Currently only getModifierState only supports Alt, Control, Shift, Meta query modifiers. Introduce support for the following query modifier fields: Accel, AltGraph, CapsLock, Fn, NumLock, OS, ScrollLock, Symbol. This leaves: FnLock, Hyper, Super, SymbolLock specified but not supported. These keys typically are not generated on modern OS and keyboards. Nor does FireFox support them.
Docs: https://w3c.github.io/DOM-Level-3-Events-key/#keys-modifier