Wow, that's awesome. As soon as I get to drop IE8 support, I'll think about using that.
I had no idea you could use % within transforms that referred to the size of the element itself (since all other % in CSS refer to the size of a parent element).
I think it's nice to see. It means the spec is probably pretty close to being finalized, because in order for that to happen, there needs to be at least two browser implementations.
There's also no way to get back to the "beginning" from one of these info pages. Neither "the current state of html5 forms" nor the "wufoo" wordmark at the top are links.
The only reason I marked it as "supported" is that it's "supported" like like any other purely semantic tag is (e.g. <nav>) It's just in with the forms stuff because it's specifically for content coming out of form calculations. I didn't even test the "for" attribute stuff, but wouldn't be surprised if that's not supported.