https://webaudio.github.io/web-audio-api/#dom-mediastreamaudiosourcenode-mediastreamaudiosourcenode step 1 says:

If context is not an AudioContext, throw an NotSupportedError exception and abort these steps.

but the arg is declared as AudioContext in the IDL, so how could it not be an AudioContext? Is it possible this dates back to before BaseAudioContext existed?