In effect, nobody. Common copyright law would suggest that some interest in the spec vests in the individuals who wrote it and in the W3C (due to language in the contract it signs with its members).
Fortunately, all of those who have any claim to a piece of the copyright share a common set of goals: that the specification be distributed as widely as possible, and that its integrity be preserved.
Copyright © 1998, Tim Bray. All rights reserved.