(dict-arg-default) Validation error at line 13 in argument-dict-default.webidl, inside `interface X -> constructor -> argument union`:
  constructor(optional Union union);
                             ^ Optional dictionary arguments must have a default value of `{}`.
(dict-arg-default) Validation error at line 14 in argument-dict-default.webidl, inside `interface X -> operation x -> argument dict`:
  undefined x(optional Dict dict);
                            ^ Optional dictionary arguments must have a default value of `{}`.
(dict-arg-default) Validation error at line 16 in argument-dict-default.webidl, inside `interface X -> operation y -> argument union`:
 (boolean or Dict) union);
                   ^ Optional dictionary arguments must have a default value of `{}`.
(dict-arg-default) Validation error at line 18 in argument-dict-default.webidl, inside `interface X -> operation z -> argument union`:
  undefined z(optional Union union);
                             ^ Optional dictionary arguments must have a default value of `{}`.
(dict-arg-default) Validation error at line 22 in argument-dict-default.webidl, inside `interface X -> iterable -> argument union`:
DOMString>(optional Union union);
                          ^ Optional dictionary arguments must have a default value of `{}`.