Although the style sheet mechanism illustrated in the previous
examples provides full operational support, it must be considered
too low level to support more complex hypermedia document
formats. A more generic, declarative approach is needed, to
allow for interoperability between hypermedia applications. Such
a generic, declarative approach has been proposed by the HyTime
standard
The ability to identify (ranges of) elements within an
arbitrary document is a critical issue for all hypermedia
systems, since addressing is a prerequisite for hyperlinking and
alignment. In the Web, addressing is based on the location of
the data on a specific server. Research to make the addressing
mechanism independent of the location of the data is still in its
infancy. Given a certain document, one must be able to address
fragments within the document. In the Dexter Hypertext Reference
model
Having powerful addressing mechanisms is not sufficient to
fully support hyperlinking. To facilitate document exchange, a
hypermedia framework needs to support standardized syntactic
elements defining both contextual links (i.e. links
defined by the source anchor) as well as independent links
(i.e. the definition of the link is independent from the
definition of its anchors). Independent links may be stored in
another document or link base. HyTime provides architectural
forms for both type of links. These links can be bidirectional
(allowing traversal from source to destination and vice versa)
and multi-headed (allowing multiple source and destination
anchors). On the Web, support for bidirectional links will
require the development of some kind of (globally) distributed
link service, since links can no longer be embodied in the
(source) document. The Amsterdam Hypermedia Model
From a HyTime perspective, presenting a document requires the
ability to position multimedia components in a (n-dimensional)
coordinate space. Note that for text-based documents, the exact
positions of the document's elements can usually be calculated by
combining the logical structure (e.g. a title) and the declarative
rendering information in the style sheet (e.g. which font and
indentation to use for titles). This mechanism relies on the
fact that the basic logical structure of the common document
formats (e.g. letters, articles, books) is well understood by
both application developers and authors. For hypermedia
documents, there is still little understanding of such fundamental structural
elements. Another issue is the temporal
synchronization between the time dependent elements of a
hypermedia presentation. These dependencies are often hard to
express in a style sheet. In contrast to the
specification of fonts and indentation, knowledge of the temporal
dependencies is often required to understand the semantics of the
document. Such important dependencies should be
defined by the content and structure, and not by an accompanying
style sheet. HyTime provides mechanisms to wrap multimedia
objects in an event. Each event has an extent
(i.e. an n-dimensional bounding box) which can be positioned in a
finite coordinate space. Although these generic
mechanisms can be used to specify spatial alignment and temporal
scheduling in a uniform manner, the resulting document structures
tend to contain (too) long, complex and detailed extent
specifications. Examples of temporal constraint specification in
HyTime can be found in
Apart from textual anchors to retrieve documents by URL, the Web
provides a limited number of facilities for user interaction,
including clickable maps and forms. Both these facilities,
however, depend on server-side processing. Adding such
facilities as client-side extensions is easy to do, and allows
for using more appealing metaphors for interaction, for example
by employing 3D objects or virtual reality extensions. However,
fitting these extensions and the enriched repertoire of user
interactions they allow for, in a hypermedia model is very
difficult