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 for a web page, the web page including publisher content provided by the publisher, the publisher content for display in the web page, and that upon execution cause the client device perform operations comprising: instantiating separate content item environments in the web page served by a publisher server, each separate content item environment being an environment in which to display content items provided by a content server that is different from the 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 of the 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 first content items for display in the primary content item environment and, for each secondary content item environment, respective second content items for display in each of the secondary content item environments; receiving, from the content server, the respective first and second content items, and first content item environment instructions in response to the primary content item request, wherein the first content item environment instructions upon execution cause the client device perform operations comprising: displaying, in the primary content item environment, the respective first 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 second content items for display in the secondary content item environments.