We plan to change initEvent() to also accept a single argument. This would be a dictionary. However, each event interface comes with its own properties so each would require its own dictionary. To avoid having to overload initEvent() for each new event interface and to avoid respecifying how type/cancelable/bubbles and other properties work each time it would be nice if inheritance was supported.

That way you could have a base InitEvent dictionary and CustomEvent could have a InitCustomEvent inherited dictionary specifying just detail. Etc.