[kps]

> For example, you can't have an ! key.

I know that was probably just meant as an illustration, but you can have an ‘!’ key — it's 07:00CF “Keypad !”¹.

Linux will ignore it² though, because being Linux they had to NIH their own key codes, and Windows will ignore it³ because it wasn't on the IBM PC keyboard in 1981.

¹ https://www.usb.org/sites/default/files/documents/hut1_12v2....

² https://elixir.bootlin.com/linux/v5.8.9/source/drivers/hid/h...

³ http://download.microsoft.com/download/1/6/1/161ba512-40e2-4...