The first rule of animating on the web: don’t animate width and height. It forces the browser to recalculate a bunch of stuff and it’s slow (or “expensive” as they say). If you can get away with it, animating any transform property is faster (and “cheaper”).
Butttt, transform can be tricky. Check out how complex this menu open/close animation becomes in order to make it really performant. Rik Schennink blogs about another tricky situation: border-radius. When you animate the scale of an element in one direction, you get a squishy effect where the corners don’t maintain their nice radius. The solution? 9-slice scaling:
This method allows you to scale the element and stretch image 2, 4, 6, and 8, while linking 1, 3, 7, and 9 to their respective corners using absolute positioning. This results in corners that aren’t stretched when scaled.
It’s like the 2020 version of sliding doors.