Unprefixed -webkit-mask behavior differences

### What is the issue with the Compatibility Standard?

See 
https://docs.google.com/document/d/14TtdFgYJ6TJuVzSE2Ra_gtuy20VNRgIKcbwiF3hg9aQ/preview#heading=h.cqull3z0pfaq for a great write up of the differences. It seems like Safari has done something similar, so we probably need to update -webkit-mask here (and maybe file an issue for Firefox to do the same, if there isn't already one).