(this is probably not new, but just in case…) Q: How to transition to height:auto? A: Transition max-height instead! jsfiddle.net/leaverou/zwvNY/

— Lea Verou (@LeaVerou) May 15, 2011

via Brad Frost:

This technique is insanely badass. I use it for all my height-animating needs, including accordions.