It would great to target all form fields (like you can with heading tags, body, etc)

One reason is due to how some mobile web browsers handle form fields.

By default, for instance, on mobile safari it places a small border-radius (4px-ish?) to the form field, even if it's not set or set at initial.

So, right now for each type of form and form field, I set a class regardless of any styling, so that I can set it to border-radius: 0px; to ensure that on mobile browsers there is no rounding.