I've worked with people who did things like this. It's not fun for anyone else on the team to try and figure out why you're using specific pseudo selectors on tags in a child inside a sibling in some label to make an accordion that would take 3 lines in Javascript.
Shouldn't pure CSS animations have better performance?
But that's changing once web animations (https://w3c.github.io/web-animations/) hits mainstream anyway - both CSS & JS get access to the same engine.