<script>

 ! function( userAgent ) {

     var screen_w = parseInt(window.screen.width),

         scale = screen_w / 640;

     if ( /Android (\d+\.\d+)/.test( userAgent ) ) {

         var version = parseFloat( RegExp.$1 );

         document.write( version > 2.3

             ? '<meta name="viewport" content="width=640, minimum-scale = ' + scale + ", maximum-scale = " + scale + ', target-densitydpi=device-dpi">'

             : '<meta name="viewport" content="width=640, target-densitydpi=device-dpi">' );

     } else {

         document.write( '<meta name="viewport" content="width=640, user-scalable=no, target-densitydpi=device-dpi">' );

     }

 }( navigator.userAgent );

 </script>