1. A method comprising: receiving a content item request for a content item to display in a content item display environment in a resource, the resource hosted by a device belonging to a first domain of a first publisher and the content item display environment hosted by a device belonging to a second domain, different from the first domain, of a second publisher different from the first publisher, wherein the first domain includes a publisher side file, hosted by a device belonging to the first domain, that facilitates display of a content item from the second publisher in the content item display environment; in response to receiving the content item request, providing, by one or more data processors, the content item along with measuring instructions, the measuring instructions being operable to interact with the publisher side file to generate a child display environment in the content item display environment, the child display environment being a hierarchical child of the content item display environment in a document object model representation of the resource, wherein the measuring instructions interact with the resource through the child display environment to cause measurement data for the content item display environment to be generated, and wherein the measuring instructions are prevented from interacting with the resource through the content item display environment; and receiving the measurement data, the measurement data specifying a configuration of the content item display environment and the resource.