Discourse knowledge in device independent document formatting

Most document structures define layout structures which implicitly define semantic relationships between content elements. While document structures for text are well established (books, reports, papers etc.), models for time based documents such as multimedia and hypermedia are relatively new and lack established document structures. Traditional document description languages convey domain-dependent semantic relationships implicitly, using domain-independent mark-up for expressing layout. This works well for textual documents a,s for example, CSS and HTML demonstrate. True device independence, however, sometimes requires a change of document model to maintain the content semantics. To achieve this we need explicit information about the discourse role of the content element. We propose a model in which content is marked-up with the discourse role it plays in the document. This way the formatter has knowledge about the function of a content element so it can make appropriate lay out choices.