1. A system that serves content in a container document comprising: a container server that identifies one or more modules for use with a container document, receives a module specification from a remote server, and serves the container document with module data based on the module specification; wherein the module specification comprises a content element and one or more optional preference elements that enable the container server to provide preferences to the module; and wherein the content element for one of the modules has a value that identifies a content location reference where code may be found to generate data for the module.