Update masks to reflect new working draft

The post for css masks doesn't reflect the fact that the feature has gained new life with the advent of the W3C working draft. I suggest changing the status to "caution", but even if you guys are hesitant to do that, I hope you will at least alter the text to reflect the feature's new status (and hopeful future).

By the way, I must be a real dummy, because I can't figure out how you guys add the "View browser share" link to caniuse.com (where applicable), so I just stuck the link in the "moreurl" line. If you merge this commit, please change it so that the caniuse.com link is put in the proper place.