One of the ideas behind HTML5 is that while there is some concept of validity and well-formedness, essentially any random stream bytes describes exactly one DOM tree, in some cases the resulting tree is surprising, but even then should be same across all conformant parsers (modulo scripting support).
The end result is that validation is not that much interesting anymore, because the idea was that valid (X)HTML document should parse the same accross all browsers (which it mostly did, but that did not say much about how it was actually rendered).
The end result is that validation is not that much interesting anymore, because the idea was that valid (X)HTML document should parse the same accross all browsers (which it mostly did, but that did not say much about how it was actually rendered).