The idea that HTML is hard to parse isn't exactly controversial; the HTML spec has a long description of the exact rules you're required to implement to parse HTML and it's a bunch of special cases and error handling. Certainly I wouldn't call it "heuritics" any more since everything is well specified, but equally certainly the rules are derived from heuristics that browsers implemented and are now required to maintin compatibility.

I'm having difficulty unpacking your claim about a DTD "parsing" HTML5, but I find it hard to believe that a primarilly DTD-based parser would get all the rules right around, say, foster-parenting, or handling of -- in script elements, or pseudo-entities or any of the the other weird quirks that are specific to HTML and not shared with SGML languages. Perhaps you can link the code and details about the claimed test suite pass rate?