1. A computer-implemented method, comprising: identifying, at a client computer, requests in a web page source document, the web page source document defining content item environments in a web page, and associating each content item environment with a corresponding request in the web page source document; for each request: determining, at the client computer, if any of the requests have generated a content item request; and in response to determining that a content item request has not been generated by any of the requests: generating, at the client device, a combined content item request for content items for all requests in the web page source document, the content items including a corresponding content item responsive to each request; receiving, at the client device, the content items in response to the combined content item request; and storing in a data store, at the client device, the content items received; wherein the content items are advertisements.