Perfect horizontal AND vertical centering in CSS, at any width or height!
Works with percentage based width/height, min-/max- width, images, position: fixed and even variable content heights.
Updated Aug 13: Added a comparison table, an explanation, more documentation, and a Modernizr test for variable height
Updated Aug 16: Minor corrections and clarifications
Updated Jun 6, 2014: Small corrections regarding Windows Phone and blurry rendering with the transforms method