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<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>
Media Source Extensions™
</title>
<script src="https://www.w3.org/Tools/respec/respec-w3c" class="remove"></script>
<script src="media-source.js" class="remove"></script>
<script class="remove">
var respecConfig = {
specStatus: "ED",
shortName: "media-source-2",
prevRecURI: "https://www.w3.org/TR/2016/REC-media-source-20161117/",
editors: [
{
name: "Jean-Yves Avenard",
mailto: "[email protected]",
company: "Apple Inc.",
companyURL: "https://www.apple.com/",
w3cid: "115886",
},
{
name: "Mark Watson",
company: "Netflix Inc.",
companyURL: "https://www.netflix.com/",
w3cid: "46379",
},
],
formerEditors: [
{
name: "Matthew Wolenetz",
retiredDate: "2024-02-01",
mailto: "[email protected]",
company: "W3C Invited Expert",
},
{
name: "Jerry Smith",
retiredDate: "2017-09-01",
company: "Microsoft Corporation",
companyURL: "https://www.microsoft.com/",
},
{
name: "Aaron Colwell",
retiredDate: "2015-04-01",
company: "Google Inc.",
companyURL: "https://www.google.com/",
},
{
name: "Adrian Bateman",
retiredDate: "2015-04-01",
company: "Microsoft Corporation",
companyURL: "https://www.microsoft.com/",
},
],
mseDefGroupName: "media-source",
// name of the WG
group: "media",
github: "w3c/media-source",
caniuse: "mediasource",
testsuiteURI: "https://wpt.live/media-source/",
wgPublicList: "public-media-wg",
preProcess: [mediaSourcePreProcessor],
postProcess: [mediaSourcePostProcessor],
};
</script>
<link rel="stylesheet" href="mse.css">
</head>
<body data-cite="html dom url fileapi infra">
<section id="abstract">
This specification extends {{HTMLMediaElement}} [[HTML]] to allow JavaScript to generate
media streams for playback. Allowing JavaScript to generate streams facilitates a variety of
use cases like adaptive streaming and time shifting live streams.
</section>
<section id="sotd">
<p>
On top of editorial updates, substantive changes since publication as a W3C Recommendation
in <a href="https://www.w3.org/TR/2016/REC-media-source-20161117/">November 2016</a> are:
</p>
<ul>
<li>the addition of a {{SourceBuffer/changeType()}} method to switch among codecs or
bytestreams
</li>
<li>the possibility to create and use {{MediaSource}} objects off the main thread in
dedicated workers
</li>
<li>the removal of the {{URL/createObjectURL()}} extension to the {{URL}} object following
its integration in the File API [[FILEAPI]]
</li>
<li>the addition of {{ManagedMediaSource}}, {{ManagedSourceBuffer}}, and
{{BufferedChangeEvent}} interfaces supporting power-efficient streaming and active buffered
media cleanup by the user agent
</li>
</ul>
<p>
For a full list of changes made since the previous version, see the <a href=
"https://github.com/w3c/media-source/commits/main">commits</a>.
</p>
<p>
The working group maintains <a href="https://github.com/w3c/media-source/issues">a list of
all bug reports that the editors have not yet tried to address</a>.
</p>
<p>
Implementors should be aware that this specification is not stable. <strong>Implementors
who are not taking part in the discussions are likely to find the specification changing
out from under them in incompatible ways.</strong> Vendors interested in implementing this
specification before it eventually reaches the Candidate Recommendation stage should track
the <a href="https://github.com/w3c/media-source">GitHub repository</a> and take part in
the discussions.
</p>
</section>
<section id="introduction" class="informative">
<h2>
Introduction
</h2>
<p>
This specification allows JavaScript to dynamically construct media streams for
<audio> and <video>. It defines a MediaSource object that can serve as a source
of media data for an HTMLMediaElement. MediaSource objects have one or more
{{SourceBuffer}} objects. Applications append data segments to the {{SourceBuffer}}
objects, and can adapt the quality of appended data based on system performance and other
factors. Data from the {{SourceBuffer}} objects is managed as track buffers for audio,
video and text data that is decoded and played. Byte stream specifications used with these
extensions are available in the byte stream format registry [[MSE-REGISTRY]].
</p>
<figure>
<a href=
'https://w3c.github.io/media-source/pipeline_model_description.html#pipelinedesc'><picture><img src="pipeline_model.svg"
alt="Media Source Pipeline Model Diagram"></picture></a>
<figcaption>
Media Source Pipeline Model Diagram
</figcaption>
</figure>
<section>
<h3>
Goals
</h3>
<p>
This specification was designed with the following goals in mind:
</p>
<ul>
<li>Allow JavaScript to construct media streams independent of how the media is fetched.
</li>
<li>Define a splicing and buffering model that facilitates use cases like adaptive
streaming, ad-insertion, time-shifting, and video editing.
</li>
<li>Minimize the need for media parsing in JavaScript.
</li>
<li>Leverage the browser cache as much as possible.
</li>
<li>Provide requirements for byte stream format specifications.
</li>
<li>Not require support for any particular media format or codec.
</li>
</ul>
<p>
This specification defines:
</p>
<ul>
<li>Normative behavior for user agents to enable interoperability between user agents and
web applications when processing media data.
</li>
<li>Normative requirements to enable other specifications to define media formats to be
used within this specification.
</li>
</ul>
</section>
</section>
<section>
<h2>
Definitions
</h2>
<dl>
<dt>
<dfn>Active Track Buffers</dfn>
</dt>
<dd>
<p>
The [=track buffers=] that provide [=coded frames=] for the {{AudioTrack/enabled}}
{{HTMLMediaElement/audioTracks}}, the {{VideoTrack/selected}}
{{HTMLMediaElement/videoTracks}}, and the <a def-id="texttrackmode-showing"></a> or
<a def-id="texttrackmode-hidden"></a> {{HTMLMediaElement/textTracks}}. All these
tracks are associated with {{SourceBuffer}} objects in the
{{MediaSource/activeSourceBuffers}} list.
</p>
</dd>
<dt>
<dfn>Append Window</dfn>
</dt>
<dd>
<p>
A [=presentation timestamp=] range used to filter out [=coded frames=] while
appending. The append window represents a single continuous time range with a single
start time and end time. Coded frames with [=presentation timestamp=] within this
range are allowed to be appended to the {{SourceBuffer}} while coded frames outside
this range are filtered out. The append window start and end times are controlled by
the {{SourceBuffer/appendWindowStart}} and {{SourceBuffer/appendWindowEnd}}
attributes respectively.
</p>
</dd>
<dt>
<dfn class="export">Coded Frame</dfn>
</dt>
<dd>
<p>
A unit of media data that has a [=presentation timestamp=], a [=decode timestamp=],
and a [=coded frame duration=].
</p>
</dd>
<dt>
<dfn>Coded Frame Duration</dfn>
</dt>
<dd>
<p>
The duration of a [=coded frame=]. For video and text, the duration indicates how
long the video frame or text SHOULD be displayed. For audio, the duration represents
the sum of all the samples contained within the coded frame. For example, if an audio
frame contained 441 samples @44100Hz the frame duration would be 10 milliseconds.
</p>
</dd>
<dt>
<dfn>Coded Frame End Timestamp</dfn>
</dt>
<dd>
<p>
The sum of a [=coded frame=] [=presentation timestamp=] and its [=coded frame
duration=]. It represents the [=presentation timestamp=] that immediately follows the
coded frame.
</p>
</dd>
<dt>
<dfn>Coded Frame Group</dfn>
</dt>
<dd>
<p>
A group of [=coded frames=] that are adjacent and have monotonically increasing
[=decode timestamps=] without any gaps. Discontinuities detected by the [=coded frame
processing=] algorithm and {{SourceBuffer/abort()}} calls trigger the start of a new
coded frame group.
</p>
</dd>
<dt>
<dfn>Decode Timestamp</dfn>
</dt>
<dd>
<p>
The decode timestamp indicates the latest time at which the frame needs to be decoded
assuming instantaneous decoding and rendering of this and any dependant frames (this
is equal to the [=presentation timestamp=] of the earliest frame, in [=presentation
order=], that is dependant on this frame). If frames can be decoded out of
[=presentation order=], then the decode timestamp MUST be present in or derivable
from the byte stream. The user agent MUST run the [=append error=] algorithm if this
is not the case. If frames cannot be decoded out of [=presentation order=] and a
decode timestamp is not present in the byte stream, then the decode timestamp is
equal to the [=presentation timestamp=].
</p>
</dd>
<dt>
<dfn class="export">Initialization Segment</dfn>
</dt>
<dd>
<p>
A sequence of bytes that contain all of the initialization information required to
decode a sequence of [=media segments=]. This includes codec initialization data,
[=Track ID=] mappings for multiplexed segments, and timestamp offsets (e.g., edit
lists).
</p>
<p class="note">
The [=byte stream format specifications=] in the byte stream format registry
[[MSE-REGISTRY]] contain format specific examples.
</p>
</dd>
<dt>
<dfn class="export">Media Segment</dfn>
</dt>
<dd>
<p>
A sequence of bytes that contain packetized & timestamped media data for a portion of
the <a def-id="media-timeline"></a>. Media segments are always associated with the
most recently appended [=initialization segment=].
</p>
<p class="note">
The [=byte stream format specifications=] in the byte stream format registry
[[MSE-REGISTRY]] contain format specific examples.
</p>
</dd>
<dt>
<dfn id="mediasource-object-url">MediaSource object URL</dfn>
</dt>
<dd>
<p>
A {{MediaSource}} object URL is a unique [=blob URL=] created by
{{URL/createObjectURL()}}. It is used to attach a {{MediaSource}} object to an
HTMLMediaElement.
</p>
<p>
These URLs are the same as a [=blob URLs=], except that anything in the definition of
that feature that refers to {{File}} and {{Blob}} objects is hereby extended to also
apply to {{MediaSource}} objects.
</p>
<p>
The [=origin=] of the MediaSource object URL is the [=relevant settings object=] of
[=this=] during the call to {{URL/createObjectURL()}}.
</p>
<p class="note">
For example, the [=origin=] of the MediaSource object URL affects the way that the
media element is <a href=
"https://html.spec.whatwg.org/multipage/canvas.html#security-with-canvas-elements">consumed
by canvas</a>.
</p>
</dd>
<dt>
<dfn id="parent-media-source">Parent Media Source</dfn>
</dt>
<dd>
<p>
The parent media source of a {{SourceBuffer}} object is the {{MediaSource}} object
that created it.
</p>
</dd>
<dt>
<dfn id="presentation-start-time">Presentation Start Time</dfn>
</dt>
<dd>
<p>
The presentation start time is the earliest time point in the presentation and
specifies the initial <a def-id="videoref" name="current-playback-position" id=
"current-playback-position">playback position</a> and <a def-id="videoref" name=
"earliest-possible-position" id="earliest-possible-position">earliest possible
position</a>. All presentations created using this specification have a presentation
start time of 0.
</p>
<p class="note">
For the purposes of determining if {{HTMLMediaElement}}'s
{{HTMLMediaElement/buffered}} contains a {{TimeRanges}} that includes the current
playback position, implementations MAY choose to allow a current playback position at
or after [=presentation start time=] and before the first {{TimeRanges}} to play the
first {{TimeRanges}} if that {{TimeRanges}} starts within a reasonably short time,
like 1 second, after [=presentation start time=]. This allowance accommodates the
reality that muxed streams commonly do not begin all tracks precisely at
[=presentation start time=]. Implementations MUST report the actual buffered range,
regardless of this allowance.
</p>
</dd>
<dt>
<dfn id="presentation-interval">Presentation Interval</dfn>
</dt>
<dd>
<p>
The presentation interval of a [=coded frame=] is the time interval from its
[=presentation timestamp=] to the [=presentation timestamp=] plus the [=coded frame
duration|coded frame's duration=]. For example, if a coded frame has a presentation
timestamp of 10 seconds and a [=coded frame duration=] of 100 milliseconds, then the
presentation interval would be [10-10.1). Note that the start of the range is
inclusive, but the end of the range is exclusive.
</p>
</dd>
<dt>
<dfn id="presentation-order">Presentation Order</dfn>
</dt>
<dd>
<p>
The order that [=coded frames=] are rendered in the presentation. The presentation
order is achieved by ordering [=coded frames=] in monotonically increasing order by
their [=presentation timestamps=].
</p>
</dd>
<dt>
<dfn id="presentation-timestamp" data-export="">Presentation Timestamp</dfn>
</dt>
<dd>
<p>
A reference to a specific time in the presentation. The presentation timestamp in a
[=coded frame=] indicates when the frame SHOULD be rendered.
</p>
</dd>
<dt>
<dfn id="random-access-point" data-export="">Random Access Point</dfn>
</dt>
<dd>
<p>
A position in a [=media segment=] where decoding and continuous playback can begin
without relying on any previous data in the segment. For video this tends to be the
location of I-frames. In the case of audio, most audio frames can be treated as a
random access point. Since video tracks tend to have a more sparse distribution of
random access points, the location of these points are usually considered the random
access points for multiplexed streams.
</p>
</dd>
<dt>
<dfn>SourceBuffer byte stream format specification</dfn>
</dt>
<dd>
<p>
The specific [=byte stream format specification=] that describes the format of the
byte stream accepted by a {{SourceBuffer}} instance. The [=byte stream format
specification=], for a {{SourceBuffer}} object, is initially selected based on the
|type:DOMString| passed to the {{MediaSource/addSourceBuffer()}} call that created
the object, and can be updated by {{SourceBuffer/changeType()}} calls on the object.
</p>
</dd>
<dt>
<dfn>`SourceBuffer` configuration</dfn>
</dt>
<dd>
<p>
A specific set of tracks distributed across one or more {{SourceBuffer}} objects
owned by a single {{MediaSource}} instance.
</p>
<p>
Implementations MUST support at least 1 {{MediaSource}} object with the following
configurations:
</p>
<ul>
<li>A single SourceBuffer with 1 audio track and/or 1 video track.
</li>
<li>Two SourceBuffers with one handling a single audio track and the other handling a
single video track.
</li>
</ul>
<p>
MediaSource objects MUST support each of the configurations above, but they are only
required to support one configuration at a time. Supporting multiple configurations
at once or additional configurations is a quality of implementation issue.
</p>
</dd>
<dt>
<dfn>Track Description</dfn>
</dt>
<dd>
<p>
A byte stream format specific structure that provides the [=Track ID=], codec
configuration, and other metadata for a single track. Each track description inside a
single [=initialization segment=] has a unique [=Track ID=]. The user agent MUST run
the [=append error=] algorithm if the [=Track ID=] is not unique within the
[=initialization segment=].
</p>
</dd>
<dt>
<dfn>Track ID</dfn>
</dt>
<dd>
<p>
A Track ID is a byte stream format specific identifier that marks sections of the
byte stream as being part of a specific track. The Track ID in a [=track
description=] identifies which sections of a [=media segment=] belong to that track.
</p>
</dd>
</dl>
</section>
<section id="mediasource" data-dfn-for="MediaSource">
<h2>
<dfn>MediaSource</dfn> interface
</h2>
<p>
The {{MediaSource}} interface represents a source of media data for an
{{HTMLMediaElement}}. It keeps track of the {{MediaSource/readyState}} for this source as
well as a list of <a>SourceBuffer</a> objects that can be used to add media data to the
presentation. MediaSource objects are created by the web application and then attached to
an HTMLMediaElement. The application uses the <a>SourceBuffer</a> objects in
{{MediaSource/sourceBuffers}} to add media data to this source. The HTMLMediaElement
fetches this media data from the <a>MediaSource</a> object when it is needed during
playback.
</p>
<p>
Each {{MediaSource}} object has a <dfn data-dfn-for="MediaSource">[[\live seekable
range]]</dfn> internal slot that stores a <a def-id="normalized-timeranges-object"></a>. It
is initialized to an empty {{TimeRanges}} object when the {{MediaSource}} object is
created, is maintained by {{MediaSource/setLiveSeekableRange()}} and
{{MediaSource/clearLiveSeekableRange()}}, and is used in [[[#htmlmediaelement-extensions]]]
to modify {{HTMLMediaElement}}'s {{HTMLMediaElement/seekable}} behavior.
</p>
<p>
Each {{MediaSource}} object has a <dfn data-dfn-for="MediaSource">[[\has ever been
attached]]</dfn> internal slot that stores a {{boolean}}. It is initialized to false when
the {{MediaSource}} object is created, and is set true in the extended
{{HTMLMediaElement}}'s [=resource fetch algorithm=] as described in the [=attaching to a
media element=] algorithm. The extended [=resource fetch algorithm=] uses this internal
slot to conditionally fail attachment of a {{MediaSource}} using a {{MediaSourceHandle}}
set on a {{HTMLMediaElement}}'s {{HTMLMediaElement/srcObject}} attribute.
</p>
<pre class="idl">
enum ReadyState {
"closed",
"open",
"ended",
};
</pre>
<dl data-dfn-for="ReadyState">
<dt>
<dfn>closed</dfn>
</dt>
<dd>
Indicates the source is not currently attached to a media element.
</dd>
<dt>
<dfn>open</dfn>
</dt>
<dd>
The source has been opened by a media element and is ready for data to be appended to the
{{SourceBuffer}} objects in {{MediaSource}}'s {{MediaSource/sourceBuffers}}.
</dd>
<dt>
<dfn>ended</dfn>
</dt>
<dd>
The source is still attached to a media element, but {{MediaSource}}'s
{{MediaSource/endOfStream()}} has been called.
</dd>
</dl>
<aside class="issue" data-number="276">
<p>
Consider adding a "`closing`" {{MediaSource/ReadyState}} to indicate the source is in the
process of being concurrently detached from a media element. This would be useful for
some implementations of {{MediaSource}} and {{SourceBuffer}} in
{{DedicatedWorkerGlobalScope}}.
</p>
</aside>
<pre class="idl">
enum EndOfStreamError {
"network",
"decode",
};
</pre>
<dl data-dfn-for="EndOfStreamError">
<dt>
<dfn>network</dfn>
</dt>
<dd>
<p>
Terminates playback and signals that a network error has occurred.
</p>
<p class="note">
JavaScript applications SHOULD use this status code to terminate playback with a
network error. For example, if a network error occurs while fetching media data.
</p>
</dd>
<dt>
<dfn>decode</dfn>
</dt>
<dd>
<p>
Terminates playback and signals that a decoding error has occurred.
</p>
<p class="note">
JavaScript applications SHOULD use this status code to terminate playback with a decode
error. For example, if a parsing error occurs while processing out-of-band media data.
</p>
</dd>
</dl>
<pre class="idl">
[Exposed=(Window,DedicatedWorker)]
interface MediaSource : EventTarget {
constructor();
[SameObject, Exposed=DedicatedWorker]
readonly attribute MediaSourceHandle handle;
readonly attribute SourceBufferList sourceBuffers;
readonly attribute SourceBufferList activeSourceBuffers;
readonly attribute ReadyState readyState;
attribute unrestricted double duration;
attribute EventHandler onsourceopen;
attribute EventHandler onsourceended;
attribute EventHandler onsourceclose;
static readonly attribute boolean canConstructInDedicatedWorker;
SourceBuffer addSourceBuffer(DOMString type);
undefined removeSourceBuffer(SourceBuffer sourceBuffer);
undefined endOfStream(optional EndOfStreamError error);
undefined setLiveSeekableRange(double start, double end);
undefined clearLiveSeekableRange();
static boolean isTypeSupported(DOMString type);
};
</pre>
<h3>
<dfn>handle</dfn> attribute
</h3>
<p>
Contains a handle useful for attachment of a dedicated worker {{MediaSource}} object to an
{{HTMLMediaElement}} via {{HTMLMediaElement/srcObject}}. The handle remains the same object
for this {{MediaSource}} object across accesses of this attribute, but it is distinct for
each {{MediaSource}} object.
</p>
<p class="note">
This specification may eventually enable visibility of this attribute on {{MediaSource}}
objects on the main Window context. If so, specification care will be necessary to prevent
potential backwards incompatible changes, such as could happen if exceptions were thrown on
accesses to this attribute.
</p>
<p>
On getting, run the following steps:
</p>
<ol>
<li>If the handle for this {{MediaSource}} object has not yet been created, then run the
following steps:
<ol>
<li>Let |created handle:MediaSourceHandle| be the result of creating a new
{{MediaSourceHandle}} object and associated resources, linked internally to this
{{MediaSource}}.
</li>
<li>Update the attribute to be |created handle|.
</li>
</ol>
</li>
<li>Return the {{MediaSourceHandle}} object that is this attribute's value.
</li>
</ol>
<h3>
<dfn>sourceBuffers</dfn> attribute
</h3>
<p>
Contains the list of {{SourceBuffer}} objects associated with this {{MediaSource}}. When
{{MediaSource}}'s {{MediaSource/readyState}} equals {{ReadyState/""closed""}} this list
will be empty. Once {{MediaSource/readyState}} transitions to {{ReadyState/""open""}}
SourceBuffer objects can be added to this list by using {{MediaSource/addSourceBuffer()}}.
</p>
<h3>
<dfn>activeSourceBuffers</dfn> attribute
</h3>
<p>
Contains the subset of {{MediaSource/sourceBuffers}} that are providing the
{{VideoTrack/selected}} video track, the {{AudioTrack/enabled}} audio track(s), and the
<a def-id="texttrackmode-showing"></a> or <a def-id="texttrackmode-hidden"></a> text
track(s).
</p>
<p>
{{SourceBuffer}} objects in this list MUST appear in the same order as they appear in the
{{MediaSource/sourceBuffers}} attribute; e.g., if only sourceBuffers[0] and
sourceBuffers[3] are in {{MediaSource/activeSourceBuffers}}, then activeSourceBuffers[0]
MUST equal sourceBuffers[0] and activeSourceBuffers[1] MUST equal sourceBuffers[3].
</p>
<p class="note">
Section [[[#active-source-buffer-changes]]] describes how this attribute gets updated.
</p>
<h3>
<dfn>readyState</dfn> attribute
</h3>
<p>
Indicates the current state of the <a>MediaSource</a> object. When the <a>MediaSource</a>
is created {{MediaSource/readyState}} MUST be set to {{ReadyState/""closed""}}.
</p>
<h3>
<dfn>duration</dfn> attribute
</h3>
<p>
Allows the web application to set the presentation duration. The duration is initially set
to NaN when the <a>MediaSource</a> object is created.
</p>
<p>
On getting, run the following steps:
</p>
<ol>
<li>If the {{MediaSource/readyState}} attribute is {{ReadyState/""closed""}} then return
NaN and abort these steps.
</li>
<li>Return the current value of the attribute.
</li>
</ol>
<p>
On setting, run the following steps:
</p>
<ol>
<li>If the value being set is negative or NaN then throw a {{TypeError}} exception and
abort these steps.
</li>
<li>If the {{MediaSource/readyState}} attribute is not {{ReadyState/""open""}} then throw
an {{InvalidStateError}} exception and abort these steps.
</li>
<li>If the {{SourceBuffer/updating}} attribute equals true on any {{SourceBuffer}} in
{{MediaSource/sourceBuffers}}, then throw an {{InvalidStateError}} exception and abort
these steps.
</li>
<li>Run the [=duration change=] algorithm with |new duration:unrestricted double| set to
the value being assigned to this attribute.
<p class="note">
The [=duration change=] algorithm will adjust |new duration| higher if there is any
currently buffered coded frame with a higher end time.
</p>
<p class="note">
{{SourceBuffer/appendBuffer()}} and {{MediaSource/endOfStream()}} can update the
duration under certain circumstances.
</p>
</li>
</ol>
<h3>
<dfn>canConstructInDedicatedWorker</dfn> attribute
</h3>
<p>
Returns true.
</p>
<p class="note">
This attribute enables main thread and dedicated worker feature detection of support for
creating and using a {{MediaSource}} object in a dedicated worker, and mitigates the need
for higher latency detection polyfills like attempting creation of a {{MediaSource}} object
from a dedicated worker, especially if the feature is not supported.
</p>
<h3>
<dfn>addSourceBuffer()</dfn> method
</h3>
<p>
Adds a new <a>SourceBuffer</a> to {{MediaSource/sourceBuffers}}.
</p>
<ol class="algorithm">
<li>If |type:DOMString| is an empty string then throw a {{TypeError}} exception and abort
these steps.
</li>
<li>If |type| contains a MIME type that is not supported or contains a MIME type that is
not supported with the types specified for the other {{SourceBuffer}} objects in
{{MediaSource/sourceBuffers}}, then throw a {{NotSupportedError}} exception and abort these
steps.
</li>
<li>If the user agent can't handle any more SourceBuffer objects or if creating a
SourceBuffer based on |type| would result in an unsupported [=SourceBuffer configuration=],
then throw a {{QuotaExceededError}} exception and abort these steps.
<p class="note">
For example, a user agent MAY throw a {{QuotaExceededError}} exception if the media
element has reached the {{HTMLMediaElement/HAVE_METADATA}} readyState. This can occur
if the user agent's media engine does not support adding more tracks during playback.
</p>
</li>
<li>If the {{MediaSource/readyState}} attribute is not in the {{ReadyState/""open""}} state
then throw an {{InvalidStateError}} exception and abort these steps.
</li>
<li>Let |buffer| be a new instance of a {{ManagedSourceBuffer}} if [=this=] is a
{{ManagedMediaSource}}, or a {{SourceBuffer}} otherwise, with their respective associated
resources.
</li>
<li>Set |buffer|'s {{SourceBuffer/[[generate timestamps flag]]}} to the value in the
"Generate Timestamps Flag" column of the [[[MSE-REGISTRY]]] entry that is associated with
|type|.
</li>
<li>If |buffer|'s {{SourceBuffer/[[generate timestamps flag]]}} is true, set |buffer|'s
{{SourceBuffer/mode}} to {{AppendMode/"sequence"}}. Otherwise, set |buffer|'s
{{SourceBuffer/mode}} to {{AppendMode/"segments"}}.
</li>
<li>[=List/Append=] |buffer| to [=this=]'s {{MediaSource/sourceBuffers}}.
</li>
<li>[=Queue a task=] to [=fire an event=] named {{addsourcebuffer}} at [=this=]'s
{{MediaSource/sourceBuffers}}.
</li>
<li>Return |buffer|.
</li>
</ol>
<h3>
<dfn>removeSourceBuffer()</dfn> method
</h3>
<p>
Removes a {{SourceBuffer}} from {{MediaSource/sourceBuffers}}.
</p>
<ol class="algorithm">
<li>If |sourceBuffer:SourceBuffer| specifies an object that is not in
{{MediaSource/sourceBuffers}} then throw a {{NotFoundError}} exception and abort these
steps.
</li>
<li>If the |sourceBuffer|.{{SourceBuffer/updating}} attribute equals true, then run the
following steps:
<ol>
<li>Abort the [=buffer append=] algorithm if it is running.
</li>
<li>Set the |sourceBuffer|.{{SourceBuffer/updating}} attribute to false.
</li>
<li>[=Queue a task=] to [=fire an event=] named {{abort}} at |sourceBuffer|.
</li>
<li>[=Queue a task=] to [=fire an event=] named {{updateend}} at |sourceBuffer|.
</li>
</ol>
</li>
<li>Let |SourceBuffer audioTracks list:AudioTrackList| equal the {{AudioTrackList}} object
returned by |sourceBuffer|.{{SourceBuffer/audioTracks}}.
</li>
<li>If the |SourceBuffer audioTracks list| is not empty, then run the following steps:
<ol>
<li>For each {{AudioTrack}} object in the |SourceBuffer audioTracks list|, run the
following steps:
<ol>
<li>Set the {{AudioTrack/sourceBuffer}} attribute on the {{AudioTrack}} object to
null.
</li>
<li>Remove the {{AudioTrack}} object from the |SourceBuffer audioTracks list|.
<p class="note">
This should trigger {{AudioTrackList}} [[HTML]] logic to [=queue a task=] to
[=fire an event=] named [=AudioTrackList/removetrack=] using {{TrackEvent}}
with the {{TrackEvent/track}} attribute initialized to the {{AudioTrack}}
object, at the |SourceBuffer audioTracks list|. If the {{AudioTrack/enabled}}
attribute on the {{AudioTrack}} object was true at the beginning of this
removal step, then this should also trigger {{AudioTrackList}} [[HTML]] logic
to [=queue a task=] to [=fire an event=] named [=AudioTrackList/change=] at the
|SourceBuffer audioTracks list|.
</p>
</li>
<li>Use the [=mirror if necessary=] algorithm to run the following steps in
{{Window}}, to remove the {{AudioTrack}} object (or instead, the {{Window}} mirror
of it if the {{MediaSource}} object was constructed in a
{{DedicatedWorkerGlobalScope}}) from the media element:
<ol>
<li>Let |HTMLMediaElement audioTracks list:AudioTrackList| equal the
{{AudioTrackList}} object returned by the {{HTMLMediaElement/audioTracks}}
attribute on the HTMLMediaElement.
</li>
<li>Remove the {{AudioTrack}} object from the |HTMLMediaElement audioTracks
list|.
<p class="note">
This should trigger {{AudioTrackList}} [[HTML]] logic to [=queue a task=]
to [=fire an event=] named [=AudioTrackList/removetrack=] using
{{TrackEvent}} with the {{TrackEvent/track}} attribute initialized to the
{{AudioTrack}} object, at the |HTMLMediaElement audioTracks list|. If the
{{AudioTrack/enabled}} attribute on the {{AudioTrack}} object was true at
the beginning of this removal step, then this should also trigger
{{AudioTrackList}} [[HTML]] logic to [=queue a task=] to [=fire an event=]
named [=AudioTrackList/change=] at the |HTMLMediaElement audioTracks list|.
</p>
</li>
</ol>
</li>
</ol>
</li>
</ol>
</li>
<li>Let |SourceBuffer videoTracks list:VideoTrackList| equal the {{VideoTrackList}} object
returned by |sourceBuffer|.{{SourceBuffer/videoTracks}}.
</li>
<li>If the |SourceBuffer videoTracks list| is not empty, then run the following steps:
<ol>
<li>For each {{VideoTrack}} object in the |SourceBuffer videoTracks list|, run the
following steps:
<ol>
<li>Set the {{VideoTrack/sourceBuffer}} attribute on the {{VideoTrack}} object to
null.
</li>
<li>Remove the {{VideoTrack}} object from the |SourceBuffer videoTracks list|.
<p class="note">
This should trigger {{VideoTrackList}} [[HTML]] logic to [=queue a task=] to
[=fire an event=] named [=VideoTrackList/removetrack=] using {{TrackEvent}}
with the {{TrackEvent/track}} attribute initialized to the {{VideoTrack}}
object, at the |SourceBuffer videoTracks list|. If the {{VideoTrack/selected}}
attribute on the {{VideoTrack}} object was true at the beginning of this
removal step, then this should also trigger {{VideoTrackList}} [[HTML]] logic
to [=queue a task=] to [=fire an event=] named [=VideoTrackList/change=] at the
|SourceBuffer videoTracks list|.
</p>
</li>
<li>Use the [=mirror if necessary=] algorithm to run the following steps in
{{Window}}, to remove the {{VideoTrack}} object (or instead, the {{Window}} mirror
of it if the {{MediaSource}} object was constructed in a
{{DedicatedWorkerGlobalScope}}) from the media element:
<ol>
<li>Let |HTMLMediaElement videoTracks list:VideoTrackList| equal the
{{VideoTrackList}} object returned by the {{HTMLMediaElement/videoTracks}}
attribute on the HTMLMediaElement.
</li>
<li>Remove the {{VideoTrack}} object from the |HTMLMediaElement videoTracks
list|.
<p class="note">
This should trigger {{VideoTrackList}} [[HTML]] logic to [=queue a task=]
to [=fire an event=] named [=VideoTrackList/removetrack=] using
{{TrackEvent}} with the {{TrackEvent/track}} attribute initialized to the
{{VideoTrack}} object, at the |HTMLMediaElement videoTracks list|. If the
{{VideoTrack/selected}} attribute on the {{VideoTrack}} object was true at
the beginning of this removal step, then this should also trigger
{{VideoTrackList}} [[HTML]] logic to [=queue a task=] to [=fire an event=]
named [=VideoTrackList/change=] at the |HTMLMediaElement videoTracks list|.
</p>
</li>
</ol>
</li>
</ol>
</li>
</ol>
</li>
<li>Let |SourceBuffer textTracks list:TextTrackList| equal the {{TextTrackList}} object
returned by |sourceBuffer|.{{SourceBuffer/textTracks}}.
</li>
<li>If the |SourceBuffer textTracks list| is not empty, then run the following steps:
<ol>
<li>For each {{TextTrack}} object in the |SourceBuffer textTracks list|, run the
following steps:
<ol>
<li>Set the {{TextTrack/sourceBuffer}} attribute on the {{TextTrack}} object to
null.
</li>
<li>Remove the {{TextTrack}} object from the |SourceBuffer textTracks list|.
<p class="note">
This should trigger {{TextTrackList}} [[HTML]] logic to [=queue a task=] to
[=fire an event=] named [=TextTrackList/removetrack=] using {{TrackEvent}} with
the {{TrackEvent/track}} attribute initialized to the {{TextTrack}} object, at
the |SourceBuffer textTracks list|. If the {{TextTrack/mode}} attribute on the
{{TextTrack}} object was <a def-id="texttrackmode-showing"></a> or <a def-id=
"texttrackmode-hidden"></a> at the beginning of this removal step, then this
should also trigger {{TextTrackList}} [[HTML]] logic to [=queue a task=] to
[=fire an event=] named [=TextTrackList/change=] at the |SourceBuffer
textTracks list|.
</p>
</li>
<li>Use the [=mirror if necessary=] algorithm to run the following steps in
{{Window}}, to remove the {{TextTrack}} object (or instead, the {{Window}} mirror
of it if the {{MediaSource}} object was constructed in a
{{DedicatedWorkerGlobalScope}}) from the media element:
<ol>
<li>Let |HTMLMediaElement textTracks list:TextTrackList| equal the
{{TextTrackList}} object returned by the {{HTMLMediaElement/textTracks}}
attribute on the HTMLMediaElement.
</li>
<li>Remove the {{TextTrack}} object from the |HTMLMediaElement textTracks
list|.
<p class="note">
This should trigger {{TextTrackList}} [[HTML]] logic to [=queue a task=] to
[=fire an event=] named [=TextTrackList/removetrack=] using {{TrackEvent}}
with the {{TrackEvent/track}} attribute initialized to the {{TextTrack}}
object, at the |HTMLMediaElement textTracks list|. If the
{{TextTrack/mode}} attribute on the {{TextTrack}} object was <a def-id=
"texttrackmode-showing"></a> or <a def-id="texttrackmode-hidden"></a> at
the beginning of this removal step, then this should also trigger
{{TextTrackList}} [[HTML]] logic to [=queue a task=] to [=fire an event=]
named [=TextTrackList/change=] at the |HTMLMediaElement textTracks list|.
</p>
</li>
</ol>
</li>
</ol>
</li>
</ol>
</li>
<li>If |sourceBuffer| is in {{MediaSource/activeSourceBuffers}}, then remove |sourceBuffer|
from {{MediaSource/activeSourceBuffers}} and [=queue a task=] to [=fire an event=] named
{{removesourcebuffer}} at the <a>SourceBufferList</a> returned by
{{MediaSource/activeSourceBuffers}}.
</li>
<li>Remove |sourceBuffer| from {{MediaSource/sourceBuffers}} and [=queue a task=] to [=fire
an event=] named {{removesourcebuffer}} at the <a>SourceBufferList</a> returned by
{{MediaSource/sourceBuffers}}.
</li>
<li>Destroy all resources for |sourceBuffer|.
</li>
</ol>
<h3>
<dfn>endOfStream()</dfn> method
</h3>
<p>
Signals the end of the stream.
</p>
<ol class="algorithm">
<li>If the {{MediaSource/readyState}} attribute is not in the {{ReadyState/""open""}} state
then throw an {{InvalidStateError}} exception and abort these steps.
</li>
<li>If the {{SourceBuffer/updating}} attribute equals true on any {{SourceBuffer}} in
{{MediaSource/sourceBuffers}}, then throw an {{InvalidStateError}} exception and abort
these steps.
</li>
<li>Run the [=end of stream=] algorithm with the error parameter set to
|error:EndOfStreamError|.
</li>
</ol>
<h3>
<dfn>setLiveSeekableRange()</dfn> method
</h3>
<p>
Updates {{MediaSource/[[live seekable range]]}} that is used in section
[[[#htmlmediaelement-extensions]]] to modify {{HTMLMediaElement}}'s
{{HTMLMediaElement/seekable}} behavior.
</p>
<ol class="method-algorithm">
<li>If the {{MediaSource/readyState}} attribute is not {{ReadyState/""open""}} then throw
an {{InvalidStateError}} exception and abort these steps.
</li>
<li>If |start:double| is negative or greater than |end:double|, then throw a {{TypeError}}
exception and abort these steps.
</li>
<li>Set {{MediaSource/[[live seekable range]]}} to be a new <a def-id=
"normalized-timeranges-object"></a> containing a single range whose start position is
|start| and end position is |end|.
</li>
</ol>
<h3>
<dfn>clearLiveSeekableRange()</dfn> method
</h3>
<p>
Updates {{MediaSource/[[live seekable range]]}} that is used in section
[[[#htmlmediaelement-extensions]]] to modify {{HTMLMediaElement}}'s
{{HTMLMediaElement/seekable}} behavior.
</p>
<ol class="method-algorithm">
<li>If the {{MediaSource/readyState}} attribute is not {{ReadyState/""open""}} then throw
an {{InvalidStateError}} exception and abort these steps.
</li>
<li>If {{MediaSource/[[live seekable range]]}} contains a range, then set
{{MediaSource/[[live seekable range]]}} to be a new empty {{TimeRanges}} object.
</li>
</ol>
<h3>
<dfn>isTypeSupported()</dfn> method
</h3>
<p>
Check to see whether the <a>MediaSource</a> is capable of creating <a>SourceBuffer</a>
objects for the specified MIME type.
</p>
<ol class="method-algorithm">
<li>If |type:DOMString| is an empty string, then return false.
</li>