[META] Erase NEW_XML (and Markup*Base abstractions) from every corner of the earth
It's just complicating the HTML parser code and impeding further development.
We might need to rework the WebVTT parser.
I believe this is basically done now. The only "Markup" file remaining is:
I'm sure there is a bunch of cleanup we'll do in the various HTML parser classes now that we can see what we're doing (with the code all in one place again), but I'm not sure we need this meta bug anymore.