PHP:

<?php
$browse = getenv("HTTP_USER_AGENT");
if (preg_match("/MSIE/i", "$browse")){
print "\t\t<LINK REL='stylesheet' TYPE='text/css' HREF='css/ie5.css'>\n"; }
elseif (preg_match("/Mozilla/i", "$browse")){
print "\t\t<LINK REL='stylesheet' TYPE='text/css' HREF='css/netscape.css'>\n"; }
else {
print "\t\t<LINK REL='stylesheet' TYPE='text/css' HREF='css/ie5.css'>\n"; }
?>

Javascript:

<SCRIPT LANGUAGE="JavaScript">
if(document.layers){
document.write("<link rel='stylesheet' href='***' type='text/css'>");
}
if(document.all){
document.write("<link rel='stylesheet' href='***' type='text/css'>");
}
if(document.getElementById){
document.write("<link rel='stylesheet' href='***' type='text/css'>");
}
</SCRIPT>

ASP ?