AVC (H.264) WebCodecs Registration W3C Group Note Draft , 8 June 2026 More details about this document This version: https://www.w3.org/TR/2026/DNOTE-webcodecs-avc-codec-registration-20260608/ Latest published version: https://www.w3.org/TR/webcodecs-avc-codec-registration/ Editor's Draft: https://w3c.github.io/webcodecs/avc_codec_registration.html Previous Versions: https://www.w3.org/TR/2025/DNOTE-webcodecs-avc-codec-registration-20250514/ History: https://www.w3.org/standards/history/webcodecs-avc-codec-registration/ Feedback: GitHub Editors: Paul Adenot ( Mozilla ) Eugene Zemtsov ( Google LLC ) Former Editors: Bernard Aboba ( Microsoft Corporation ) Chris Cunningham ( Google LLC ) Participate: Git Repository. File an issue. Version History: https://github.com/w3c/webcodecs/commits Copyright © 2026 World Wide Web Consortium . W3C ® liability , trademark and permissive document license rules apply. Abstract This registration is entered into the [webcodecs-codec-registry] . It describes, for AVC (H.264), the (1) fully qualified codec strings , (2) the codec-specific EncodedVideoChunk [[internal data]] bytes, (3) the VideoDecoderConfig.description bytes, (4) the values of EncodedVideoChunk [[type]] , (5) the codec-specific extensions to VideoEncoderConfig , and (6) the codec-specific extensions to VideoEncoderEncodeOptions . The registration is not intended to include any information on whether a codec format is encumbered by intellectual property claims. Implementers and authors are advised to seek appropriate legal counsel in this matter if they intend to implement or use a specific codec format. Implementers of WebCodecs are not required to support the AVC / H.264 codec. This registration is non-normative. Status of this document This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C standards and drafts index . Feedback and comments on this specification are welcome. GitHub Issues are preferred for discussion on this specification. Alternatively, you can send comments to the Media Working Group’s mailing-list, [email protected] ( archives ). This draft highlights some of the pending issues that are still to be discussed in the working group. No decision has been taken on the outcome of these issues including whether they are valid. This document was published by the Media Working Group as a Group Note Draft using the Note track . Group Note Drafts are not endorsed by W3C nor its Members. This is a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to cite this document as other than a work in progress. The W3C Patent Policy does not carry any licensing requirements or commitments on this document. This document is governed by the 18 August 2025 W3C Process Document . Table of Contents 1 Fully qualified codec strings 2 EncodedVideoChunk data 3 VideoDecoderConfig description 4 EncodedVideoChunk type 5 VideoEncoderConfig extensions 5.1 AvcEncoderConfig 5.2 AvcBitstreamFormat 6 VideoEncoderEncodeOptions extensions 6.1 VideoEncoderEncodeOptionsForAvc 7 Privacy Considerations 8 Security Considerations Conformance Document conventions Index Terms defined by this specification Terms defined by reference References Normative References Non-Normative References IDL Index 1. Fully qualified codec strings The codec string begins with the prefix "avc1." or "avc3.", with a suffix of 6 characters as described respectively in Section 3.4 of [rfc6381] and Section 5.4.1 of [iso14496-15] . 2. EncodedVideoChunk data EncodedVideoChunk [[internal data]] is expected to be an access unit as defined in [ITU-T-REC-H.264] section 7.4.1.2. NOTE: An access unit contains exactly one primary coded picture. If the bitstream is in avc format, [[internal data]] is assumed to be in canonical format, as defined in [iso14496-15] section 5.3.2. If the bitstream is in annexb format, [[internal data]] is assumed to be in in Annex B format, as defined in [ITU-T-REC-H.264] Annex B. NOTE: Since [[internal data]] is inherently byte-aligned, implementations are not required to recover byte-alignment. 3. VideoDecoderConfig description If the description is present, it is assumed to be an AVCDecoderConfigurationRecord , as defined by [iso14496-15] , section 5.3.3.1, and the bitstream is assumed to be in avc format. NOTE: This format is commonly used in .MP4 files, where the player generally has random access to the media data. If the description is not present, the bitstream is assumed to be in annexb format. NOTE: "annexb" format is described in greater detail by [ITU-T-REC-H.264] , Annex B. This format is commonly used in live-streaming applications, where including the SPS and PPS data periodically allows users to easily start from the middle of the stream. 4. EncodedVideoChunk type If an EncodedVideoChunk ’s [[type]] is key , and the bitstream is in avc format, then the EncodedVideoChunk is expected to contain a primary coded picture that is an instantaneous decoding refresh (IDR) picture. NOTE: If the bitstream is in avc format, parameter sets necessary for decoding are included in VideoDecoderConfig.description . If an EncodedVideoChunk ’s [[type]] is key , and the bitstream is in annexb format, then the EncodedVideoChunk is expected to contain both a primary coded picture that is an instantaneous decoding refresh (IDR) picture, and all parameter sets necessary to decode all video data NAL units in the EncodedVideoChunk . 5. VideoEncoderConfig extensions partial dictionary VideoEncoderConfig { AvcEncoderConfig avc ; }; avc , of type AvcEncoderConfig Contains codec specific configuration options for the AVC (H.264) codec. 5.1. AvcEncoderConfig dictionary AvcEncoderConfig { AvcBitstreamFormat format = "avc"; }; format , of type AvcBitstreamFormat , defaulting to "avc" Configures the format of output EncodedVideoChunk s. See AvcBitstreamFormat . 5.2. AvcBitstreamFormat enum AvcBitstreamFormat { "annexb" , "avc" , }; The AvcBitstreamFormat determines the location of AVC parameter sets, and mechanisms for packaging the bitstream. annexb SPS and PPS data are included periodically throughout the bitstream. NOTE: This format is described in greater detail by [ITU-T-REC-H.264] , Annex B. This format is commonly used in live-streaming applications, where including the SPS and PPS data periodically allows users to easily start from the middle of the stream. avc SPS and PPS data are not included in the bitstream and are instead emitted via the [[output callback]] as the VideoDecoderConfig.description of the EncodedVideoChunkMetadata.decoderConfig . NOTE: This format is described in greater detail by [iso14496-15] , section 5.3. This format is commonly used in .MP4 files, where the player generally has random access to the media data. 6. VideoEncoderEncodeOptions extensions partial dictionary VideoEncoderEncodeOptions { VideoEncoderEncodeOptionsForAvc avc ; }; avc , of type VideoEncoderEncodeOptionsForAvc Contains codec specific encode options for the [ITU-T-REC-H.264] codec. 6.1. VideoEncoderEncodeOptionsForAvc dictionary VideoEncoderEncodeOptionsForAvc { unsigned short ? quantizer ; }; quantizer , of type unsigned short , nullable Sets per-frame quantizer value. In [ITU-T-REC-H.264] the quantizer threshold can be varied from 0 to 51. 7. Privacy Considerations Please refer to the section Privacy Considerations in [WEBCODECS] . 8. Security Considerations Please refer to the section Security Considerations in [WEBCODECS] . Conformance Document conventions Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification. All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119] Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example" , like this: This is an example of an informative example. Informative notes begin with the word “Note” and are set apart from the normative text with class="note" , like this: Note, this is an informative note. Index Terms defined by this specification "annexb" , in § 5.2 annexb , in § 5.2 "avc" , in § 5.2 avc dict-member for VideoEncoderConfig , in § 5 dict-member for VideoEncoderEncodeOptions , in § 6 enum-value for AvcBitstreamFormat , in § 5.2 AvcBitstreamFormat , in § 5.2 AvcEncoderConfig , in § 5.1 format , in § 5.1 quantizer , in § 6.1 VideoEncoderEncodeOptionsForAvc , in § 6.1 Terms defined by reference [WEBCODECS] defines the following terms: "key" EncodedVideoChunk VideoEncoderConfig VideoEncoderEncodeOptions [[internal data]] [[output callback]] [[type]] decoderConfig description [WEBIDL] defines the following terms: unsigned short References Normative References [RFC2119] S. Bradner. Key words for use in RFCs to Indicate Requirement Levels . March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119 [WEBCODECS] Paul Adenot; Eugene Zemtsov. WebCodecs . 5 May 2026. WD. URL: https://www.w3.org/TR/webcodecs/ [WEBIDL] Edgar Chen; Timothy Gu. Web IDL Standard . Living Standard. URL: https://webidl.spec.whatwg.org/ Non-Normative References [ISO14496-15] ISO/IEC 14496-15:2024 Information technology — Coding of audio-visual objects — Part 15: Carriage of network abstraction layer (NAL) unit structured video in the ISO base media file format . October 2024. URL: https://www.iso.org/standard/89118.html [ITU-T-REC-H.264] H.264 : Advanced video coding for generic audiovisual services . June 2019. URL: https://www.itu.int/rec/T-REC-H.264 [RFC6381] R. Gellens; D. Singer; P. Frojdh. The 'Codecs' and 'Profiles' Parameters for "Bucket" Media Types . August 2011. Proposed Standard. URL: https://www.rfc-editor.org/rfc/rfc6381 [WEBCODECS-CODEC-REGISTRY] Paul Adenot; Eugene Zemtsov. WebCodecs Codec Registry . 12 February 2026. DRY. URL: https://www.w3.org/TR/webcodecs-codec-registry/ IDL Index partial dictionary VideoEncoderConfig { AvcEncoderConfig avc ; }; dictionary AvcEncoderConfig { AvcBitstreamFormat format = "avc"; }; enum AvcBitstreamFormat { "annexb" , "avc" , }; partial dictionary VideoEncoderEncodeOptions { VideoEncoderEncodeOptionsForAvc avc ; }; dictionary VideoEncoderEncodeOptionsForAvc { unsigned short ? quantizer ; };