Font size in CSS - % or em?

Question

When setting the size of fonts in CSS, should I be using a percent value (%) or em? Can you explain the advantage?

Answer 1

Yahoo User Interface library (http://developer.yahoo.com/yui/) has a nice set of base css classes used to "reset" the browser specific settings so that the basis for displaying the site is same for all (supported) browsers.

With YUI one is supposed to use percentages.