Key Press
The bottom border is the key travel. Two pixels do all the work.
Usage
⌘K
Meant for
Examples
On a Kbd
The key goes down and its bottom edge shortens with it, so the cap looks like it has depth rather than just sliding. Two properties, one keyframe — anything more and it stops reading as a key.
⌘K
The class is wm wm-key-press — the first half is the marker the trigger and reduced-motion rules hook into, the second is the effect.