> I foresee all of this CSS/HTML stuff as eventually being declared a sort of legacy format and adding a standard way to ship pluggable rendering engines/language runtimes.

I doubt this is going to happen as long as backwards compatibility continues to be W3C's north star. That's why all current browsers can still render the first website created by TBL in 1989.

Sure, official support for certain extensions should happen but HTML/CSS will always be at the core.