Define when :drop applies

[zcorpan]

We need some glue here for :drop and :drop() pseudo-classes, so it's well-defined when they apply for HTML.

https://drafts.csswg.org/selectors-4/#drag-pseudos
https://html.spec.whatwg.org/multipage/scripting.html#pseudo-classes

I think these are not yet implemented though?
http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4518

[fantasai]

These were dropped since the corresponding HTML feature was dropped: w3c/csswg-drafts#2257 So I think this issue should be closed.
Please file an issue against Selectors if you think they need to be re-added. :)

[domenic]

Thanks @fantasai!