I would be 100% for this as lemmy seems to be using the browser to play it and actually is able to work with it (unlike a certain chat app that only has electron/react native clients and is actively pissing off their users with a username migration). Animated WebP also seems to strike the perfect middleground between the old animated gifs and mp4/webm allowing for animated images to use modern video compression and encoding transparently.
If you're unsure on how well this would handle: modern browsers have had support for this for years so unless you're on something like gnu icecat or pale moon you should be fine.