If you track through the grammar, you get (removing paths irrelevant to this problem):
InterfaceMember → AttributeOrOperationOrIterator
AttributeOrOperationOrIterator → Qualifier AttributeOrOperationRest
Qualifier → "static" | "stringifier"
AttributeOrOperationRest → OperationRest
OperationRest → OptionalIdentifier "(" ArgumentList ")" ";"
So putting all of that together, it's impossible to specify a return type for static or stringifier operations. It should probably be ReturnType OperationRest in there.
*** This bug has been marked as a duplicate of bug 18424 ***