<script>
(function (doc, win) {
         var docEl = doc.documentElement,
             resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
             recalc = function () {
                 var clientWidth = docEl.clientWidth;
                 if (!clientWidth) return;
                 if(clientWidth>=640){
                     docEl.style.fontSize = '100px';
                }else{
                     docEl.style.fontSize = 100 * (clientWidth / 640) + 'px';
                }
            };
 
         if (!doc.addEventListener) return;
         win.addEventListener(resizeEvt, recalc, false);
        doc.addEventListener('DOMContentLoaded', recalc, false);
   })(document, window);
</script>