setRem.js

//移动设备设置rem, 参考m.taobao.com
;function (window) {
    var v = window.devicePixelRatio;
    var dpr = v ? (v >= 3 ? 3 : (v >= 2 ? 2 : 1)) : 1;
    var scale = 1 / dpr;
    var doc = window.document;
    var html = doc.documentElement;
    var viewport = doc.createElement('meta');

    viewport.setAttribute('name', 'viewport');
    viewport.setAttribute('content', 'initial-scale=' + scale + ', maximum-scale=' + scale + ', minimum-scale=' + scale + ', user-scalable=no');

    if (html.firstElementChild) {
        html.firstElementChild.appendChild(viewport);
    }
    else {
        var m = doc.createElement('div');
        m.appendChild(viewport);
        doc.write(m.innerHTML);
    }

    var t;

    window.addEventListener('resize', function () {
        clearTimeout(t);
        t = setTimeout(setRem, 300);
    }, false);

    window.addEventListener('orientationchange', function () {
        setRem();
    });

    doc.addEventListener('DOMContentLoaded', function () {
        setRem();
    });

    function setRem() {
        // .getBoundingClientRect().width 比 .clientWidth更精准
        var width = html.getBoundingClientRect().width;

        // 等价于 20 * (width / 320)
        // 开发基准rootFontSize * (deviceWidth / 开发基准pageSize)
        html.style.fontSize = (width / 16) + 'px';
    }

}(window);