These rules were copied from normalize.css 3, and are mostly redundant.
* `optgroup` isn't used in rustdoc at all
* `textarea` is only used for the "copy" button, so it's not visible
* The remaining buttons and inputs mostly have styles set anyway.
* We should never be setting `color` without also setting the
background to something. Otherwise, you get white-on-gray
text. That seems to be [the reason] why `normalize.css` changed this.
[the reason]: https://github.com/necolas/normalize.css/pull/502