I believe that you might find this link very useful:
http://www.devarticles.com/c/a/PHP/PHP-Bro...CSS-Generation/In addition to this what you can do is obtain browser type of the user and contain this in a variable. This uses the following function browser_get_agent(). Then when you know which browser the user has you can check it against the CSS files that you have, each one being for a different browser. An example would be (be aware that all HTML tags must be the same in the CSS files and that the file is to be saved as a .php file) :
<?php
if (browser_get_platform()=='Mac') {
INCLUDE YOUR MAC STYLESHEET HERE
}
if (browser_get_agent()=='IE') {
INCLUDE YOUR IE STYLESHEET HERE
}
if (browser_get_agent()=='MOZILLA') {
INCLUDE YOUR FF STYLESHEET HERE
}
else{
INCLUDE YOUR DEFAULT STRLESHEET HERE, THIS ONLY APPLIES WHEN THE USER HAS A BROWSER WHICH DOES NOT MATCH ANY OF THE ABOVE
}
?>