设计稿750px移动端字体自适应，1rem=100px

<script type="text/javascript">
    //JS监听浏览器文字大小代码
    "use strict";
    (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 = (clientWidth / 3.75) + 'px';
            };

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