VAST-2.0-ad-sequence
Mixed use of sequence attribute across <Ad> elements in a pod
Short answer
When serving a pod (playlist) of ads, all <Ad> elements should either all have a sequence attribute or none should. Mixing sequenced and unsequenced ads leads to undefined playback order.
Why this matters in production
This rule is a strong risk signal. Tags with this issue often still parse, but they become brittle across SDKs, SSAI resolvers, and CTV environments. Structure rules matter because wrapper resolution, creative selection, and event handling all depend on the XML tree appearing in the exact place the spec expects. Affected scope: VAST 2.0 and later (all versions).
How to fix
Decide whether this is a pod: if so, add a sequence attribute to every <Ad>; otherwise remove sequence from all of them.
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