1. A computer system for presenting content fragments within an electronic document, comprising: a request subsystem receiving requests from a software application configured to display the electronic document, each request requesting to present a content fragment within the electronic document, the requests received by the request subsystem each specifying a fragment identifier included in the electronic document, the fragment identifier identifying the content fragment; a fragment retrieval subsystem, for each request received by the request subsystem, retrieving from a fragment store implemented as a cloud-based service contents of the content fragment identified by the fragment identifier specified by the request; an accessing subsystem, for each request received by the request subsystem, accessing a presentation identifier associated with the content fragment identified by the fragment identifier specified by the request; the fragment retrieval subsystem, for each request received by the request subsystem, retrieving from the fragment store contents of a presentation fragment identified by the presentation identifier accessed by the accessing subsystem for the request, the contents of the presentation fragment including code for presenting content within the electronic document, the code being decoupled from the software application generating the requests; an invocation subsystem, for each request received by the request subsystem, invoking the code included in the contents of the presentation fragment retrieved by the fragment retrieval subsystem for the request to present the contents of the content fragment identified by the fragment identifier specified by the request; a detection subsystem, for each presentation fragment whose contents are retrieved by the fragment retrieval subsystem, detecting whether the contents of the presentation fragment include a reference specifying a fragment identifier identifying a further presentation fragment; the fragment retrieval subsystem, for each presentation fragment whose contents are detected by the detection subsystem to include a reference specifying a fragment identifier identifying a further presentation fragment, retrieving from the fragment store the further presentation fragment; and the invocation system, for each further presentation fragment, invoking code among the contents of the further presentation fragment as part of invoking the code among the contents of the presentation fragment whose contents contain the detected reference to the further presentation fragment.