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

As Galwegian mentions, px is the most reliable for web typography, as everything else you do on the page is mostly laid out in reference to a computer monitor. The problem with absolute sizes is that some browsers (IE) won't scale pixel-value elements on a web-page, so when you try to zoom in/out, everything adjusts except for those elements.

I do not know whether IE8 handles this properly, but all other browser vendors handle pixels just fine and it is still a minority case where a user needs to enlarge/diminish text (this text box on SO perhaps being the exception). If you want to get really dirty, you could always add a javascript function for making your text size larger and offer a "small"/"larger" button to the user.

Answer 2

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.