This site doesn't seem to be suggesting absolute centering [1] when it would be the easiest way to center things. "margin: auto" seems to be the way that the designers of CSS 2.1 intended to center things. 90% of the time, absolute centering is the easiest way to center in CSS; you should prefer it when possible. [1]: http://codepen.io/shshaw/full/gEiDt
Would this technique work to center, say, an tag with undeclared dimensions inside of a div? Or do you have to jump through hoops?
In my experience, most of the time when I do want to center something vertically, I have a defined height (which can be a percentage!), so absolute centering is the solution of choice.