itystreams-vocabulary/">Activity
        Vocabulary</a> that conforms to a subset of [[!JSON-LD]] syntax
        constraints but does not require JSON-LD processing. While other
        serialization forms are possible, such alternatives are not discussed
        by this document.
      </p>

      <p>
        When serialized, absent properties are represented by either (a)
        setting the property value to null, or (b) by omitting the property
        declaration altogether at the option of the publish