Formalize parser popping an element off the stack of open elements callback

[zcorpan]

What is the issue with the HTML Standard?

See discussion in #11653 (comment)

We should formalize this in the HTML parser and in https://html.spec.whatwg.org/#parsing-xhtml-documents

@whatwg/html-parser

[josepharhar]

To continue the thread from the previous issue

Presumably this is what WebKit (and I guess Chromium) call finishParsingChildren()?

Yes, this is how I implemented selectedoption updating.

Right, I think we want to make "popping" an algorithm.

This sounds good to me