At the moment we only care about the MIME type, but we're expected to assess the requested codecs as well. MediaPlayerPrivateGStreamerMSE already does that to a degree, but the underlying requestMediaKeySystemAccess() algorithm still fails. I'll provide more information on this once I have it. https://trac.webkit.org/browser/trunk/Source/WebCore/platform/graphics/gstreamer/mse/MediaPlayerPrivateGStreamerMSE.cpp#L753
Possibly fixed by bug #191191
(In reply to Philippe Normand from comment #1) > Possibly fixed by bug #191191
<rdar://problem/104092705>