Methods, and systems, including computer programs encoded on computer-readable storage mediums, including a method for determining a configuration of a content item display environment. The method includes receiving a content item request for a content item to display in a content item display environment in a resource, the resource hosted on a first domain and the content item display environment hosted on a second domain, wherein the first domain includes a publisher side file; providing the content item along with measuring instructions operable to interact with the publisher side file to generate a child display environment, wherein the measuring instructions interact with the resource through the child display environment to cause measurement data to be generated, wherein the measuring instructions are prevented from interacting with the resource through the content item display environment; and receiving the measurement data specifying a configuration of the content item display environment and the resource.