code: function ieRgbaHack(decl, i) {
    //十六进制不足两位自动补 0
    function pad(str) {
      return str.length == 1 ? '0' + str : '' + str
    }
    if (
      (decl.prop == 'background' || decl.prop == 'background-color') &&
      decl.value.match(reRGBA)
    ) {
      // rgba 转换为 AARRGGBB
      var colorR = pad(parseInt(RegExp.$1).toString(16))
      var colorG = pad(parseInt(RegExp.$2).toString(16))
      var colorB = pad(parseInt(RegExp.$3).toString(16))
      var colorA = pad(parseInt(RegExp.$4 * 255).toString(16))
      var ARGB = "'" + '#' + colorA + colorR + colorG + colorB + "'"
  
      // 插入IE半透明滤镜
      var reBefore = decl.raws.before.replace(reBLANK_LINE, '$1')
      insertDecl(decl, i, {
        raws: {
          before: reBefore
        },
        prop: 'filter',
        value:
          'progid:DXImageTransform.Microsoft.gradient(startColorstr=' +
          ARGB +
          ', endColorstr=' +
          ARGB +
          ')'
      })
  
      // IE9 rgba 和滤镜都支持，插入 :root hack 去掉滤镜
      var newSelector = ':root ' + decl.parent.selector
  
      var nextrule = postcss.rule({
        selector: newSelector
      })
      decl.parent.parent.insertAfter(decl.parent, nextrule)
      nextrule.append({
        prop: 'filter',
        value: 'none\\9'
      })
    }
  }
docs: IE rgba hack
  background rgba 转换为 IE ARGB