Change the URL for CSS documentation

Apparently [cssdocs.org](http://cssdocs.org) only support CSS 2.1 (and below) properties. If you try to look up any CSS3 property, like `text-shadow`, you’ll be redirected to a Google search. [Mozilla’s CSS Reference](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference), on the other hand, supports all CSS properties including those new in CSS3.

I know we’re able to change the URL’s ourselves, but I think the default CSS documentation should include even the newest properties now that CSS3 is so common and supported by most browsers.

Feel free to not merge the pull request if you disagree! :)