I also use some javascript to delay rendering until the browser window stays on an image. That could very well be introducing some wonkiness.
Current Big Sur release, not a Beta.
From what I can find, Catalina should still be supported for more than a year from now, right?
Edit: well, that's bizarre. Caniuse.com [0] lists the same restriction, WebP is only available from Big Sur onwards. And then people wonder why nobody likes developing for Safari.
I suppose Apple doesn't want to include a software fallback for platforms where hardware isn't available to discourage developers from using the format. It's not like Apple would need to write any complicated decoding algorithms when there's already an open source implementation that's free to use.
I'm just saying it's not unprecedented to have the same browser version supporting different audio/video/image formats depending on OS or hardware.
For image formats it's not as usual, but if someone is making a site only supporting webp then I'd assume they'd look up some support matrixes beforehand.
VP9 and WebP patents are all granted freely by Google. WebP images aren't as terrible for power consumption even in software because the browser doesn't decode 60 of them every second.
Regarding the examples, when it comes to Ogg Vorbis there is no issue with software decoding (it's software everywhere), AV1 was granted software decode support on some OS'es but not others in the exact same browser, version and hardware (and IIRC it's the encoding perf that is especially bad for AV1, not decoding) and the chrome on android issue also shows how legal or hardware issues are not the only things limiting what is available (it can also be bugs that just stay around).
All I'm saying is that testing for browser version A is not a surefire test. Either check the support or test on browser A version B on OS C version D on hardware E (for each of the A/B/C/D/E variants) or just stick with what works across the board. Either way you probably won't end up shipping a site with only webp support.
So Safari gets free accelerated handling of new formats and codecs but only when the underlying OS supports them.