I agree; a HTML document without CSS should be perfectly usable and OK (and ideally CSS should never be required; if CSS is disabled it ought to still work OK, too).

However, if CSS specifies some things and omits others that are related to it (one example is specifying the font size for one heading level but not another one; another example is specifying the background colour without specifying the foreground colour or vice-versa), then it makes sense to be a warning.