Natural language offers a range of clues for identifying discourse segments. Hypermedia documents contain explicit clues to this process. HTML acts as a meta-language that describes the relationships between content bearing objects. One simple example is the relationship between heading, italicised and bolded objects and discourse salience. Another is the association of text that follows a conclusion or abstract heading with that discourse intention.
Sentences exist in the wider context of a discourse.
By discourse, I mean a deliberate concatenation of semantic units. The arrangement of semantic units within a discourse is meaningful. The structure determines the contribution that each literal semantic unit makes towards the final discourse meaning. The organisation of a hyper-document reflects a discourse structure. The interpretation of a hyper-document is often influenced by its web context.