if (navigator.appVersion.indexOf("Win") != -1) { // WINDOWS
   if (document.layers) {
      document.write("<LINK REL=stylesheet TYPE='text/css' HREF='/tes/css/n4.css'>"); // NN4+
   } else if (document.all) {
      document.write("<LINK REL=stylesheet TYPE='text/css' HREF='/tes/css/e4.css'>"); // MSIE4+
   } else if (document.getElementsByTagName) {
      document.write("<LINK REL=stylesheet TYPE='text/css' HREF='/tes/css/n6.css'>"); // NN6
   }
} else if (navigator.appVersion.indexOf("Mac") != -1) { // MAC
   document.write("<LINK REL=stylesheet TYPE='text/css' HREF='/tes/css/m.css'>");
} else if (navigator.appVersion.indexOf("X11") != -1) { // UNIX
   document.write("<LINK REL=stylesheet TYPE='text/css' HREF='/tes/css/u.css'>");
} else {
   // not a v 4 or +
}