From Colin: "I get occasional (and entirely legitimate) questions about support and adoption levels of HTML5. Can we really go ahead using it? How do the various browsers handle the various new features? Here are two extremely useful websites that address those questions:
As an entry point I recommend clicking the 'Tell me more' link in the first paragraph of the latter site."
Piotr adds: "Dive Into HTML 5 is a free online book (coincidentally written using HTML5) which teaches basics of HTML5 and some techniques on how to gracefully fall back into the "old" html when browser doesn't support HTML5."
And yes, we know that some people argue that it's not HTML5, it's just HTML. We're ok with just linking to this document as a response to that.