From Bugzilla Helper:
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.10) Gecko/20050717

Description of problem:
A style of the form
div {
 font: 12px Arial, Helvetica, sans-serif;
 color:#666666;
 line-height:20px;
 margin-top:0px;
 }
produces rendering in fixed-tiny-size font.
The fault is in the 12px, but any ??px produces the same thing, regardless of
the face. Styles like this are widespread. The bug occurs with Mozilla builds that work fine on FC3 (fontconfig-2.2.3-5). That is why I conclude it is fontconfig, but it possibly might be some other aspect of the font/X system.

Version-Release number of selected component (if applicable):
fontconfig-2.2.3-13

How reproducible:
Always

Steps to Reproduce:
1. Use FC4 standard distribution with, e.g. firefox
2. Go to http://silas.psfc.mit.edu/fontsizebug.html
3. Enjoy! (NOT)
  

Additional info: