1. A computer system for presenting content fragments, comprising: a request subsystem configured to receive requests each to present a content fragment, the requests received by the request subsystem each specifying a fragment identifier identifying the content fragment; a fragment retrieval subsystem configured to, for each request received by the request subsystem, retrieve from a fragment store contents of the content fragment identified by the fragment identifier specified by the request; an accessing subsystem configured to, for each request received by the request subsystem, access a fragment identifier associated with the content fragment identified by the fragment identifier specified by the request; the fragment retrieval subsystem being further configured to, for each request received by the request subsystem, retrieve from the fragment store contents of a presentation fragment identified by the fragment identifier accessed by the accessing subsystem for the request; and an invocation subsystem configured to, for each request received by the request subsystem, invoke code among the contents of the presentation fragment retrieved by the fragment retrieval subsystem for the request to present the contents of the content fragment retrieved by the identified by the fragment identifier specified by the request.