Interesting. I remember (a few years ago) finding out about them through Wikipedia and that I would need a Chrome extension to use them (they had an example). So, I installed it to view it, but I never saw any other usage of it outside of that page/article.
I guess there’s not really much space savings[0] over an animated GIF even with an optimized PNG and max compression. It does explain why WebP is replacing GIF though[a]: being a video format, it can compress much better.
Off topic: Is YouTube still using VP8? Or have they switched to AV1 (the basis of AVIF)?
[a]: The popular GIF sharing site Giphy actually uses WebP if your browser supports it. Otherwise just browsing their site would use hundreds of megabytes (which isn’t good for mobile users with a data cap)