I don't really understand this objection. Sure, HTML 5 isn't officially a standard, but neither is -webkit-whatever; in both cases, you can use whichever non-standard you like, subject to browser support. The advantage of the W3C draft is that it is a non-standard that the browser makers are all working towards.
The objection is that the standardization process is too slow. Not a little bit too slow, but so glacially slow it's almost indistinguishable from abandonment. So, progress of key web technologies needs to be put into vendors's hands.
CSS border-radius is still not a W3C recommendation. It's been a draft for nine and a half years. So far. It was last updated one year ago.
With a standardization process that broken, vendors need to take things into their own hands, and them doing so, especially webkit doing so, has been an astounding success, IMHO.
Yes, but why does it matter that the standardization process is slow? What is the practical relevance of something moving from "Candidate Recommendation" to "Recommendation" status? Why are draft standards any worse than vendor extensions?
Prefixed attributes bring features to developers while they are still experimental, which means we don't have to wait a decade before trying them out.
HTML is a living standard and was last updated yesterday:
http://whatwg.org/html
The problem is not the webkit vendor attributes, the problem is that proper standards aren't able to keep up.
HTML5 is still not a proper W3C standard (or "recommendation"), it's still just a working draft, and was last updated in May of 2011. It's been six years. So far.