h5页面640版面rem换算

      var root = document.getElementsByTagName("html")[0],
        NATIVE_W = 640;
      function updateSize() {
        var w = window.innerWidth;
        var cw = w / (NATIVE_W / 100);
        root.style.fontSize = cw + "px";
      }
      window.onload = function () {
        updateSize();
        document.getElementsByTagName("body")[0].style.display = "block";
      };
      window.onresize = function () {
        updateSize();
      };