根据窗口宽度大小，确定html的fontsize大小，之后用rem作为尺寸单元

htmlFontSized.js

<script>
        (function(doc, win) {
            var docEl = doc.documentElement,
                resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
                recalc = function() {
                    var clientWidth = docEl.clientWidth;
                    if (!clientWidth)
                        return;
                    //由屏幕宽度得到basefont大小,并将其写入html标签中,文档总的尺寸由rem确定
                    docEl.style.fontSize = 100 * (clientWidth / 640) + 'px';
                    doc.body.style.display = "block";
                    doc.getElementsByTagName('html')[0].removeAttribute('class');
                };

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