(function (doc, win) {
  var docEI = doc.documentElement,
    design_width = (/(iPhone|iPad|iPod|iOS|Android|webOS|BlackBerry)/i.test(navigator.userAgent)) ? 750 : 1920,
    resizeEvt = 'orientationchange' in window ? 'orientataionchange' : 'resize',
    recalc = function () {
      var clientWidth = docEI.clientWidth
      if (!clientWidth) return
      //1920下的根元素大小为假设100px，然后根据下面计算公式算出等比下当前页面的根元素大小。
      docEI.style.fontSize = 100 * (clientWidth / design_width) + 'px'
    }

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