rem移动端适配

引入如下js内容（按照750px设计稿宽度配置）：

<script type="text/javascript">
    (function (doc, win) {
      let docEl = doc.documentElement
      let resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize'
      let recalc = function () {
        let clientWidth = docEl.clientWidth
        if (!clientWidth) return
        if (clientWidth >= 750) {
          docEl.style.fontSize = '100px'
        } else {
          docEl.style.fontSize = 100 * (clientWidth / 750) + 'px'
        }
      }
      if (!doc.addEventListener) return
      win.addEventListener(resizeEvt, recalc, false)
      recalc()
      doc.addEventListener('DOMContentLoaded', recalc, false)
    })(document, window)
  </script>