移动端布局（rem单位）

<meta name="viewport" content="width=device-width,initial-scale=1.0,maximum-scale=1.0,user-scalable=0">

JS设置HTML的font-size

(function(doc, win) {
var docEl = doc.documentElement,
resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
recalc = function() {
var clientWidth = docEl.clientWidth > 640 ? 640 : docEl.clientWidth;
if (!clientWidth) return;
docEl.style.fontSize = 20 * (clientWidth / 320) + 'px';
};

if (!doc.addEventListener) return;
win.addEventListener(resizeEvt, recalc, false);
doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window);