The editor’s draft [1] contains this definition:

MediaStream implements EventTarget;

However recent specs (including, for example: HTML, DOM Core, WebSockets, Web Audio, Battery Status, File Writer, Indexed DB, HTML Media, SVG, XHR, etc. etc. etc.) don’t declare EventTarget as a mixin interface but extend it directly.

It will simplify implementations and give programmers a more consistent API if MediaStream also extends EventTarget. So I suggest removing that implements clause and changing the definition of MediaStream to read:

interface MediaStream : EventTarget {

[1] <http://dev.w3.org/2011/webrtc/editor/getusermedia.html#idl-def-MediaStream>