No, actually! There's no metadata option in .WebM to tell it to loop! It's not in the spec (which is just a subset of matroska).

The format literally does not support looping. Whereas the .WebP spec does provide a looping option:

https://datatracker.ietf.org/doc/rfc9649/

It's in the ANIM chunk which specifies the "loop count". If it's zero, loop forever.

WebM has no built-in loop flag 🤷