[whatwg] Why are we merging Document and HTMLDocument again?

[Boris Zbarsky]

I'm trying to understand why Document and HTMLDocument got merged.  In 
practice, UAs don't do that, and doing it imposes a performance penalty 
on manipulating, say, XMLHttpRequest response documents (because the 
name getter on HTMLDocument makes property access slower on that object).

So why are we doing this?
-Boris