CSS3 Tip: Root Em

A new relative measurement in CSS3 is called a rem (for root em) developed by that bases font size on the size of the root (html) element. If you specify the size of the html element (presumably to 100%), all elements that are specified in rem measurements will be relative to that size, not their inherited size.

This gets rid of the compounding issue that makes ems potentially aggravating. The drawback is that IE8 and earlier and other older browsers do not support rems, so you need to provide a fallback font size in pixels. Browsers that support rems will use the last declaration in the stack.

Reference: http://www.sitepoint.com/css3-rem-units/