Vertically align anything, even if you don't know the parent's height.
This solution is my variation of Sebastian Ekström's vertical alignment solution (http://zerosixthree.se/vertical-align-anything-with-just-3-lines-of-css/).
The CSS property display:flex; on the grandparent forces the parent to have a height, thereby alleviating the need to explicitly declare a height on the parent itself. This also solves the need to absolutely position anything, causing elements to be removed from the normal flow.