AVIF despite being supported by most browsers is sadly very much not a good image format and has multiple downsides compared to previous image formats.
some of the downsides (not all) include:

• Poorly performing lossless mode that always loses when compared to WebP and sometimes even worse then PNG.
• You can not see a single pixel of an AVIF image until it is 100% downloaded unlike every single other format supported by web browsers (BMP, GIF, JPEG, PNG, and WebP).

The compression rate depends on the specific situation of the image. Since I transitioned from PNG to AVIF, the size of AVIF has been between 0.1x and 1.1x that of PNG, so we defaulted to AVIF.

Poorly performing lossless mode that always loses when compared to WebP and sometimes even worse then PNG.

You can just use WebP lossless instead. It's an excellent format unlike its lossy variant.

You can not see a single pixel of an AVIF image until it is 100% downloaded unlike every single other format supported by web browsers (BMP, GIF, JPEG, PNG, and WebP).

I believe it matters more whether the entirely rendered image looks prettier, less noisy, and less dirty. AVIF is the best at hiding the compression noise.

AVIF (compared to WebP) also enables 10-bit and 12-bit depth sources, HDR, screen content tooling, and lossy 4:2:2 and 4:4:4. This is in addition to the overall efficiency improvements.

It'd be nice for GitHub to natively support AVIF, so we don't have to work it around by renaming AVIFs to ".jpg" upon upload.

Here's an example of an HDR AVIF. It's a 4K screenshot, yet it's only 252 KB!

Is AVIF on the road too?

https://github.com/orgs/community/discussions/169478#discussioncomment-14116729

support for .webp is already in the works and coming soon. I will note down your request for .avif as well as we consider additional extensions.