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https://dvcs.w3.org/hg/dap/raw-file/default/media-stream-capture/ImageCapture.html#widl-ImageCapture-onphoto "Register/unregister for photo events of type BlobEvent. The handler should expect to get a BlobEvent object as its first parameter." You don't register for certain event interface, like BlobEvent, you add handler for certain event type. And same event type can be used with many event interfaces. It just means that event.type is "photo". target.dispatchEvent(new MouseEvent("photo")); is perfectly valid. [1] https://bugzilla.mozilla.org/show_bug.cgi?id=916643#c44