> Unfortunately browsers don't allow those in image contexts
The fact that we have the <img> element at all is bad. HTML has since the early days a perfectly capable <object> which can even be nested to provide fallback, but browser support was always spotty.
The Acid2 test famously used <object> to shame browser vendors into supporting it at least to some extent.
AVIF is only starting to become widespread so can't be used without fallback if you care about your users. Not sure how it compares to AV1 quality/compression wise but hopefully its not as gimped as webp and there will encoders that aren't as crap as libwebp that almost everyone uses.