var browserName=navigator.appName; 
if (browserName=="Netscape")
 { 
 document.write('<link rel="stylesheet" type="text/css" href="ff.css">')
 }
else if (browserName=="Opera")
 {
 document.write('<link rel="stylesheet" type="text/css" href="ff.css">');
 }
else if (browserName=="Microsoft Internet Explorer")
 {
 document.write('<link rel="stylesheet" type="text/css" href="ie.css">');
 }
else
 {
 alert("What ARE you browsing with here? Browser FAIL!!!");
 }