To create the animated webp file: Rendered the image sequence. I got a bunch of static webp inages inside the output folder.
To create a single animated webp file from this, I used the webpmux command from libwebp:
webpmux \
-frame ./anim10001.webp +3+0+0+0-b \
-frame ./anim10002.webp +3+0+0+0-b \
-frame ./anim10003.webp +3+0+0+0-b \
…
-o output.webp
the 3 means 3 milliseconds per frame.