A helpful first step (regardless of what happens after) would be for someone to create an uptodate list of all the CSS that's actually in use.
ie. Enwiki's en:Wikipedia:Catalogue of CSS classes has been "outofdate" for many years, which is regularly frustrating - eg. when I'm trying to determine where-the-hell "class:foo" is defined I find myself opening up 15 tabs, but often don't find the target.
I'm not expert enough to attempt a comprehensive summary, else I'd try. I beg of someone, to do this. Here, there, anywhere.
(And, it would help this RfC, by showing the scope more clearly.)