This seems suspicious. Keyboard events of some kind must have been supported since IE 4 and maybe earlier. Can you do some digging?

Running a little test code (see below), it isn't until IE 9 that I see the event argument to onkeydown being an event, let alone a KeyboardEvent. In IE 8 and below, it is undefined.

document.onkeydown = function(event) {
  alert(event);
}

@vinyldarkscratch if you omit the event argument, is alert(event) still undefined? IE4 and later had window.event (per BCD) and maybe the argument just shadows that.

Alternatively, you could try the addEventListener. It looks like the event is fired, just need to see what the event object looks like...

It appears that in IE 10 and below, window.event is some sort of proprietary MSEventObj (but then a regular Event in IE 8), of which I can't get any of the KeyboardEvent properties. Additionally, it seems that the keydown event is only supported in IE 9 and above, despite the fact I'm also using attachEvent for IE 8 and lower, and that onkeydown is present in earlier IE versions.

With all of this in mind, I think that setting this event API to IE 9 is correct.

Thanks @vinyldarkscratch! If we have a lot of event data to work through, and if some big events rewrite was in IE9, maybe we should go with a <=IE9 range and not bother testing earlier. Or just <=IE11 would be fine honestly.

Just checking to see if there's anything I need to do to get this PR ready for merging?

I had suggested a <=9 range, but given that I just merged the MouseEvent PR this seems very plausible too. Nobody will care if it's right anyway, who's supporting IE8 now?