So, how many different parsers did you want to see in the browser again?
You'd still need to support HTML4 somewhere. Supporting HTML5 separately just means duplicating the common parts of the parser. The simplicity boat has already sailed.
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.
Not really true in this case. WebKit's previous HTML parser is similar in complexity to the HTML5 parser. Adding a second HTML parser would have been more code, more complexity, and a more complex test matrix.
You'd still need to support HTML4 somewhere. Supporting HTML5 separately just means duplicating the common parts of the parser. The simplicity boat has already sailed.