A document object may be transmitted to a plurality of user devices. The document object may include at least one field for display of a content object of a group of content objects. The field may be associated with an identifier corresponding to the group of content objects. When the document object is accessed, then the access may trigger a request to a server, which may select a content object of the group of content objects using a content object identification function. The content object identification function may be dynamically updated based at least in part on the document object being accessed by one or more of the plurality of user devices. Responsive to the request, a unique content object identifier corresponding to the selected content object may be transmitted to the user device and displayed at the accessed document object.