Usage is 0.0406% I wonder if we can start thinking about deprecating this since it was never implemented in FireFox.

That is fairly high usage from something seemingly obscure and not interoperable. Understanding the typical usage here would help, in particular are those 0.0406% broken in Firefox, or is there fallback code? In the worst case, there's some widely used code that uses UA sniffing to only try initKeyboardEvent in non-Gecko and something else in Gecko.

It looks like there are only 1898 hits in httparchive, which is a small number given the usage, so possibly usage is concentrated in some fairly large site behind a login that isn't captured in httparchive.

https://storage.googleapis.com/blink-httparchive-export/initKeyboardEvent.json is an export of those hits, and https://github.com/foolip/unhar/blob/master/unjson can be used to split it into individual files for detailed analysis. (I haven't tried it yet, so it may break with this input.)

AFAIK works in Firefox nightly:
> event = new KeyboardEvent("onkeydown", null);
onkeydown { target: null, key: "", charCode: 0, keyCode: 0 }
> event.initKeyboardEvent("onkeyup");
undefined
> event
onkeyup { target: null, key: "", charCode: 0, keyCode: 0 }

It should be "keydown" and "keyup", not "onkeydown" and "onkeyup" (this is not Internet Explorer 8 and earlier ;)).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5e13fac3f90eaed55ceac453466ea3610d9f8b5c

commit 5e13fac3f90eaed55ceac453466ea3610d9f8b5c
Author: Rob Buis <rob.buis@samsung.com>
Date: Tue Oct 03 16:24:19 2017

Sync initKeyboardEvent with the spec

This change syncs initKeyboardEvent with the spec:
https://w3c.github.io/uievents/#idl-interface-KeyboardEvent-initializers

Behavior matches Firefox.

Bug:  692703 
Change-Id: I8f2129b92df678c963b8759587b29ca97a70a06e
Reviewed-on: https://chromium-review.googlesource.com/673370
Commit-Queue: Rob Buis <rob.buis@samsung.com>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506063}
[add] https://crrev.com/5e13fac3f90eaed55ceac453466ea3610d9f8b5c/third_party/WebKit/LayoutTests/external/wpt/uievents/legacy/Event-subclasses-init-expected.txt
[add] https://crrev.com/5e13fac3f90eaed55ceac453466ea3610d9f8b5c/third_party/WebKit/LayoutTests/external/wpt/uievents/legacy/Event-subclasses-init.html
[modify] https://crrev.com/5e13fac3f90eaed55ceac453466ea3610d9f8b5c/third_party/WebKit/Source/core/events/KeyboardEvent.idl