Web updates

HTML 90a60b2a0dc7

Remove "control group" concept from the focus model The control group concept existed to give special focus behavior to <dialog> elements, treating them on par with Documents in terms of allowing focus to shift inside of them. However, this was never implemented in user agents. This change removes the control group concept, instead using Document directly. This allows some simplifications, e.g. because the first focusable area of a non-empty Document is always the Document's viewport. The change uncovered some areas of the spec that were potentially wrong or unclear when considering focus around documents. We will follow up on that in But for now we keep the same algorithms, just without <dialog>s. Fixes #2171.

Takayoshi Kochi