rem布局教程最详细（菜菜出品，必出菜品第二季）

//rem.js
    (function (doc, win) {
 
      var isAndroid = win.navigator.appVersion.match(/android/gi);
      var isIPhone = win.navigator.appVersion.match(/iphone/gi);
 
      var scale = 1.0;
      var ratio = 1;
      if(isIPhone) {
        if (window.devicePixelRatio == 2) {
          scale *= 0.5;
          ratio *= 2;
        }
        if (window.devicePixelRatio == 3) {
          scale *= (1/3);
          ratio *= 3;
        }
      }
      var text = '<meta name="viewport" content="initial-scale=' + scale + ', maximum-scale=' + scale + ',' + ' minimum-scale=' + scale + ', width=device-width,' + ' user-scalable=no" />';
      document.write(text);
 
      var docEl = doc.documentElement
      var resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize'
      var recalc = function () {
        var clientWidth = docEl.clientWidth
        if (!clientWidth) return
        docEl.style.fontSize = 100 * (clientWidth / 750)  + 'px'
 
        // 解决部分rem特别大的问题
        var docElFontSize = docEl.style.fontSize.replace(/px/gi, '')
        var computedFontSize = win.getComputedStyle(docEl)['font-size'].replace(/px/gi, '')
        docElFontSize != computedFontSize && (docEl.style.fontSize = docElFontSize * docElFontSize / computedFontSize + 'px')
      }
      if (!doc.addEventListener) return
      recalc()
      win.addEventListener(resizeEvt, recalc, false)
    })(document, window);