CTL launches wireless keyboard and mouse that are certified to work with Chromebooks
If you have a Chromebook or other Chrome OS device, and you want to use an external keyboard and/or mouse with it, pretty much any of them will work. Whether the keyboard and/or mouse connect via USB or Bluetooth, they should be compatible with Chrome OS -- even if support is not listed by the manufacturer.
With all of that said, why are some accessories officially certified as "Works with Chromebooks?" The simple answer is, marketing -- especially when it comes to mice. For keyboards, however, there can be value in having a layout designed for Chrome OS rather than Windows or Mac. Such keyboards can be hard to find.