h5bp/html5-boilerplate — pull request #1436 by jeffpowrs (2013-09-23 19:23:43+00:00)
Title: Helper Class for Fluid Vertical Alignment.

Adding helper classes for vertically aligning a child inside it's container without needing to define fixed heights. I find this useful for rapid prototyping fluid sites. This is supported in IE8+ and all other browsers that support pseudo-elements.

Reference:
http://css-tricks.com/centering-in-the-unknown/