(Edited: current resolved-on proposal is at #626 (comment))

https://drafts.csswg.org/css-transitions/#animtype-rect

It would be incredibly useful to be able to transition to (or from) height: auto. As it stands now, you need to jump through a bunch of hoops in JavaScript to simulate it. This is used frequently for transitioning in alert boxes, opening accordions, etc, but it seems silly that JavaScript is required.

I’m sure there must be discussion about this in the old mailing list, but I’d love to be privy to it here.