the same reason with RTCDataChannelEvent, and also keep the MediaStream untouched.

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13.2 RTCMediaStreamEvent

The addstream and removestream events use the RTCMediaStreamEvent interface.

Firing a stream event named e with a MediaStream stream means that an event with the name e, which does not bubble (except where otherwise stated) and is not cancelable (except where otherwise stated), and which uses the RTCMediaStreamEvent interface with the stream attribute set to stream, must be created and dispatched at the given target.

[Constructor(DOMString type, RTCMediaStreamEventInit eventInitDict)]
interface RTCMediaStreamEvent : Event {
    readonly attribute MediaStream? stream;
};
dictionary RTCMediaStreamEventInit : EventInit {
    MediaStream stream;
};
13.2.1 Attributes

stream of type MediaStream, readonly, nullable
The stream attribute represents the MediaStream object associated with the event.

13.2.2 Dictionary RTCMediaStreamEventInit Members

stream of type MediaStream

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