I wrote a 'scratch off' card[1] using the CSS paintlet API a little while ago to see what it was like to work with. There were a few annoying quirks (basically Chrome only, lots of talk without much forward motion, not much that you can't do with a plain old canvas) but there's a lot of potential.

[1] Working example: https://scratchy.ooer.com/ - click and drag to scratch off the symbols. The code lives at https://github.com/onion2k/scratchy