In more detail
AVIF (AV1 Image File Format) stores AV1-compressed frames in the same HEIF box container used by HEIC / HEIF. It supports lossy and lossless coding, an alpha channel, 8-, 10- and 12-bit colour, HDR and animated sequences. It is slower to encode than JPEG or WebP, and very large images are split into tiles internally.
Why it matters
At the same visual quality, AVIF files are usually smaller than both JPEG and WebP, which saves bandwidth on image-heavy pages. Current versions of the major browsers display it, but some older software, email clients and photo viewers cannot open it, so a JPEG copy is often needed for sharing or uploading elsewhere. Websites commonly offer AVIF with a JPEG fallback in a <picture> element.
Example
An AVIF file starts with an ftyp box naming the brand avif. The first bytes of one written by an image library were:
00 00 00 20 66 74 79 70 61 76 69 66 ...ftypavif
The first four bytes are the box length (here 32); bytes 4 to 11 read ftypavif. An animated AVIF uses the brand avis instead.