http://dev.sopili.net/2014/03/viewport-setup-same-viewport-width-with.html

gistfile1.html

<meta name="viewport" id="viewport" />
<script>
var viewport_width = 400;
(function(target_width){
var device_width = screen.width || 320,
    ratio = (device_width/target_width);
// desktop or some old mobile browser
if(device_width > 414 || device_width < 320)
    ratio = 1.0;
document.getElementById('viewport').content = "width=device-width,user-scalable=1,initial-scale=" + ratio;
})(viewport_width);
</script>