var OPER=navigator.userAgent.indexOf("Opera");

if (document.all) { // Explorer

document.write('<link rel="stylesheet" href="css/main_ie.css" type="text/css">');

}

else if ( (document.layers) || (OPER>-1) || (document.getElementById) ) { 

document.write('<link rel="stylesheet" href="css/main.css" type="text/css">');

}