Writing a post on any CSS selector or property (or even any new one coming up) would produce a long (and boring) blog. But :focus-within is different – and it’s something I have been waiting for forever. To me, it’s more than another pseudo-class, it’s a game-changer in many ways. And it’s in the stable versions of both Chrome and Firefox (and others). See the MDN page on :focus-within for a full (and esp. up2date) info on browser support.
What :focus-within does is quite simple, actually: it allows styling the parent(s) of the element that has the focus. Opposed to :focus (which references the element having the focus itself), it can be the direct parent, or the parent’s parent or the parent of the parent’s parent or… (you get it).