Currently the TrackDefaultList constructor only throws an exception when multiple TrackDefault objects with the same type and an empty byteStreamTrackID are passed in. This leads to ambiguity in the track default kind/language/label algorithms when multiple defaults with the same type and byteStreamTrackID are present.

I propose that step 1 in the constructor be changed to the following:
"If trackDefaults contains two or more TrackDefault objects with the same type and the same byteStreamTrackID, then throw an INVALID_ACCESS_ERR and abort these steps."

This should cover the original empty string case and prevent the ambiguous situation as well.