font-family: BitStream Vera Sans, Verdana, Arial, Helvetica, sans-serif;
The choice of 11 point BitStream Vera Sans causes some display errors for Mac users that can be seen on http://postfixadmin.sourceforge.net/ in the text "However, it may contain bugs." toward the bottom of the page. The 11pt font causes text overlapping when rendered bold (either <b> or <strong>) or italic, so that the "it may contain bugs" text is itself squashed like a bug ;)
At least, that's the rendering I get from the chosen fonts for Firefox on Mac OS X. Setting the font size to 10 or 12 resolves the issue for me. So this is probably as much a bug in the font "BitStream Vera Sans" on Mac OS X, since if I put arial, verdana or helvetica ahead I get fine rendering at 11pt.