Using CSS Transitions on Auto Dimensions

We’ve all been there. You’ve got an element you want to be able to collapse and expand smoothly using CSS transitions, but its expanded size needs to be content-dependent. You’ve set transition: height 0.2s ease-out. You’ve created a collapsed CSS class that applies height: 0. You try it out, and… the height doesn’t transition. It snaps between the two sizes as if transition had never been set.

This helpful post came too late for me, I was there yesterday and I wasted a good few minutes trying to get my head around this frustrating feature. Just think, I could have used that wasted time to do something proper useful, like making another cup of coffee.