The question “What is HTML5?” has gotten more complicated. While W3C continues work on a full specification of HTML5, the Web Hypertext Application Technology Working Group (WHATWG) is pursuing a “living standard” approach that is frequently updated. Both groups are reassuring us that this doesn’t constitute a rift, but certainly it will make things tricky when resolving the fine points of the standard(s). Ian Hickson has gone into some detail on the W3C site about the relationship between the WHATWG HTML living standard and the W3C HTML5 specification.
The WHATWG “HTML Living Standard” site significantly has no version number.
Considering that HTML5 is already widely implemented even though it won’t be finalized till the year after next, it’s unlikely this will add any further confusion. By the time it becomes a W3C Recommendation, many implementers will doubtless have moved beyond it to new features.