http://heycam.github.io/webidl/#idl-overloading explicitly disallows (with an example and everything) things like this:

  interface A {
    void f();
  };

  partial interface A {
    void f(float x);
  };

but I think we have specs doing this (e.g. shadow DOM) and this seems an OK thing to me in general, as long as people are careful.