MP3 WebCodecs Registration

W3C Group Draft Note,

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Paul Adenot (Mozilla)
Bernard Aboba (Microsoft Corporation)
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This registration is entered into the [webcodecs-codec-registry]. It describes, for MP3, the (1) fully qualified codec strings, (2) the AudioDecoderConfig.description bytes, (3) the codec-specific EncodedAudioChunk [[internal data]] bytes, and (4) the values of EncodedAudioChunk [[type]].

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 MP3 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 technical reports index at

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, (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 Draft Note using the Note track.

Group Draft Notes 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 work in progress.

The W3C Patent Policy does not carry any licensing requirements or commitments on this document.

This document is governed by the 2 November 2021 W3C Process Document.

1. Fully qualified codec strings

This codec has multiple equivalent codec strings:

2. EncodedAudioChunk data

EncodedAudioChunk [[internal data]] is expected to be a "frame", as described in the section of the [MP3] specification.

3. AudioDecoderConfig description

description is not used for this codec.

The sampleRate and numberOfChannels members are ignored.

4. EncodedAudioChunk type

The [[type]] for an EncodedAudioChunk containing mp3 is always "key".

NOTE: Once the initialization has succeeded, any mp3 packet can be decoded at any time without error, but this might not result in the expected audio output.

5. Privacy Considerations

Please refer to the section Privacy Considerations in [WEBCODECS].

6. Security Considerations

Please refer to the section Security Considerations in [WEBCODECS].


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.


Terms defined by reference


Normative References

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL:
Paul Adenot; Bernard Aboba. WebCodecs. 6 July 2023. WD. URL:

Informative References

ISO/IEC 13818-3:1998 - Information technology — Generic coding of moving pictures and associated audio information — Part 3: Audio. 1998-04. URL:
Chris Cunningham; Paul Adenot; Bernard Aboba. WebCodecs Codec Registry. 10 October 2022. NOTE. URL: