[maxst]

>I've found that animated WebP can produce files around 10% the size of GIFs.

For nicely optimized GIFs like these https://iwdrm.tumblr.com/ then WebP really struggles. Try to convert some of them to WebP and you'll see the result is often bigger than the original GIF.

[catalogia]

I don't know much about webp (my build of ffmpeg doesn't support it), but those gifs don't seem so small to me. Using the first gif on that page:

    Input #0, gif, from 'tumblr_nxt634tytr1qe0eclo3_r1_500.gif':
      Duration: 00:00:08.64, start: 0.000000, bitrate: 1434 kb/s
        Stream #0:0: Video: gif, bgra, 500x269, 16.67 fps, 16.67 tbr, 100 tbn, 100 tbc

vs

    Input #0, matroska,webm, from 'bar.webm':
      Duration: 00:00:08.64, start: 0.000000, bitrate: 252 kb/s
        Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv), 500x269, SAR 1:1 DAR 500:269, 16.67 fps, 16.67 tbr, 1k tbn, 1k tbc (default)

1434kb/s for the gif, vs 252 kb/s for the VP9 webm. Even this highly optimized gif is only highly optimized by gif standards.

[maxst]

I'm talking about WebP vs GIF. Not about video.