1. A computer storage medium encoded with instructions that when executed by a client device cause the client device to request content items from a content server and process the content items upon receipt, the instructions comprising: content item environment definition instructions that upon execution cause the client device perform operations comprising: defining separate content item environments in a web page served by a publisher server, the separate content item environments belonging to a domain of the content server that is different from the domain to which the publisher server belongs, and one of the content item environments being defined as a primary content item environment and each of the other content item environments being defined as secondary content item environments, and each content item environment displays one or more content items received from the content server; generating a primary content item request to the content server for the primary content item environment, the primary content item request being a request for respective content items for display in the primary content item environment and respective content items for display in each of the secondary content item environments; receiving first content item environment instructions in response to the primary content item request and that upon execution cause the client device perform operations comprising: displaying, in the primary content item environment, the respective content items received from the content server for display in the primary content item environment; and providing from the primary content item environment to each secondary content item environment the respective content items for display in the secondary content item environments.