Author here: I'm also very much looking forward to using AVIF (and also JPEG XL), but it's another tooling question. I'm really hoping that Pixelmator Pro (now owned by Apple) will support them both soon. Photoshop (which I've recently dumped) supports WebP and AVIF, but not through the "Save for Web" or "Export As" flows. Having a nice export flow where you can easily compare compression vs. quality levels is really important to me, and I suspect to many others.

When it comes to animations, is there much of a reason to use avif over webm? I suppose an avif fits into CSS and <img> without further code changes, but that's about it as far as I can think.

Since all modern browsers support AVIF, I'd skip WEBP and just use AVIF (possibly with a fallback for JPG/PNG).

I just switched to AVIF after several years of using WebP. Main reason for me is HDR support, but the smaller encoding is nice. Just beware of lossless AVIF, that's a trap.