"when the modifier is activated differently" confused me while reading. IMHO it needs to be more explicitely stated that this is about the modifier being pressed on its own, as key, not as modifier to another key.
Perhaps "Different browsers + platforms report when a modifier key is activated itself differently." or better.

Should be also noted (at least as hint) with the API dox of KeyboardHandler.bind, because that is where people will look if adding another binding, but might not see this.