if (navigator.platform.indexOf('Mac') > -1) {
    //mac
    if (navigator.appName.indexOf('Microsoft') > -1) {
      //explorer

      document.write('<LINK HREF="css/mac_ie.css" REL="styleSheet" TYPE="text/css">');
  
    } else {
      //netscape

      document.write('<LINK HREF="css/mac_ns.css" REL="styleSheet" TYPE="text/css">');
  
    }
  } else {
    //windows or unix
    if (navigator.appName.indexOf('Microsoft') > -1) {
      //explorer

      document.write('<LINK HREF="css/win_ie.css" REL="styleSheet" TYPE="text/css">');
  
    } else {
      //netscape

      document.write('<LINK HREF="css/win_ns.css" REL="styleSheet" TYPE="text/css">');
  
    }
  }