Multimedia Research Group,
University of Southampton, SO17 1BJ, U.K.
XML is aimed at allowing the author to express the structure and high-level meaning of a document or group of documents. By itself it does not attempt to answer the question "what should this document look like" or "how should this document behave". This is the domain of the style language, XSL , which must map the abstract document structure onto an actual presentation. XLL similarly is aimed at allowing the author to express the inter-relationships between the elements of a document or a group of documents. The process of rendering a link as an artifact in the user's presentation is a problem which has to be addressed by some part of the overall system, but apart from very general descriptions of when and where the link ends are to be activated it is not tackled by the XLL definition.
An open hypermedia environment provides hypermedia facilities not to a single hypertext viewer, but as a set of services that can be used by any third party viewer, so that all the desktop applications become hypertext-enabled. Consequently, the semantics of the data contained in the documents is ignored. The viewer presents the information, but the OHS is concerned only with links: the positioning of their anchors (attached to real positions in the data) is interpreted by the viewers. The semantics of the links, on the other hand, are not understood by the viewers but are held privately by the OHS. Data handling and link handling are two (largely) independent activities, and the author of an information application must work with both of these systems to produce an integrated result.
The World Wide Web consists of a collection of data servers and client browsers. The majority data format on the WWW is HTML, whose semantics are published by the W3C and incorporated into every WWW browser. The link semantics are defined as a part of the data semantics, and understood and implemented (fairly) uniformly by each browser. There is no controlling information application in the WWW: there are only viewers and data servers. There is no application to understand that a table-of-contents link could be used to generate a high-level summary of the data as it is delivered in a set of entities. Instead this feature could be emulated by a CGI script (i.e., a dedicated process located on a WWW server) or by a viewer's assistant (i.e., a dedicated process located on the local machine).
The brave new WWW is composed of XML documents. It is the SGML world's standardised document representations and standardised specifications of non-standardised document semantics together with the WWW world's distributed client-server framework. The complication is that the standardised document semantics do not include behaviour semantics for links. This part is still very much application dependent, but in the WWW there is no defining application!
One solution is to create applications: specific programs that manipulate specific information architectures (DTDs) in specific ways with specific behaviours. But it is more likely that general browsing applications are the order of the day, and so sophisticated ways of processing links must be made available to mirror the sophisticated methods of processing document contents using XSL. The application is likely to become scattered among various components: the general document browser, the DTD, the style sheets and the scripts. The information architecture that the author uses is likely to be partly defined by an external agency (standards group, user community, publishing company) and partly defined by personal modifications. The specifications of the semantics of the document data therefore are likely to be amalgamated from many sources spread across the WWW perhaps even more widely than the data inside the documents themselves.
 Bray, T. and De Rose, S., Extensible Markup Language (XML): Part 2. Linking, W3C Working Draft July-31-97, 1997, http://www.w3.org/TR/WD-xml-link
 Carr, L., De Roure, D., Hall W., and Hill G., The distributed link service: a tool for publishers, authors and readers, The Web Journal, 1(1): 647656, 1995, O'Reilly and Associates.