The Hypermedia Design Model (HDM) is a hypertext model being developed as part of the HYTEA project by an European Consortium [Garzotto et al., 1991]. The basic features of HDM include the representation of hypertext applications through primitives: types entities composed of hierarchies of components; different perspectives for each component; units corresponding to component-perspective pairs; bodies representing the actual contents of units; structural links relating components belonging to the same entity; application links relating components belonging to different entities; and browsing semantics determining the visualization and dynamic properties of the application. These primitives are similar to objects defined in HAM.
The HDM is concerned with authoring-in-the-large or the definition of the topology of the hypertext network. It does not deal with authoring-in-the-small or filling in the contents of nodes and their presentation. This is because, Garzotto et al., believe that systematic and rational structural decisions about the hypertext should be made before the actual hypertext is written so that a coherent and expressive application can be developed from the very beginning instead of being added later. These HDM design specifications can be translated automatically into a lower-level node and link specification resulting in the actual implementation of the topology. HDM is still evolving and requires experimentation with a large number of applications. It is a step to induce a methodology for hypertext design based on a top-down, model-based approach.