if( (-1 != navigator.userAgent.indexOf("Mac")))	
{
// Mac
document.write('<link rel="stylesheet" type="text/css" href="css/mac.css">');
}

if( (-1 != navigator.userAgent.indexOf("Win"))&&(-1 !=navigator.userAgent.indexOf("MSIE")))
{
// Internet Explorer WIN
document.write('<link rel="stylesheet" type="text/css" href="css/ie.css">');
}