Comments above at-rules

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**NOT READY TO MERGE**
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This addresses #34.

* keyframes
  - [x] test
  - [x] fix
* media
  - [x] test
  - [x] fix
* supports
  - [ ] test
  - [ ] fix
* import
  - [ ] test
  - [ ] fix
* charset
  - [ ] test
  - [ ] fix
* namespace
  - [ ] test
  - [ ] fix
* document
  - [ ] test
  - [ ] fix
* page
  - [ ] test
  - [ ] fix