rem.js

(function (win) {
    var ratio, scaleValue, renderTime,
            document = window.document,
            docElem = document.documentElement,
            vpm = document.querySelector('meta[name="viewport"]');
    if (vpm) {
        var tempArray = vpm.getAttribute("content").match(/initial\-scale=(["']?)([\d\.]+)\1?/);
        if (tempArray) {
            scaleValue = parseFloat(tempArray[2]);
            ratio = parseInt(1 / scaleValue);
        }
    } else {
        vpm = document.createElement("meta");
        vpm.setAttribute("name", "viewport");
        vpm.setAttribute("content", "width=device-width, initial-scale=1, user-scalable=no, minimal-ui");
        docElem.firstElementChild.appendChild(vpm);
    }
    win.addEventListener("resize", function () {
        clearTimeout(renderTime);
        renderTime = setTimeout(initPage, 300);
    }, false);
    win.addEventListener("pageshow", function (e) {
        e.persisted && (clearTimeout(renderTime), renderTime = setTimeout(initPage, 300));
    }, false);
    "complete" === document.readyState ? document.body.style.fontSize = 12 * ratio + "px" : document.addEventListener("DOMContentLoaded", function () {
        document.body.style.fontSize = 12 * ratio + "px";
    }, false);
    initPage();
    function initPage() {
        var htmlWidth = docElem.getBoundingClientRect().width;
        htmlWidth / ratio > 540 && (htmlWidth = 540 * ratio);
        win.rem = htmlWidth / 7.5;
        docElem.style.fontSize = win.rem + "px";
    }
})(window);