CSS needs to define hit testing

Hit testing is an important concept that's invoked from a number of places, but is defined nowhere. CSS should own it and spec it.

There is no WPT for hit testing on opacity: 0 elements and the opacity property does not mention hit testing, at all. At minimum, it should say that opacity does not affect hit testing; and once there is a spec it should link to that. Currently, the sole definition is "see what existing browsers do".