Script Tag

<script>
    if(navigator.userAgent.match(/Android/i)
      || navigator.userAgent.match(/webOS/i)
      || navigator.userAgent.match(/iPhone/i)
      || navigator.userAgent.match(/iPod/i)
      || navigator.userAgent.match(/BlackBerry/i)
      || navigator.userAgent.match(/Windows Phone/i)) {
        document.write('<meta name="viewport" content="width=400');
    } 
</script>