2.1.40 [HTML5:2014] Section 22.214.171.124 Synchronizing multiple media elements
V0088: The MediaController interface is not supported
The specification states:
126.96.36.199.1 Introduction Each media element can have a MediaController. A MediaController is an object that coordinates the playback of multiple media elements, for instance so that a sign-language interpreter track can be overlaid on a video track, with the two being kept in sync.
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is not supported.