Forget about HTML 6 - HTML5 is the last 'version' of HTML
HTML5 (yeah with its new logo), could be the last 'version' of HTML.
Ian Hickson, one of the key authors of the HTML5 specification noted that starting with HTML5, "..the technology is not versioned and instead we just have a living document that defines the technology as it evolves."
In my view that's obscene.
Sure there is a baseline and sure what that plan calls for is additions (not deletions), that said it is the version number or even revision number that keeps things somewhat sane for web developers. Without a target to peg a web dev effort against, developers will always be running on a treadmill that will never stop.
While I don't agree with the approach, for better or for worse, as Hickson points out - HTML5 has already been operating like this for some time. There is no final HTML5 spec as I've written many time - and there likely never will be one either.