This is a thread to review the CSS properties that, once discovered, change the very way pages get set up. One of mine is background-size. Once this became widespread I stopped using various image tricks to make sure the background would look fine in the corner.
Multi-backgrounds are certainly a close second, and transitions are a third.
I think the next big one for me will be CSS-grid. I've done experiments with it so far and it's promising, but not widespread enough to depend on yet.