Add support for AVIF as a core media type

[HadrienGardeur]

Section

https://www.w3.org/TR/epub-33/#sec-core-media-types

Describe the problem

In 2020, I opened a similar issue related to WebP: #1344

Back in 2020, AVIF didn't had enough widespread support to be considered. It has been part of the baseline support in browsers since 2024: https://caniuse.com/avif

With widespread adoption on the Web and on all modern OS, plus clear benefits in terms of compression and file size, we should consider adding AVIF support to EPUB 3.4.

Describe the fix or new feature you propose

AVIF should be listed under our core media types:

• image/avif
• Spec: https://aomediacodec.github.io/av1-avif/latest-approved.html
• AVIF Images

[iherman]

Just a piece of data: I have cloned our official test for webp by converting the image into avif and modified the EPUB content accordingly. The generated epub renders properly in Thorium and Apple Books. I.e., if we accept this additional format, it has already passed the CR exit criteria😀 (except for epubcheck, which flags the avif file, as it should, as erroneous because it does not have a fallback).

[iherman]

With my staff contact hat on, I looked at

1. the official spec, which seems to be stable (published in 2022);
2. the organization publishing it, i.e., Alliance for Open Media;
3. the general adoption of AVIF (@HadrienGardeur referred to caniuse for browsers; the relevant Wikipedia article also lists other tools and libraries)

and I believe that, per the W3C Normative References document, we can normatively refer to the AVIF spec. To be on the safe side, I cc Chris Lilley and François Daoust who are more familiar with the format and the background to see if they have something to add.

cc @svgeesus @tidoust

[tidoust]

That looks reasonable to me but I note that normative references are evaluated at transitions on the Recommendation track in practice. So you won't get a definitive answer today. Evaluation will be done by the team that reviews an upcoming transition request.

(FWIW, having a quick look at spec data, I don't see any other W3C spec that normatively references a spec from the Alliance for Open Media for now)

[svgeesus]

(FWIW, having a quick look at spec data, I don't see any other W3C spec that normatively references a spec from the Alliance for Open Media for now)

Probably because most document specs don't normatively mandate required media formats.

AOM has the same sort of relationship to ISO MPEG as the Independent JPEG Group does to ISO JPEG - making a publicly available spec, sample media, tests, libraries, and pushing for adoption.

I note that AVIF is proposed as an Investigation Effort for Interop2026 (not a focus area, because the number of WPT is too low).

[iherman]

Thanks @svgeesus and @tidoust.

[iherman]

This was discussed during the pmwg meeting on 02 October 2025.

View the transcript

AVIF files in the package

<shiestyle> w3c/epub-specs#2794

w3c/epub-specs#2794

Hadrien: A few years ago I added a similar issue about webp
… it is the fastest growing format
… widely used today
… images is a funny case, since we have a whitelist for them
… unlike other media types
… so it is a strong choice to add

ivan: We have at least two implementations at this point (Thorium and Mac's Books)

<kimberg> not sure offhand

ivan: so this is probably just an editing issue

shiestyle: In Japan many RSes don't support even webp
… so I think it is good to add avif

ivan: Let's do a formal proposal

Proposed:Add AVIF to the EPUB package

<sueneu> +1

ivan: and I am happy to do the PR

<shiestyle> +1

<ivan> +1

<Hadrien> +1

<toshiakikoike> +1

<duga> +1

<gautierchomel> +1

<Dale> +1

<wendyreid> +1

<MasakazuKitahara> +1

<CharlesL> +1

RESOLUTION: Add AVIF to the EPUB package

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