An Empty VAST Wrapper Is Schema-Valid in 4.4. It Was Not in 2.0.

A VAST wrapper with no AdSystem, no VASTAdTagURI and no Impression validates against the VAST 4.4 draft schema. The same document has been invalid in every version from 2.0 through 4.2. It is one line of XSD, and it is almost certainly a side effect of the CTV Ad Portfolio restructure rather than a decision anyone made on purpose. I have filed it with IAB Tech Lab. This post is the working, because the reproduction is short enough that anyone can check it in about a minute. The change In vast_4.4.xsd on master, both vastInLine_type and vastWrapper_type wrap their children in a single compositor: an xs:choice with minOccurs zero and maxOccurs unbounded. That looks harmless. It is the idiom people reach for when they want to say "these children may appear in any order". What it actually says is stronger than that. In XSD, the cardinality on the compositor governs the content model, and the minOccurs on the individual child elements only describes a single selection from the choice. Set the choice itself to zero-or-more and every constraint underneath it stops binding. So the children still declare minOccurs="1". They are still, in effect, optional. The compositor in question <!-- vast_4.4.xsd, vastWrapper_type and vastInLine_type --> <xs:choice minOccurs= "0" maxOccurs= "unbounded" > <xs:element name= "AdSystem" type= "vastAdSystem_type" /> <xs:element name= "VASTAdTagURI" type= "vastURIElement_type" /> <xs:element name= "Impression" type= "vastImpression_type" /> <xs:element name= "Creatives" type= "vastCreatives_type" /> <!-- ... --> </xs:choice> Three consequences, not one The empty wrapper is the headline, but the compositor gives up three separate guarantees at once. Each is reproducible with xmllint against the published schema. What now validates in 4.4 Everything is optional. An empty <Wrapper/> validates. So does an empty <InLine/> , with no AdSystem, no AdTitle, no Impression and no Creatives. Everything repeats. maxOccurs="unbounded" on the choice means any