algorithm.

2. 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 task to fire a simple event named timeupdate at the media element.

3. If the media element's paused attribute is true, run the following substeps:

   1. Change the value of paused to false.

   2. Queue a task to fire a simple event named play at the element.

   3. If the media element's readyState attribute has the value HAVE_NOTHING, HAVE_METADATA, or HAVE_CURRENT_DATA, queue a task to fire a simple event named waiting at the element.

      Otherwise, the media element's readyState attribute has the value HAVE_FUTURE_DATA or HAVE_ENOUGH_DATA; queue a task to fire a simple event named playing at the element.

4. Set the media element's autoplaying flag to false.

---

When the pause() method is invoked, the user agent must run the following steps:

1. If the media element's networkState attribute has the value NETWORK_EMPTY, invoke the media element's resource selection algorithm.

2. Set the media element's autoplaying flag to false.

3. If the media element's paused attribute is false, run the following steps:

   1. Change the value of paused to true.

   2. Queue a task to fire a simple event named timeupdate at the element.

   3. Queue a task to fire a simple event named pause at the element.

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When a media element is potentially playing and its Document is a fully active Document, its current playback position must increase monotonically at playbackRate units of media time per unit time of wall clock time.

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.

When the playbackRate is negative (playback is backwards), any corresponding audio must be muted. When the playbackRate is so low or so high that the user agent cannot play audio usefully, the corresponding audio must also be muted. If the playbackRate is not 1.0, the user agent may apply pitch adjustments to the audio as necessary to render it faithfully.

The 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).

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 to which no references exist has reached a point where no further audio remains to be played for that element (e.g. because the element is paused, or because the end of the clip has been reached, or because its playbackRate is 0.0) may the element be garbage collected.

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When the current playback position of a media element changes (e.g. due to playback or seeking), the user agent must run the following steps. 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 certain cues to be skipped over as the user agent rushes ahead to "catch up".)

1. Let current cues be an ordered list of cues, initialized to contain all the cues of all the hidden, showing, or showing by default 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, in text track cue order.

2. Let other cues be an ordered list of cues, initialized to contain all the cues of hidden, showing, and showing by default text tracks of the media element that are not present in current cues, also in text track cue order.

3. 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 task to fire a simple 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.

4. If all of the cues in current cues have their text track cue active flag set, and none of the cues in other cues have their text track cue active flag set, then abort these steps.

5. 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 both their text track cue active flag set and their text track cue pause-on-exit flag set, then immediately act as if the element's pause() method had been invoked. (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.)

6. Let affected tracks be a list of text tracks, initially empty.

7. For each text track cue in other cues that has its text track cue active flag set, in list order, queue a task to fire a simple event named exit at the TextTrackCue object, and add the cue's text track to affected tracks, if it's not already in the list.

8. For each text track cue in current cues that does not have its text track cue active flag set, in list order, queue a task to fire a simple event named enter at the TextTrackCue object, and add the cue's text track to affected tracks, if it's not already in the list.

9. For each text track in affected tracks, in the order they were added to the list (which will match the relative order of the text tracks in the media element's list of text tracks), queue a task to fire a simple event named cuechange at the TextTrack object, and, if the text track has a corresponding track element, to then fire a simple event named cuechange at the track element as well.

10. 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.

11. Run the rules for updating the text track rendering of each of the text tracks in affected tracks that are showing or showing by default.

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.

When a media element is removed from a Document, if the media element's networkState attribute has a value other than NETWORK_EMPTY then the user agent must act as if the pause() method had been invoked.

If the media element's Document stops being a fully active document, then the playback will stop until the document is active again.

The seeking attribute must initially have the value false.

When the user agent is required to seek to a particular new playback position in the media resource, 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 ⌛.

1. If the media element's readyState is HAVE_NOTHING, then raise an INVALID_STATE_ERR exception (if the seek was in response to a DOM method call or setting of an IDL attribute), and abort these steps.

2. 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.

3. Set the seeking IDL attribute to true.

4. 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 asynchronously. With the exception of the steps marked with ⌛, they could be aborted at any time by another instance of this algorithm being invoked.

5. 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.

6. If the new playback position is less than the earliest possible position, let it be that position instead.

7. 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 abort these steps.

8. Set the current playback position to the given new playback position.

9. Queue a task to fire a simple event named seeking at the element.

10. Queue a task to fire a simple event named timeupdate at the element.

11. 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 queue a task to fire a simple event named waiting at the element.

12. 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.

13. Await a stable state. The synchronous section consists of all the remaining steps of this algorithm. (Steps in the synchronous section are marked with ⌛.)

14. ⌛ Set the seeking IDL attribute to false.

15. ⌛ Queue a task to fire a simple 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.

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).

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.

When a media resource contains data that the user agent recognises 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:

1. 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.

2. Set the new text track's kind, label, and language based on the semantics of the relevant data, as defined by the relevant specification.

3. Populate the new text track's list of cues with the cues parsed so far, folllowing the guidelines for exposing cues, and begin updating it dynamically as necessary.

4. Set the new text track's readiness state to the value that most correctly describes the current state, and begin updating it dynamically as necessary.

   For example, if the relevant data in the media resource has been fully parsed and completely describes the cues, then the text track would be loaded. On the other hand, if the data for the cues is interleaved with the media data, and the media resource as a whole is still being downloaded, then the loading state might be more accurate.

5. 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.

6. Leave the text track list of cues empty, and associate with it the rules for updating the text track rendering appropriate for the format in question.

7. Add the new text track to the media element's list of text tracks.

When a media element is to forget the media element's media-resource-specific text tracks, the user agent must remove from the media element's list of text tracks all the media-resource-specific text tracks.

4.8.10.10.3 Sourcing out-of-band text tracks

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	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 added, removed, or changed, 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 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.

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.

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.

When a text track corresponding to a track element is added to a media element's list of text tracks, the user agent must set the text track mode appropriately, as determined by the following conditions:

If the text track kind is subtitles or captions and the user has indicated an interest in having a track with this text track kind, text track language, and text track label enabled, and there is no other text track in the media element's list of text tracks with a text track kind of either subtitles or captions whose text track mode is showing

If the text track kind is descriptions and the user has indicated an interest in having text descriptions with this text track language and text track label enabled, and there is no other text track in the media element's list of text tracks with a text track kind of descriptions whose text track mode is showing

If the text track kind is chapters and the text track language is one that the user agent has reason to believe is appropriate for the user, and there is no other text track in the media element's list of text tracks with a text track kind of chapters whose text track mode is showing

Let the text track mode be showing.

If there is a text track in the media element's list of text tracks whose text track mode is showing by default, the user agent must furthermore change that text track's text track mode to hidden.

If the track element has a default attribute specified, and there is no other text track in the media element's list of text tracks whose text track mode is showing or showing by default

Let the text track mode be showing by default.

Otherwise

Let the text track mode be disabled.

When a text track corresponding to a track element is created with text track mode set to hidden, showing, or showing by default, and when a text track corresponding to a track element is created with text track mode set to disabled and subsequently changes its text track mode to hidden, showing, or showing by default for the first time, the user agent must immediately and synchronously 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 step in that section is marked with ⌛.

1. Set the text track readiness state to loading.

2. Let URL be the track URL of the track element.

3. Asynchronously run the remaining steps, while continuing with whatever task was responsible for creating the text track or changing the text track mode.

4. Download: If URL is not the empty string, and its origin is the same as the media element's Document's origin, then fetch URL, from the media element's Document's origin, with the force same-origin flag set.

   The tasks queued by the fetching algorithm on the networking task source to process the data as it is being fetched must examine the resource's Content Type metadata, once it is available, if it ever is. If no Content Type metadata is ever available, or if the type is not recognised as a text track format, then the resource's format must be assumed to be unsupported (this causes the load to fail, as described below). If a type is obtained, and represents a supported text track format, then the resource's data must be passed to the appropriate parser as it is received, with the text track list of cues being used for that parser's output.

   If the fetching algorithm fails for any reason (network error, the server returns an error code, a cross-origin check fails, etc), or if URL is the empty string or has the wrong origin as determined by the condition at the start of this step, or if the fetched resource is not in a supported format, then queue a task to first change the text track readiness state to failed to load and then fire a simple event named error at the track element; and then, once that task is queued, move on to the step below labeled monitoring.

   If the fetching algorithm does not fail, then, when it completes, queue a task to first change the text track readiness state to loaded and then fire a simple event named load at the track element; and then, once that task is queued, move on to the step below labeled monitoring.

   If, while the fetching algorithm is active, either:

   • the track URL changes so that it is no longer equal to URL, while the text track mode is set to hidden, showing, or showing by default; or
   • the text track mode changes to hidden, showing, or showing by default, while the track URL is not equal to URL

   ...then the user agent must run the following steps:

   1. Abort the fetching algorithm.

   2. Queue a task to fire a simple event named abort at the track element.

   3. Let URL be the new track URL.

   4. Jump back to the top of the step labeled download.

   Until one of the above circumstances occurs, the user agent must remain on this step.

5. Monitoring: Wait until the track URL is no longer equal to URL, at the same time as the text track mode is set to hidden, showing, or showing by default.

6. Wait until the text track readiness state is no longer set to loading.

7. Await a stable state. The synchronous section consists of the following step. (The step in the synchronous section is marked with ⌛.)

8. ⌛ Set the text track readiness state to loading.

9. End the synchronous section, continuing the remaining steps asynchronously.

10. Jump to the step labeled download.

The tracks attribute of media elements must return an array host object for objects of type TextTrack that is fixed length and read only. The same object must be returned each time the attribute is accessed. [WEBIDL]

The array must contain 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.

interface TextTrack {
  readonly attribute DOMString kind;
  readonly attribute DOMString label;
  readonly attribute DOMString language;

  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 Function onload;
  readonly attribute Function onerror;

  const unsigned short OFF = 0;
  const unsigned short HIDDEN = 1;
  const unsigned short SHOWING = 2;
           attribute unsigned short mode;

  readonly attribute TextTrackCueList cues;
  readonly attribute TextTrackCueList activeCues;

  readonly attribute Function oncuechange;
};

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 readyState attribute must return the numeric value corresponding to the text track readiness state of the text track that the TextTrack object represents, as defined by the following list:

NONE (numeric value 0)

The text track not loaded state.

LOADING (numeric value 1)

The text track loading state.

LOADED (numeric value 2)

The text track loaded state.

ERROR (numeric value 3)

The text track failed to load state.

The mode attribute, on getting, must return the numeric value corresponding to the text track mode of the text track that the TextTrack object represents, as defined by the following list:

OFF (numeric value 0)

The text track disabled mode.

HIDDEN (numeric value 1)

The text track hidden mode.

SHOWING (numeric value 2)

The text track showing and showing by default modes.

On setting, if the new value is not either 0, 1, or 2, the user agent must throw an INVALID_ACCESS_ERR exception. Otherwise, if the new value isn't equal to what the attribute would currently return, the new value must be processed as follows:

If the new value is 0

Set the text track mode of the text track that the TextTrack object represents to the text track disabled mode.

If the new value is 1

Set the text track mode of the text track that the TextTrack object represents to the text track hidden mode.

If the new value is 2

Set the text track mode of the text track that the TextTrack object represents to the text track showing mode.

If the mode had been showing by default, this will change it to showing, even though the value of mode would appear not to change.

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 start times occur before the earliest possible position when the script started, in text track cue order. Otherwise, it must return null. When an object is returned, the same 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. When an object is returned, the same 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.

interface MutableTextTrack : TextTrack {
 void addCue(in TextTrackCue cue);
 void removeCue(in TextTrackCue cue);
};

The addTrack(kind, label, language) method of media elements, when invoked, must run the following steps:

1. If kind is not one of the following strings, then throw a SYNTAX_ERR exception and abort these steps:

   • subtitles
   • captions
   • descriptions
   • chapters
   • metadata

2. If the label argument was omitted, let label be the empty string.

3. If the language argument was omitted, let language be the empty string.

4. Create a new text track, 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 text track hidden mode, and its text track list of cues to an empty list.

5. Add the new text track to the media element's list of text tracks.

The addCue(cue) method of MutableTextTrack objects, when invoked, must run the following steps:

1. If cue is null, then throw an INVALID_ACCESS_ERR exception and abort these steps.

2. If the given cue is already associated with a text track other than the method's MutableTextTrack object's text track, then throw an INVALID_STATE_ERR exception and abort these steps.

3. Associate cue with the method's MutableTextTrack object's text track, if it is not currently associated with a text track.

4. If the given cue is already listed in the method's MutableTextTrack object's text track's text track list of cues, then throw an INVALID_STATE_ERR exception.

5. Add cue to the method's MutableTextTrack object's text track's text track list of cues.

The removeCue(cue) method of MutableTextTrack objects, when invoked, must run the following steps:

1. If cue is null, then throw an INVALID_ACCESS_ERR exception and abort these steps.

2. If the given cue is not associated with the method's MutableTextTrack object's text track, then throw an INVALID_STATE_ERR exception.

3. If the given cue is not currently listed in the method's MutableTextTrack object's text track's text track list of cues, then throw a NOT_FOUND_ERR exception.

4. Remove cue from the method's MutableTextTrack object's text track's text track list of cues.

var sfx = new Audio('sfx.wav');
var sounds = a.addTrack('metadata');
// add sounds we care about
sounds.addCue(new TextTrackCue('dog bark', 12.783, 13.612, '', '', '', true));
sounds.addCue(new TextTrackCue('kitten mew', 13.612, 15.091, '', '', '', true));

function playSound(id) {
  sfx.currentTime = sounds.getCueById(id).startTime;
  sfx.play();
}

sfx.oncanplaythrough = function () {
  playSound('dog bark');
}
window.onbeforeunload = function () {
  playSound('kitten mew');
  return 'Are you sure you want to leave this awesome page?';
}

interface TextTrackCueList {
  readonly attribute unsigned long length;
  getter TextTrackCue (in unsigned long index);
  TextTrackCue getCueById(in DOMString id);
};

A TextTrackCueList object represents a dynamically updating list of text track cues in a given order.

The length attribute must return the number of cues in the list represented by the TextTrackCueList object.

The supported property indicies 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 indicies.

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.

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.

interface TextTrackCue {
  readonly attribute TextTrack track;
  readonly attribute DOMString id;

  readonly attribute double startTime;
  readonly attribute double endTime;
  readonly attribute boolean pauseOnExit;


  DOMString getCueAsSource();
  DocumentFragment getCueAsHTML();

  readonly attribute Function onenter;
  readonly attribute Function onexit;
};

The track attribute must return the TextTrack object of the text track with which the text track cue that the TextTrackCue object represents is associated, if any; or null otherwise.

The id attribute must return the text track cue identifier of the text track cue that the TextTrackCue object represents.

The startTime attribute must return the text track cue start time of the text track cue that the TextTrackCue object represents, in seconds.

The endTime attribute must return the text track cue end time of the text track cue that the TextTrackCue object represents, in seconds.

The pauseOnExit attribute 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.

The direction attribute must return the text track cue writing direction of the text track cue that the TextTrackCue object represents.

The getCueAsSource() method must return the raw text track cue text.

The getCueAsHTML() method must convert the text track cue text to a DocumentFragment for the media element's Document, using the appropriate rules for doing so.

4.8.10.10.5 Event definitions

The following are the event handlers that must be supported, as IDL attributes, by all objects implementing the TextTrack interface:

Event handler	Event handler event type
onload	load
onerror	error
oncuechange	cuechange

The following are the event handlers that must be supported, as IDL attributes, by all objects implementing the TextTrackCue interface:

Event handler	Event handler event type
onenter	enter
onexit	exit

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. full-screen 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, 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.

Where possible (specifically, for starting, stopping, pausing, and unpausing playback, for seeking, 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.

For the purposes of listing chapters in the media resource, only text tracks in the media element's list of text tracks showing or showing by default and whose text track kind is chapters should be used. Each cue in such a text track represents a chapter starting at the cue's start time. The name of the chapter is the text track cue text, interpreted literally.

The controls IDL attribute must reflect the content attribute of the same name.

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The volume attribute must return the playback volume of any audio portions of the media element, in the range 0.0 (silent) to 1.0 (loudest). Initially, the volume must 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. On setting, if the new value is in the range 0.0 to 1.0 inclusive, the attribute must be set to the new value and the playback volume must be correspondingly adjusted as soon as possible after setting the attribute, 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. If the new value is outside the range 0.0 to 1.0 inclusive, then, on setting, an INDEX_SIZE_ERR exception must be raised instead.

The muted attribute must return true if the audio channels are muted and false otherwise. Initially, the audio channels should not be muted (false), but user agents may remember the last set value across sessions, on a per-site basis or otherwise, so the muted state may start as muted (true). On setting, the attribute must be set to the new value; if the new value is true, audio playback for this media resource must then be muted, and if false, audio playback must then be enabled.

Whenever either the muted or volume attributes are changed, the user agent must queue a task to fire a simple event named volumechange at the media element.

When a video element is created, if it has an audio attribute specified, the user agent must split the attribute's value on spaces; if any of the tokens are an ASCII case-insensitive match for the string muted, the user agent must then set the muted attribute to true, overriding any user preference.

<video src="adverts.cgi?kind=video" controls autoplay loop audio=muted></video>

interface TimeRanges {
  readonly attribute unsigned long length;
  double start(in unsigned long index);
  double end(in unsigned long index);
};

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 raise INDEX_SIZE_ERR exceptions 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:

• The start of a range must be greater than the end of all earlier ranges.
• The start of a range must be less than the end of that same range.

In other words, the ranges in such an object are ordered, don't overlap, aren't empty, and don't touch (adjacent ranges are folded into one bigger range).

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.

4.8.10.14 Security and privacy considerations

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.

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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 or chapter titles. This version of the API does not expose such information. Future extensions to this API will likely reuse a mechanism such as CORS to check that the embedded content's site has opted in to exposing such information. [CORS]

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 confidentiality breach.

4.8.10.16 Best practices for implementors of media elements

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-bit-rate 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 bit rate 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.

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When a media element that is paused is removed from a document and not reinserted before the next time the event loop spins, 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.)