compositionstart event is defined in DOM Level3 Events. It is fired when indirect text inputting starts.
Yes, but Gecko doesn't allow to cancel
compositionstart, see Notes.
Launch text composition system, but Gecko fires
compositionstart when the system starts composition actually.
Spec of DOM Level3 Events said
compositionstart is cancelable, however, Gecko doesn't allow to cancel
Gecko fires this event when IME starts composition and some platforms don't have API for cancelling the composition forcibly. Gecko cannot know whether a keyboard event will start composition or not until IME actually starts composition.
Therefore, Gecko cannot support