An event timeline track MUST carry:
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a Section 5.1.12 attribute with a value of "eventtimeline".
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a Section 5.1.21 attribute which contains an array of all track names to which the event timeline track applies.
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a Section 5.1.25 attribute with a value of "application/json".
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an Section 5.1.13 attribute declaring the type & structure of data contained in the event timeline track.