Stop using AppendChild in the HTML parser

[kmcallister]

It's likely that we can skip much of the complex logic used to handle insertions by arbitrary JavaScript code. Also we need to merge adjacent text nodes during parsing to be spec-compliant.

[Ms2ger]

Not sure that's worth it; I like the peace of mind that using the DOM entry point gives me.

[kmcallister]

I'm not sure it's worth it either, but I'd like to do the experiment. See also #1269.

[nox]

We now merge properly and following the spec wording as long as it's not a perf issue in real world is nice. Closing.