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On this diagram ( https://dvcs.w3.org/hg/dom3events/raw-file/tip/html/DOM3-Events.html#event-interfaces ) CustomEvent inherits from Event interface. But IDL says that it doesn't (https://dvcs.w3.org/hg/dom3events/raw-file/tip/html/DOM3-Events.html#interface-CustomEvent). If CustomEvent inherits from Event then dictionary CustomEventInit should inherits from EventInit.
Looks like bug in D3E interface definition, in DOM4 is correctly http://dom.spec.whatwg.org/#interface-customevent
Fixed in changelist: https://dvcs.w3.org/hg/dom3events/rev/eab211d2898a Thanks for the report!