I started messing around with the new WordPress.org twenty twelve theme. In the notes at the top of style.css, I found this:— This stylesheet uses rem values with a pixel fallback. — (full rem note from twenty twelve style.css – http://pfhyper.com/css/rem.html) I had never heard of rem values before. I brought it up at the Coco WordPress Wednesday table and no one there was familiar with them either. It's sort of a super-charge em value in that you can define the base in the html or body element and that will always be base no matter how deep you go with parent/child relationships with your CSS. Browser support isn't bad although it's recommended to use a pixel fallback for the older browsers. Some googling led me to the W3C site where I found that there are all sorts of relative length units I'd never heard of: ch, vw, vmin, vmax, and vh.
http://www.w3.org/TR/css3-values/#relative-lengths Here are a couple of links for further rem reading.