WebM Byte Stream Format

This specification describes a byte stream format based on the WebM container format [WEBM].

It defines the MIME-type parameters used to signal codecs, and provides the necessary format specific definitions for initialization segments, media segments, and random access points required by the Byte Stream Formats section of the Media Source Extensions™ specification.

This section specifies the parameters that can be used in the MIME-type passed to isTypeSupported() or addSourceBuffer().

codecs

A comma separated list of codec IDs used to specify what codecs will be used in the byte stream.

Codec ID	Valid with "audio/webm"	Valid with "video/webm"
vorbis	true	true
opus	true	true
vp8	false	true
vp9	false	true
vp09... as described in the VP Codec ISO Media File Format Binding document [VP09CODECSPARAMETERSTRING]	false	true

Note

Implementations SHOULD support all of the codec IDs mentioned in the table above.

Note

Implementations SHOULD encourage applications to prefer the "vp09..." codec ID over "vp9". The "vp09..." format provides detailed profile and color information, enabling implementations to give more accurate answers for codec support.

Examples of valid MIME-types with a codecs parameter.

• audio/webm;codecs="vorbis"
• video/webm;codecs="vorbis"
• video/webm;codecs="vp8"
• video/webm;codecs="vp8,vorbis"
• video/webm;codecs="vp9,opus"
• video/webm;codecs="vp09.00.10.08"
• video/webm;codecs="vp09.02.10.10.01.09.16.09.01,opus"

A WebM initialization segment MUST contain a subset of the elements at the start of a typical WebM file.

The user agent MUST run the append error algorithm if any of the following conditions are not met:

1. The initialization segment MUST start with an EBML Header element, followed by a Segment header.
2. The size value in the Segment header MUST signal an "unknown size" or contain a value large enough to include the Segment Information and Track elements that follow.
3. A Segment Information element and a Track element MUST appear, in that order, after the Segment header and before any further EBML Header or Cluster elements.

The user agent MUST accept and ignore any elements other than an EBML Header or a Cluster that occur before, in between, or after the Segment Information and Track elements.

The user agent MUST source attribute values for id, kind, label and language for AudioTrack, VideoTrack and TextTrack objects as described for WebM in the in-band tracks spec [INBANDTRACKS].

A WebM media segment is a single Cluster element.

The user agent uses the following rules when interpreting content in a Cluster:

1. The TimecodeScale in the WebM initialization segment most recently appended applies to all timestamps in the Cluster
2. The Timecode element in the Cluster contains a presentation timestamp in TimecodeScale units.
3. The Cluster header MAY contain an "unknown" size value. If it does then the end of the cluster is reached when another Cluster header or an element header that indicates the start of a WebM initialization segment is encountered.

The user agent MUST run the append error algorithm if any of the following conditions are not met:

1. The Timecode element MUST appear before any Block & SimpleBlock elements in a Cluster.
2. Block & SimpleBlock elements are in time increasing order consistent with [WEBM].
3. If the most recent WebM initialization segment describes multiple tracks, then blocks from all the tracks MUST be interleaved in time increasing order. At least one block from all audio and video tracks MUST be present.

The user agent MUST accept and ignore Cues or Chapters elements that follow a Cluster element.

Either a SimpleBlock element with its Keyframe flag set, or a BlockGroup element having no ReferenceBlock elements, signals the location of a random access point for that track. The order of multiplexed blocks within a media segment MUST conform to the WebM Muxer Guidelines.

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MAY, MUST, and SHOULD in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.