wundert mich das das funktionieren soll ... der IE ist auch doof -.-

Code: 
<head>
 <link href="style1.css" rel="stylesheet" type="text/css" id="css">
 <script type="text/javascript">
  function CSS_2 () {document.getElementById("css").href = "style2.css";}
  function CSS_3 () {document.getElementById("css").href = "style3.css";}
  if (screen.width >= 1200) {
   CSS_2();
  }
  else {
   if (screen.width >= 800) {
    CSS_3();
   }
  }
 </script>
</head>