[selectors] add an :ever-been-visible pseudo-class

[heycam]

I think it would be useful to have a declarative way to apply styles (for example for starting animations), when an element has been scrolled into view. So this would be kind of like a declarative way to set up an intersection observer, and set a class name. And maybe you'd want to similarly have some kind of threshold.

I've seen plenty of sites that kick off animations (maybe fading in an image, or starting a complex animation) once an element is scrolled into view.

The reason for suggesting :ever-been-visible rather than :visible is to avoid cycles if the rules that apply when the selector matches cause the element to be moved out of view.

/cc @AmeliaBR @bkardell

[AmeliaBR]

Cross reference to the discussion about scroll timeline animations versus scroll-triggered animations in https://github.com/WICG/scroll-animations/issues/48

This would address the most common use case of scroll-triggered animations, and possibly remove the need for complex interactions of scroll-based and time-based animations.

One thing that might be needed is a way to wait to mark something as "been visible" until it reaches a certain position in the screen, so that the animation isn't triggered while the element is just barely touching the bottom edge of the viewport.

[bkardell]

Hmm.. If we added a threshold is it fixed or author provided? If the latter it seems it's multibits.. Does that matter?