What you say is true for the default, but form controls have additional UA styles applied to them that can override the default. Just like the default border s

What you say is true for the default, but form controls have additional UA styles applied to them that can override the default. Just like the default border style is "none", but that's not what form controls do. But even past that point, this is the _ua_ default. A user can always have a user stylesheet that sets whatever colors the user wants (e.g. for accessibility reasons). And not setting background without setting color and vice versa is standard best-practice web development advice and has been for 2 decades or so now.