An event timeline track MUST carry:
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a Section 5.2.4 attribute with a value of "eventtimeline".
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a Section 5.2.14 attribute which contains an array of all track names to which the event timeline track applies.
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a Section 5.2.19 attribute with a value of "application/json".
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an Section 5.2.5 attribute declaring the type & structure of data contained in the event timeline track.