多余的hack

``` CSS
audio, canvas, progress, video {
  display: inline-block;
  *display: inline;
  *zoom: 1;
}
```

这样的CSS代码，会输出为

``` CSS
audio, canvas, progress, video {
  display: inline-block;
  *display: inline;
  *zoom: 1;
  *display: inline;
  *zoom: 1;
}
```

建议在插入hack语句前先检查一下是否已有hack
