[mediaqueries-5] Detect physical keyboard (keyboard capable of shortcuts?)

From stackoverflow: https://stackoverflow.com/questions/36976117/css-media-query-for-whether-a-physical-keyboard-is-attached

Use case: I want show keyboard shortcut labels in my menu / tooltips but only when the user has a physical keyboard.

Rationale (taken directly from the above SO issue):

• nobody would use a soft-keyboard this way even if they could
• the visual display of the keyboard shortcut takes up valuable real-estate
• it's perhaps confusing, or at least inelegant, to display keyboard shortcuts in a keyboardless context

Apparently some people are doing horrible hacks like using @media (hover: hover) and (pointer: fine) { ... } to detect if there is a mouse/trackpad and assuming that means there is a keyboard too.

Start by reading the CSSWG issue discussion and the linked Stack Overflow question, including the proposed hover and pointer workaround. Determine whether a physical-keyboard media query is sufficiently specified and what observable behavior would define completion; the issue does not name implementation files or tests.