rem.js(2)

var html = document.getElementsByTagName("html")[0];

        var rootResize = function() {

        var winClient = document.documentElement.clientWidth;

        var fontSize = winClient < 640 ? winClient / 16 : 40;

        if (fontSize < 20) {

            fontSize = 20;

        }

        html.style.fontSize = fontSize + "px";

        }

        rootResize();

        window.onresize = function() {

        rootResize();

        }