HTML5 parsing is clearly defined and in most cases quite sensible. I think it was an excellent compromise.
XML is simple. Sure it's pedantic in the sense that it breaks, but html5 breaks too only subtly.
If html5 fails on some seemingly valid input (e.g. makes a strange layout when you self-close a div-tag) then it isn't lenient, it's still pedantic. It's just as pedantic as an xml standard is about closing tags, only that the specification for closing tags is dozens of pages instead of three words.
In fact, I think most developers agree that an error message would be preferable to a corrupt layout in the case of the self-cosed div.