It's not supported by IE9 which is the main target for this polyfill. I've instead gone with the following which is a little leaner:

```css
border: 1px solid #666;
border-top: 0;
```

On a side note, I noticed that the build is converting <code>#666</code> to <code>#666666</code> undoing all our work to shorten it to three characters. Any particular reason the build is doing this? If anything it should be the reverse, shortening the colours values where possible.