F99 mentions to beware "the presence of the .keycode attribute".

1. That looks like a DOM property accessor, and to the best of my knowledge, there is no (.)keycode attribute, assuming HTML is the host language. Note that DOM property accessors might yield different results than the getAttribute method for the reflected attribute.
2. DOM property names are case sensitive: keyCode.
3. keyCode has been deprecated since, eh, I'm not an archeologist. :-) A superficial search unveils a Stackoverflow question from 2016.