There are two official HTML5 specifications:
The history is a bit complex - a lot of fighting and bickering. But the end result is that we have two specifications, both with the same editor: Ian Hickson. WHATWG seems to be sort of a playground, which the editor and his contributors use to innovate. We expect that the W3C specification will ultimately be considered authoritative.
The HTML5 specs are incredibly long, but most of it is describing how user agents should deal with HTML5. Don't be afraid to use it as a reference. It can be intimidating at first, but can be very useful once you get used to it.