The documents that HTX is concerned with are written using the Hypertext Markup Language (HTML) which can be read using World Wide Web (WWW) browsers such as Mosaic and Netscape.
Of course, any page of hypertext written using HTML could be considered a document in its own right. However, most documents of substance will consist of multiple pages of HTML, joined together using hypertext links. It is convenient to gather these related pages together and to consider them as a single multi-page hypertext document. This is the sense in which the term document is used here.