This might be better to move UI Events, though.

I'd like to suggest that KeyboardEvent should have .isComposing attribute which is true if the key event is dispatched between compositionstart and compositionend. This attribute is useful for handling key events only when there is no composition.

For example:
                                       isComposing
keydown 's'                            false
compositionstart ''
compositionupdate 's'
beforeinput 's'
DOM change occurs
input 's'
keyup 's'                              true
keydown 'Enter'                        true
beforeinput 's'
DOM change occurs
input 's'
compsitionend 's'
keyup 'Enter'                          false