HTML

Living Standard — Last Updated 11 August 2026

1. 1. 1. 4.8.8 The video element
      2. 4.8.9 The audio element
      3. 4.8.10 The track element
      4. 4.8.11 Media elements
         1. 4.8.11.1 Error codes
         2. 4.8.11.2 Location of the media resource
         3. 4.8.11.3 MIME types
         4. 4.8.11.4 Network states
         5. 4.8.11.5 Loading the media resource
         6. 4.8.11.6 Offsets into the media resource
         7. 4.8.11.7 Ready states
         8. 4.8.11.8 Playing the media resource
         9. 4.8.11.9 Seeking
         10. 4.8.11.10 Media resources with multiple media tracks
             1. 4.8.11.10.1 AudioTrackList and VideoTrackList objects
             2. 4.8.11.10.2 Selecting specific audio and video tracks declaratively
         11. 4.8.11.11 Timed text tracks
             1. 4.8.11.11.1 Text track model
             2. 4.8.11.11.2 Sourcing in-band text tracks
             3. 4.8.11.11.3 Sourcing out-of-band text tracks
             4. 4.8.11.11.4 Guidelines for exposing cues in various formats as text track cues
             5. 4.8.11.11.5 Text track API
             6. 4.8.11.11.6 Event handlers for objects of the text track APIs
             7. 4.8.11.11.7 Best practices for metadata text tracks
         12. 4.8.11.12 Identifying a track kind through a URL
         13. 4.8.11.13 User interface
         14. 4.8.11.14 Time ranges
         15. 4.8.11.15 The TrackEvent interface
         16. 4.8.11.16 Events summary
         17. 4.8.11.17 Security and privacy considerations
         18. 4.8.11.18 Best practices for authors using media elements
         19. 4.8.11.19 Best practices for implementers of media elements

4.8.8 The video element

Categories:

Flow content.

Phrasing content.

Embedded content.

If the element has a controls attribute: Interactive content.

Palpable content.

Contexts in which this element can be used:

Where embedded content is expected.

Content model:

If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.

If the element does not have a src attribute: zero or more source elements, then zero or more track elements, then transparent, but with no media element descendants.

Tag omission in text/html:

Neither tag is omissible.

Content attributes:

Global attributes

src — Address of the resource

crossorigin — How the element handles crossorigin requests

poster — Poster frame to show prior to video playback

preload — Hints how much buffering the media resource will likely need

autoplay — Hint that the media resource can be started automatically when the page is loaded

playsinline — Encourage the user agent to display video content within the element's playback area

loop — Whether to loop the media resource

muted — Whether to mute the media resource by default

controls — Show user agent controls

loading — Used when determining loading deferral

width — Horizontal dimension

height — Vertical dimension

Accessibility considerations:

For authors.

For implementers.

Sanitization:

Uncategorized.

DOM interface:

[Exposed=Window]
interface HTMLVideoElement : HTMLMediaElement {
  [HTMLConstructor] constructor();

  [CEReactions, Reflect] attribute unsigned long width;
  [CEReactions, Reflect] attribute unsigned long height;
  readonly attribute unsigned long videoWidth;
  readonly attribute unsigned long videoHeight;
  [CEReactions, ReflectURL] attribute USVString poster;
  [CEReactions, Reflect] attribute boolean playsInline;

};

A video element is used for playing videos or movies, and audio files with captions.

Content may be provided inside the video element. User agents should not show this content to the user; it is intended for older web browsers which do not support video, so that text can be shown to the users of these older browsers informing them of how to access the video contents.

In particular, this content is not intended to address accessibility concerns. To make video content accessible to the partially sighted, the blind, the hard-of-hearing, the deaf, and those with other physical or cognitive disabilities, a variety of features are available. Captions can be provided, either embedded in the video stream or as external files using the track element. Sign-language tracks can be embedded in the video stream. Audio descriptions can be embedded in the video stream or in text form using a WebVTT file referenced using the track element and synthesized into speech by the user agent. WebVTT can also be used to provide chapter titles. For users who would rather not use a media element at all, transcripts or other textual alternatives can be provided by simply linking to them in the prose near the video element. [WEBVTT]

The video element is a media element whose media data is ostensibly video data, possibly with associated audio data.

The video element has an internal shadow tree with no slot elements.

The src, crossorigin, preload, autoplay, loop, muted, and controls attributes are the attributes common to all media elements.

The poster attribute gives the URL of an image file that the user agent can show while no video data is available. The attribute, if present, must contain a valid non-empty URL potentially surrounded by spaces.

The image given by the poster attribute, the poster frame, is intended to be a representative frame of the video (typically one of the first non-blank frames) that gives the user an idea of what the video is like.

The playsinline attribute is a boolean attribute. If present, it serves as a hint to the user agent that the video ought to be displayed "inline" in the document by default, constrained to the element's playback area, instead of being displayed fullscreen or in an independent resizable window.

The absence of the playsinline attribute does not imply that the video will display fullscreen by default. Indeed, most user agents have chosen to play all videos inline by default, and in such user agents the playsinline attribute has no effect.

If the poster attribute is present and the loading attribute is in the Lazy state, the user agent must defer loading the poster image source data until the element's lazy load resumption steps are invoked.

<video src="1.mp4" poster="1.jpg" type="video/mp4">
<video src="2.mp4" type="video/mp4" loading="eager">
<video src="3.mp4" type="video/mp4" loading="lazy">
<video src="4.mp4" type="video/mp4" loading="lazy" autoplay>
<div id="very-large"></div> <!-- Everything after this div is below the viewport -->
<video src="5.mp4" type="video/mp4">
<video src="6.mp4" type="video/mp4" loading="lazy">
<video src="7.mp4" type="video/mp4" autoplay loading="lazy">
<video src="8.mp4" type="video/mp4" poster="8.jpg" loading="lazy">
<video src="9.mp4" type="video/mp4" preload="none" poster="9.jpg" loading="lazy">
<video src="10.mp4" type="video/mp4" preload="metadata" loading="lazy">
<video src="11.mp4" type="video/mp4" poster="11.jpg" preload="none" loading="eager">

Frames of video must be obtained from the video track that was selected when the event loop last reached step 1.

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 mode, and any audio from the media resource, at the current playback position.

Any audio associated with the media resource must, if played, be played synchronized with the current playback position, at the element's effective media volume. The user agent must play the audio from audio tracks that were enabled when the event loop last reached step 1.

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.

When a video element's media resource has a video channel, the element provides a paint source whose width is the media resource's natural width, whose height is the media resource's natural height, and whose appearance is the frame of video corresponding to the current playback position, if that is available, or else (e.g. when the video is seeking or buffering) its previous appearance, if any, or else (e.g. because the video is still loading the first frame) blackness.

video.videoWidth

✔MDN

HTMLVideoElement/videoWidth

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+

---

Opera10.5+Edge79+

---

Edge (Legacy)12+Internet Explorer9+

---

Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+

video.videoHeight

✔MDN

HTMLVideoElement/videoHeight

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+

---

Opera10.5+Edge79+

---

Edge (Legacy)12+Internet Explorer9+

---

Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+

These attributes return the natural dimensions of the video, or 0 if the dimensions are not known.

The natural width and natural 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 video element supports dimension attributes.

In the absence of style rules to the contrary, 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.

In user agents that implement CSS, the above requirement can be implemented by using the style rule suggested in the Rendering section.

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, such as fullscreen or in an independent resizable window. User agents may even trigger such a viewing mode by default upon playing a video, although they should not do so when the playsinline attribute is specified. As with 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 viewing mode, however, user agents may make full user interfaces visible, 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.

<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>

4.8.9 The audio element

Categories:

Flow content.

Phrasing content.

Embedded content.

If the element has a controls attribute: Interactive content.

If the element has a controls attribute: Palpable content.

Contexts in which this element can be used:

Where embedded content is expected.

Content model:

If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.

If the element does not have a src attribute: zero or more source elements, then zero or more track elements, then transparent, but with no media element descendants.

Tag omission in text/html:

Neither tag is omissible.

Content attributes:

Global attributes

src — Address of the resource

crossorigin — How the element handles crossorigin requests

preload — Hints how much buffering the media resource will likely need

autoplay — Hint that the media resource can be started automatically when the page is loaded

loop — Whether to loop the media resource

muted — Whether to mute the media resource by default

controls — Show user agent controls

loading — Used when determining loading deferral

Accessibility considerations:

For authors.

For implementers.

Sanitization:

Uncategorized.

DOM interface:

[Exposed=Window,
 LegacyFactoryFunction=Audio(optional DOMString src)]
interface HTMLAudioElement : HTMLMediaElement {
  [HTMLConstructor] constructor();
};

An audio element represents a sound or audio stream.

Content may be provided inside the audio element. User agents should not show this content to the user; it is intended for older web browsers which do not support audio, so that text can be shown to the users of these older browsers informing them of how to access the audio contents.

In particular, this content is not intended to address accessibility concerns. To make audio content accessible to the deaf or to those with other physical or cognitive disabilities, a variety of features are available. If captions or a sign language video are available, the video element can be used instead of the audio element to play the audio, allowing users to enable the visual alternatives. Chapter titles can be provided to aid navigation, using the track element and a WebVTT file. And, naturally, transcripts or other textual alternatives can be provided by simply linking to them in the prose near the audio element. [WEBVTT]

The audio element is a media element whose media data is ostensibly audio data.

The audio element has an internal shadow tree with no slot elements.

The src, crossorigin, preload, autoplay, loop, muted, controls, and loading attributes are the attributes common to all media elements.

audio elements without a controls attribute will not be displayed by the user agent, preventing them from lazy loading.

<audio src="1.mp3" type="audio/mpeg" controls>
<audio src="2.mp3" type="audio/mpeg" controls loading="eager">
<audio src="3.mp3" type="audio/mpeg" controls loading="lazy">
<audio src="4.mp3" type="audio/mpeg" controls loading="lazy" autoplay>
<div id="very-large"></div> <!-- Everything after this div is below the viewport -->
<audio src="5.mp3" type="audio/mpeg" controls>
<audio src="6.mp3" type="audio/mpeg" controls loading="lazy">
<audio src="7.mp3" type="audio/mpeg" controls autoplay loading="lazy">
<audio src="8.mp3" type="audio/mpeg" controls preload="metadata" loading="lazy">

audio = new Audio([ url ])

✔MDN

HTMLAudioElement/Audio

Support in all current engines.

Firefox3.5+Safari3.1+Chrome4+

---

Opera12.1+Edge79+

---

Edge (Legacy)12+Internet Explorer9+

---

Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+

Returns a new audio element, with the src attribute set to the value passed in the argument, if applicable.

4.8.10 The track element

Categories:

None.

Contexts in which this element can be used:

As a child of a media element, before any flow content.

Content model:

Nothing.

Tag omission in text/html:

No end tag.

Content attributes:

Global attributes

kind — The type of text track

src — Address of the resource

srclang — Language of the text track

label — User-visible label

default — Enable the track if no other text track is more suitable

Accessibility considerations:

For authors.

For implementers.

Sanitization:

Uncategorized.

DOM interface:

[Exposed=Window]
interface HTMLTrackElement : HTMLElement {
  [HTMLConstructor] constructor();

  [CEReactions] attribute DOMString kind;
  [CEReactions, ReflectURL] attribute USVString src;
  [CEReactions, Reflect] attribute DOMString srclang;
  [CEReactions, Reflect] attribute DOMString label;
  [CEReactions, Reflect] attribute boolean default;

  const unsigned short NONE = 0;
  const unsigned short LOADING = 1;
  const unsigned short LOADED = 2;
  const unsigned short ERROR = 3;
  readonly attribute unsigned short readyState;

  readonly attribute TextTrack track;
};

The track element allows authors to specify explicit external timed text tracks for media elements. It does not represent anything on its own.

The kind attribute is an enumerated attribute with the following keywords and states:

The src attribute gives the URL of the text track data. The value must be a valid non-empty URL potentially surrounded by spaces. This attribute must be present.

The element has an associated track URL (a string), initially the empty string.

If the element's track URL identifies a WebVTT resource, and the element's kind attribute is not in the chapters metadata or metadata state, then the WebVTT file must be a WebVTT file using cue text. [WEBVTT]

The srclang attribute gives the language of the text track data. The value must be a valid BCP 47 language tag. This attribute must be present if the element's kind attribute is in the subtitles state. [BCP47]

The label attribute gives a user-readable title for the track. This title is used by user agents when listing subtitle, caption, and audio description tracks in their user interface.

The value of the label attribute, if the attribute is present, must not be the empty string. Furthermore, there must not be two track element children of the same media element whose kind attributes are in the same state, whose srclang attributes are both missing or have values that represent the same language, and whose label attributes are again both missing or both have the same value.

The default attribute is a boolean attribute, which, if specified, indicates that the track is to be enabled if the user's preferences do not indicate that another track would be more appropriate.

Each media element must have no more than one track element child whose kind attribute is in the subtitles or captions state and whose default attribute is specified.

Each media element must have no more than one track element child whose kind attribute is in the description state and whose default attribute is specified.

Each media element must have no more than one track element child whose kind attribute is in the chapters metadata state and whose default attribute is specified.

There is no limit on the number of track elements whose kind attribute is in the metadata state and whose default attribute is specified.

track.readyState

Returns the text track readiness state, represented by a number from the following list:

track.NONE (0)

The text track not loaded state.

track.LOADING (1)

The text track loading state.

track.LOADED (2)

The text track loaded state.

track.ERROR (3)

The text track failed to load state.

track.track

Returns the track element's text track.

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

4.8.11 Media elements

HTMLMediaElement objects (audio and video, in this specification) are simply known as media elements.

enum CanPlayTypeResult { "" /* empty string */, "maybe", "probably" };
typedef (MediaStream or MediaSource or Blob) MediaProvider;

[Exposed=Window]
interface HTMLMediaElement : HTMLElement {

  // error state
  readonly attribute MediaError? error;

  // network state
  [CEReactions, ReflectURL] attribute USVString src;
  attribute MediaProvider? srcObject;
  readonly attribute USVString currentSrc;
  [CEReactions] attribute DOMString? crossOrigin;
  const unsigned short NETWORK_EMPTY = 0;
  const unsigned short NETWORK_IDLE = 1;
  const unsigned short NETWORK_LOADING = 2;
  const unsigned short NETWORK_NO_SOURCE = 3;
  readonly attribute unsigned short networkState;
  [CEReactions] attribute DOMString preload;
  readonly attribute TimeRanges buffered;
  undefined load();
  CanPlayTypeResult canPlayType(DOMString type);

  // ready state
  const unsigned short HAVE_NOTHING = 0;
  const unsigned short HAVE_METADATA = 1;
  const unsigned short HAVE_CURRENT_DATA = 2;
  const unsigned short HAVE_FUTURE_DATA = 3;
  const unsigned short HAVE_ENOUGH_DATA = 4;
  readonly attribute unsigned short readyState;
  readonly attribute boolean seeking;

  // playback state
  attribute double currentTime;
  undefined fastSeek(double time);
  readonly attribute unrestricted double duration;
  object getStartDate();
  readonly attribute boolean paused;
  attribute double defaultPlaybackRate;
  attribute double playbackRate;
  attribute boolean preservesPitch;
  readonly attribute TimeRanges played;
  readonly attribute TimeRanges seekable;
  readonly attribute boolean ended;
  [CEReactions, Reflect] attribute boolean autoplay;
  [CEReactions, Reflect] attribute boolean loop;
  Promise<undefined> play();
  undefined pause();

  // controls
  [CEReactions, Reflect] attribute boolean controls;
  attribute double volume;
  attribute boolean muted;
  [CEReactions, Reflect="muted"] attribute boolean defaultMuted;
  [CEReactions] attribute DOMString loading;

  // tracks
  [SameObject] readonly attribute AudioTrackList audioTracks;
  [SameObject] readonly attribute VideoTrackList videoTracks;
  [SameObject] readonly attribute TextTrackList textTracks;
  TextTrack addTextTrack(TextTrackKind kind, optional DOMString label = "", optional DOMString language = "");
};

The media element attributes, src, crossorigin, preload, autoplay, loop, muted, controls, and loading, apply to all media elements. They are defined in this section.

The loading attribute is a lazy loading attribute. Its purpose is to indicate the policy for loading media resources that are outside the viewport.

When the loading attribute is in the Lazy state, it takes precedence over the preload attribute by deferring data fetching.

If the autoplay attribute is present and the loading attribute is in the Lazy state, the user agent must also defer starting playback (and any associated network requests autoplay can introduce) until the element's lazy load resumption steps are invoked.

Media elements are used to present audio data, or video and audio data, to the user. This is referred to as media data in this section, since this section applies equally to media elements for audio or for video. The term media resource is used to refer to the complete set of media data, e.g. the complete video file, or complete audio file.

A media resource has an associated origin, which is either "none", "multiple", "rewritten", or an origin. It is initially set to "none".

A media resource can have multiple audio and video tracks. For the purposes of a media element, the video data of the media resource is only that of the currently selected track (if any) as given by the element's videoTracks attribute when the event loop last reached step 1, and the audio data of the media resource is the result of mixing all the currently enabled tracks (if any) given by the element's audioTracks attribute when the event loop last reached step 1.

Both audio and video elements can be used for both audio and video. The main difference between the two is simply that the audio element has no playback area for visual content (such as video or captions), whereas the video element does.

Each media element has a unique media element event task source.

4.8.11.1 Error codes

media.error

✔MDN

HTMLMediaElement/error

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+

---

Opera12.1+Edge79+

---

Edge (Legacy)12+Internet Explorer9+

---

Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+

Returns a MediaError object representing the current error state of the element.

Returns null if there is no error.

[Exposed=Window]
interface MediaError {
  const unsigned short MEDIA_ERR_ABORTED = 1;
  const unsigned short MEDIA_ERR_NETWORK = 2;
  const unsigned short MEDIA_ERR_DECODE = 3;
  const unsigned short MEDIA_ERR_SRC_NOT_SUPPORTED = 4;

  readonly attribute unsigned short code;
  readonly attribute DOMString message;
};

media.error.code

✔MDN

MediaError/code

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+

---

Opera12.1+Edge79+

---

Edge (Legacy)12+Internet Explorer9+

---

Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+

Returns the current error's error code, from the list below.

media.error.message

✔MDN

MediaError/message

Support in all current engines.

Firefox52+Safari15+Chrome59+

---

Opera?Edge79+

---

Edge (Legacy)?Internet ExplorerNo

---

Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

Returns a specific informative diagnostic message about the error condition encountered. The message and message format are not generally uniform across different user agents. If no such message is available, then the empty string is returned.

Every MediaError object has a message, which is a string, and a code, which is one of the following:

MEDIA_ERR_ABORTED (numeric value 1)

The fetching process for the media resource was aborted by the user agent at the user's request.

MEDIA_ERR_NETWORK (numeric value 2)

A network error of some description caused the user agent to stop fetching the media resource, after the resource was established to be usable.

MEDIA_ERR_DECODE (numeric value 3)

An error of some description occurred while decoding the media resource, after the resource was established to be usable.

MEDIA_ERR_SRC_NOT_SUPPORTED (numeric value 4)

The media resource indicated by the src attribute or assigned media provider object was not suitable.

4.8.11.2 Location of the media resource

The src content attribute on media elements gives the URL of the media resource (video, audio) to show. The attribute, if present, must contain a valid non-empty URL potentially surrounded by spaces.

If the itemprop attribute is specified on the media element, then the src attribute must also be specified.

The crossorigin content attribute on media elements is a CORS settings attribute.

A media provider object is an object that can represent a media resource, separate from a URL. MediaStream objects, MediaSource objects, and Blob objects are all media provider objects.

Each media element has an assigned media provider object, which is a media provider object or null, initially null.

media.srcObject [ = source ]

⚠MDN

HTMLMediaElement/srcObject

Support in one engine only.

Firefox🔰 42+Safari11+Chrome🔰 108+

---

Opera?Edge🔰 108+

---

Edge (Legacy)?Internet ExplorerNo

---

Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

Allows the media element to be assigned a media provider object.

media.currentSrc

✔MDN

HTMLMediaElement/currentSrc

Support in all current engines.

Firefox3.5+Safari3.1+Chrome3+

---

Opera12.1+Edge79+

---

Edge (Legacy)12+Internet Explorer9+

---

Firefox Android?Safari iOS3+Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+

Returns the URL of the current media resource, if any.

Returns the empty string when there is no media resource, or it doesn't have a URL.

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

There are three ways to specify a media resource: the srcObject IDL attribute, the src content attribute, and source elements. The IDL attribute takes priority, followed by the content attribute, followed by the elements.

4.8.11.3 MIME types

A media resource can be described in terms of its type, specifically a MIME type, in some cases with a codecs parameter. (Whether the codecs parameter is allowed or not depends on the MIME type.) [RFC6381]

Types are usually somewhat incomplete descriptions; for example "video/mpeg" doesn't say anything except what the container type is, and even a type like "video/mp4; codecs="avc1.42E01E, mp4a.40.2"" doesn't include information like the actual bitrate (only the maximum bitrate). Thus, given a type, a user agent can often only know whether it might be able to play media of that type (with varying levels of confidence), or whether it definitely cannot play media of that type.

A type that the user agent knows it cannot render is one that describes a resource that the user agent definitely does not support, for example because it doesn't recognize the container type, or it doesn't support the listed codecs.

The MIME type "application/octet-stream" with no parameters is never a type that the user agent knows it cannot render. User agents must treat that type as equivalent to the lack of any ex