I feel like superlupo and I looked at the current specification for “suggestions [to UA] for rendering HTML documents” and it looks like input elements should have ‘initial’ as the default value for letter spacing.

https://html.spec.whatwg.org/multipage/rendering.html#form-controls

I suppose that user settings would apply in Chrome/Firefox/? if they were not applying ‘normal’ for letter spacing of input elements, as letter-spacing is an inherited property by default.

Nice work by the way. Thank you for sharing.