- Can calc() accept device-width/height?
  - No. Numbers only.
  - Can viewport/meta viewport accept them?
    - nsContentUtils::GetViewportInfo() might be the only place..
    - nsContentUtils::ProcessViewportInfo()
    - Viewport seems unused on all but mobile
    - Maybe worth testing..

- currentColor: OK
- scrollMax: OK
- IdleAPI: FirefoxOS only, but still present w/ pref??
  - Throws "The operation is insecure" exception
  - Disabled for content. WebApp only.

- Web Activities
  - Seems unfinished and unexported to content

- RegisterContent/ProtocolHandler -> Third party supercookie??
  - asks for confirmation
