[selectors-4][css-pseudo-4] meaning of ::search-text:current(…), :past, :future

[delan]

We resolved that ::search-text (#3812) should have a :current (#10212) state for the active search result, but :current also has a functional form, where :current(x, y) matches any current element (or innermost ancestor) that also matches x, y, as well as the related pseudo-classes :past and :future.

0. Is the list of compound selectors in :current(…) a <forgiving-selector-list>? On the one hand, selector lists are normally invalid when they contain an invalid selector, but on the other hand, “like ‘:is()’” suggests it should be forgiving. If so, we should also update this note.

1. What does ::search-text:current(x, y) mean? Does it mean the innermost x or y that contains the active search result (which we probably want to forbid, since it would allow highlights to affect layout), or the ::search-text of that innermost x or y (which may have tricky :has()-like perf implications), or something else? We propose that ::search-text:current(…) be invalid, for now.

2. What do ::search-text:past and ::search-text:future mean? We propose that these also be invalid, for now, since no known UA distinguishes between search results on either “side” of :current.