This API returns a map which translates from KeyboardEvent.code values into strings that can be shown to the user to identify each key. This is not possible with existing web platform APIs because the value that should be shown to the user depends on the keyboard layouts that the user has installed and activated.
Docs: Original WICG incubation proposal: https://discourse.wicg.io/t/keyboardevent-getcharacterbykey-key-modifierlist/1857/3 Explainer: https://github.com/WICG/keyboard-map/blob/master/explainer.md