This specification defines a Media Source Extensions™ [MEDIA-SOURCE] byte stream format specification based on the WebM container format.

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This document is governed by the 1 September 2015 W3C Process Document.

1. Introduction

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 spec.

2. MIME-type parameters

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

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

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"

3. Initialization Segments

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 Tracks elements that follow.
  3. A Segment Information element and a Tracks 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 Tracks 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].

4. Media Segments

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 an 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 the WebM spec.
  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.

5. Random Access Points

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.

6. Conformance

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 are to be interpreted as described in [RFC2119].

7. Acknowledgments

The editors would like to thank Frank Galligan, and Philip Jägenstedt for their contributions to this specification.

A. References

A.1 Normative references

Matthew Wolenetz; Jerry Smith; Mark Watson; Aaron Colwell; Adrian Bateman. W3C. Media Source Extensions. 5 July 2016. W3C Candidate Recommendation. URL: https://www.w3.org/TR/media-source/
S. Bradner. IETF. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
The WebM Project. WebM Container Guidelines. 26 April 2016. URL: https://www.webmproject.org/docs/container/

A.2 Informative references

Bob Lund; Silvia Pfeiffer. W3C. Sourcing In-band Media Resource Tracks from Media Containers into HTML. URL: https://dev.w3.org/html5/html-sourcing-inband-tracks/