Simple way to transition a div's height from 0 to auto in CSS

To anyone asking why this was submitted: This is actually a breakthrough for a CSS problem that has annoyed frontend developers for a long time, as you can see in this stackoverflow question:

https://stackoverflow.com/questions/3508605/how-can-i-transi...

It does not make use of any new CSS features, but uses long existing features in a very clever way.