var doc = window.document
var docEl = doc.documentElement
var tid

function refreshRem () {
var width = docEl.getBoundingClientRect().width
if (width > 540) { // 最大宽度
width = 540
}
var rem = width / 7.5 // 将屏幕宽度分成750份，1份为1rem 375屏刚好0.5px
docEl.style.fontSize = rem + 'px'
}

window.addEventListener('resize', function () {
clearTimeout(tid)
tid = setTimeout(refreshRem, 300)
}, false)
window.addEventListener('pageshow', function (e) {
if (e.persisted) {
clearTimeout(tid)
tid = setTimeout(refreshRem, 300)
}
}, false)

refreshRem()

(function(doc, win) {

    var docEl = doc.documentElement,

        resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',

        recalc = function() {

            var clientWidth = 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);