Change the max-height, from "none" to "100px" and back to "none":

This DIV element does not have a pre-defined height.

An animation will change the max-height, from "none" to "100px".

The height of this DIV element will not exceed 100 pixels.

The max-height property is animatable in CSS.

Note: Currently the max-height property is not changing gradually in Chrome, and does not change at all in Firefox.

Note: CSS Animations do not work in Internet Explorer 9 and earlier versions.