That we're free to let `jQuery( window ).is("a")` return true at some future point (unlikely as that may be), just like `jQuery( window ).is("*")` returns true today. The explicit purpose of these tests is to verify lack of exception, and they shouldn't do more than that unless we commit to having non-elements fail *all* selectors (which, incidentally, is established on the side by this PR). If we are ready for that, I can expand them rather than restrict their scope.