### Precheck
 
* Do a quick search and make sure a bug has not yet been reported;
* do a quick check if the bug still exists in the latest patch version;
* finally, be nice and have fun!

### Environment

* clean-css version - `npm ls clean-css`:
* node.js version - `node -v`:
* operating system:

### Configuration options

```js
var CleanCSS = require('clean-css');
new CleanCSS({
  // compatibility: 'ie8',
  // format: 'keep-breaks',
  // level: 2
})
```

### Input CSS

```css
/* the shorter the better! */
```

### Actual output CSS

```css
```

### Expected output CSS

```css
```
