移动端rem源码

750为准  px/40 = rem

(function(window, document) {
    (function() {
        var viewportEl = document.querySelector('meta[name="viewport"]'),
            hotcssEl = document.querySelector('meta[name="hotcss"]'),
            dpr = window.devicePixelRatio || 1;
        if(hotcssEl) {
            var hotcssCon = hotcssEl.getAttribute("content");
            if(hotcssCon) {
                var initialDpr = hotcssCon.match(/initial\-dpr=([\d\.]+)/);
                if(initialDpr) {
                    dpr = parseFloat(initialDpr[1])
                }
            }
        }
        var scale = 1,
            content = "width=device-width, initial-scale=" + scale + ", minimum-scale=" + scale + ", maximum-scale=" + scale + ", user-scalable=no";
        if(viewportEl) {
            viewportEl.setAttribute("content", content)
        } else {
            viewportEl = document.createElement("meta");
            viewportEl.setAttribute("name", "viewport");
            viewportEl.setAttribute("content", content);
            document.head.appendChild(viewportEl)
        }
    })();
    var hotcss = {};
    hotcss.px2rem = function(px, designWidth) {
        if(!designWidth) {
            designWidth = parseInt(hotcss.designWidth, 10)
        }
        return parseInt(px, 10) * 320 / designWidth / 20
    };
    hotcss.mresize = function() {
        var innerWidth = window.innerWidth;
        if(!innerWidth) {
            return false
        }
        document.documentElement.style.fontSize = (innerWidth * 20 / 320) + "px"
    };
    hotcss.mresize();
    window.addEventListener("resize", hotcss.mresize, false);
    window.addEventListener("load", hotcss.mresize, false);
    setTimeout(function() {
        hotcss.mresize()
    }, 300);
    window.hotcss = hotcss;
})(window, document);