From Ian Hickson himself:
were then unofficially calling "HTML5", and officially calling "Web
Applications 1.0". We renamed the specification "HTML5", and the W3C began
publishing a copy of it as well. Not long after, the W3C side of this
effort decided to split their version of the spec into subspecs (e.g.
splitting out the 2D canvas API, server-sent events, postMessage, etc),
and for a while we tried to match that on the WHATWG side. The result was
an increasing confusion of versions of the spec, so we eventually went
back to just having a single spec on the WHATWG side which contains
everything I work on, which we now call the "HTML Living Standard". Over
the years, this document and the various documents on the W3C side have
slowly slightly forked, as documented at the top of the WHATWG spec.
More recently, the goals of the W3C and the WHATWG on the HTML front have
diverged a bit as well. The WHATWG effort is focused on developing the
canonical description of HTML and related technologies, meaning fixing
bugs as we find them , adding new features as they become necessary and
viable, and generally tracking implementations. The W3C effort, meanwhile,
is now focused on creating a snapshot developed according to the venerable
W3C process. This led to the chairs of the W3C HTML working group and
myself deciding to split the work into two, with a different person
responsible for editing the W3C HTML5, canvas, and microdata
specifications than is editing the WHATWG specification (me).
So no.. WHATWG is not "definitive" standard. It's not standard at all. It's introducing new features that will go far ahead of W3C HTML standard and will be implemented however browser makers feel they should (in their own way) and then they will try to patch things with W3C if the relationship doesn't collapse completely.
I don't see how any of this can be good. The one bright side was that WHATWG and W3C worked together and created one HTML standard that should have evolved in unified way. Obviously this didn't work out and now even bigger fragmentation between features will happen.