I've used Rive in a small personal project before and I really can't imagine creating or editing web animations in any other way. Apparently they also made their own vector-based feathering technique, which is also amazing:
https://rive.app/blog/introducing-vector-feathering
I do understand the appeal for an open format though. Rive seems to have their own proprietary (documented) binary format: https://rive.app/docs/runtimes/advanced-topic/format
They say on https://rive.app/pricing that the Format is OS and MIT, perhaps I have missed something?
Oh, my mistake. I shouldn't have used the word "proprietary", but too late to edit. There does seem to be community-made runtimes: https://rive.app/docs/runtimes/community-runtimes