[whatwg] "Fullscreenable" attribute.

[Alpha Omega]

I think it would be useful to add "fullscreenable" (or more refined name)
attribute to arbitrary element, so users could be able to full-screen
DOM subtrees, that document author marked as "fullscreenable".

Usage: User choses area that he wants to fullscreen, peforms UA-specific
action there("go to fullscreen" in context menu in desktop browsers,
or gesture on mobile devices for example), UA goes up in DOM tree
until it founds "fullscreenable" attribute, and then  "fullscreens"
this subtree. If "fullscreenable" attribute is not found, then it is
UA authors decision what to do - for example fullscreen entire page.

Use case: Not only solves problem with <video> tag, but also useful
for mobile UAs (users could use it to "zoom" to author defined parts,
on pages with complex layouts.), and for interactive webapps in general IMHO.