<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>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name=viewport content="width=device-width, initial-scale=1">
<title>123</title>
<style>
*{margin:0; padding:0;}
</style>
</head>
<body>
<script type="text/javascript">
var oHtml = document.getElementsByTagName('html')[0];
var screenWidth = document.documentElement.offsetWidth || document.body.offsetWidth;
var nowFont = (screenWidth / 640 )*625 + "%";
oHtml.style.fontSize = nowFont;
</script>
</body>
</html>