One thing I noticed is that selfclosingtag is emitted for `<br/>` but not for `<br>`. There's an `_isEmptyTag` function in HTMLInstrumentation that lists things like br and link, but it doesn't appear that that function is called anywhere. I'm certain there's a bug here... the question is whether we fix it here or on the parsing side in HTMLInstrumentation.

I'd say that my preference would be to drop the "selfclosingtag" special token and make use of `_isEmptyTag` in HTMLInstrumentation, because the parsing side is what has the knowledge about what is legitimate to do. `<p/>` would emit "selfclosingtag" when that's really not legit.