rem自适应，宽度计算 1rem === 40px ;

rem.js

(function(doc, win) {
    var docEl = doc.documentElement,
        resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
       recalc = function() {
           var clientWidth = docEl.clientWidth;

           if (!clientWidth) {
               return
           };
           if (clientWidth >= 640) {
               docEl.style.fontSize = '80px';
           } else {
               docEl.style.fontSize = 80 * (clientWidth / 640) + 'px';
           }
       };

    if (!doc.addEventListener) {
        return;
    };
    recalc();
    win.addEventListener(resizeEvt, recalc, false);
    doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window);