网站字体自适应，需要搭配rem单位使用

可以参考这位大佬的文章

```(function (doc, win) `{`
 var docEl = doc.documentElement,
  resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
  recalc = function () {
   var clientWidth = docEl.clientWidth;
   if (!clientWidth) return;
   if (clientWidth <= 768) {
    document.documentElement.style.fontSize = clientWidth / 7.5 + 'px';
    console.log(clientWidth, '768');
   } else if (clientWidth >= 992 && clientWidth <= 1199) {
    document.documentElement.style.fontSize = clientWidth / 9.7 + 'px';
    console.log(clientWidth, '992');
   } else if (clientWidth >= 1200 && clientWidth <= 1399) {
    document.documentElement.style.fontSize = clientWidth / 11.7 + 'px';
    console.log(clientWidth, '1200');
   } else if (clientWidth >= 1400 && clientWidth <= 1919) {
    document.documentElement.style.fontSize = clientWidth / 13.3 + 'px';
    console.log(clientWidth, '1400');
   } else if (clientWidth >= 1920 && clientWidth <= 2559) {
    document.documentElement.style.fontSize = clientWidth / 19.2 + 'px';
   } else {
    document.documentElement.style.fontSize = clientWidth / 19.2 + 'px';
   }
  };

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