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Step 3 of the MediaKeys.create() function's specification [https://dvcs.w3.org/hg/html-media/raw-file/default/encrypted-media/encrypted-media.html#dom-create] says: "If keySystem is not supported or not allowed on the origin of the media element's Document, return a promise rejected with a new DOMException whose name is "NotSupportedError"." This mentions a media element, but the MediaKeys is not yet associated with a media element at this stage; the MediaKeys object does not even exist yet! Perhaps that should be: "If keySystem is not supported or not allowed in the origin of calling context's Document..."
Thanks for the report and suggestion. https://dvcs.w3.org/hg/html-media/rev/d4cd783f02b3 changes it to: If keySystem is not supported or not allowed in the origin of the calling context's Document, return a promise rejected with a new DOMException whose name is "NotSupportedError".