[cssom] Provide access to counter values #5879
Description
Currently there is no access to counter values, even with getComputedStyle. I'm trying to implement a system that allows cross-reference to specific elements in a list, and in order to do that I'm having to count the elements myself. Since the browser has already computed the counter value (and AFAIK counters are not reflow-sensitive, unlike page numbers), it seems like it would be useful to expose them directly.