They're extremely similar. You don't get an explosion of code size. Unless you're suggesting that HTML5 should also be radically different syntactically as well?
I'm suggesting that a HTML4 parser that has complex recovery code, and a mostly-copy-and-paste HTML5 parser that doesn't isn't a big win.
Browsers are going to have a complex, ugly, recovery-enabled parser in them either way, and the effort to add HTML5 to the recovery-enabled parser isn't very big, comparatively speaking.
