Curious if Animated SVGs are also a thing. I remember seeing some Javascript based SVG animations (it was a animated chatbot avatar) - but not sure if there is any standard framework.
Nowadays, AVIF serves that purpose best I think.
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.
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.