See https://github.com/w3c/epub-specs/pull/2811: per the [WG Resolution](https://w3c.github.io/pm-wg/minutes/2025-10-02.html#eda2) AVIF (ie, image/avif) has been added to the core media types for EPUB 3.4.
See w3c/epub-specs#2811: per the WG Resolution AVIF (ie, image/avif) has been added to the core media types for EPUB 3.4.