:focus-within is a favourite of mine since it is one of the few CSS selectors where child element state is significant. Thus it is very nice for drop-downs. It also works with CSS transitions, so my pure CSS drop-downs have a 150ms easing in & out (tip: transition the visibility property, since display:none can't be delayed).

There is another, however. An element whose state depends on that of other elements, and it's even more general. A form element moves between :valid and :invalid based on its inputs, allowing us to use form:{in}valid with any of the descendant, child, sibling or adjacent combinators. A hidden required checkbox is sufficient. Radio inputs work too (tip: you can clear radios back to :invalid with an input type="reset").

The really, truly monstrous part of this, however, is that the associated input doesn't even have to be a child of the form. Using the form attribute (i.e., <input form="whatever">) means they can be anywhere on the page, and they can themselves be hidden and targeted from somewhere else on the page again, with a <label> element.

I once documented the horrifying potential of this in a company wiki, along with a lovely modal slideover that was wrapped in a <form> element and transitioned to visible based on form:valid via a hidden required radio button, and whose backdrop was a reset button, and this was rightly labelled NSFW and banned by popular acclaim from ever appearing in our HTML.