http://w3c.github.io/webcomponents/spec/custom/#extensions-to-document-interface-to-instantiate says:

  partial interface Document {
    Element createElement(DOMString localName, optional DOMString typeExtension);
    Element createElementNS(DOMString? namespace, DOMString qualifiedName, optional DOMString typeExtension);
  };

This will fail in a conformant WebIDL implementation, because for example it creates two overloads of createElement:

  [NewObject] Element createElement(DOMString localName);
  Element createElement(DOMString localName, optional DOMString typeExtension);

but WebIDL says:

  If there is more than one entry in an effective overload set that has a given
  type list length, then for those entries there MUST be an index i such that for
  each pair of entries the types at index i are distinguishable.

and in this case the overload set for type list length == 1 has two entries which are identical.

In practice, Gecko instead has this IDL:

 partial interface Document {
   [NewObject]
   Element createElement(DOMString localName, DOMString typeExtension);
   [NewObject]
   Element createElementNS(DOMString? namespace, DOMString qualifiedName, DOMString typeExtension);
 };

(note lack of "optional", which is key); I'm sorry that we didn't provide feedback on this when we implemented something different from the spec.

That said Blink's IDL here is:

    [CustomElementCallbacks, PerWorldBindings, ActivityLogging=ForIsolatedWorlds, RaisesException] Element createElement(DOMString localName, [TreatNullAs=NullString] DOMString typeExtension);

which also has a non-optional typeExtension....