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http://www.w3.org/) since 1996. XML is a ‘neutral’ format designed to
describe the structure of data, while HTML focuses on its presentation.
XML needs to be transformed into HTML (or XHTML) to display it in
a web browser. XML is particularly useful for publishing data that are
structured, for example a reading list, or a collection of catalogue
records.
To function effectively an XML document needs to conform to a set of
rules laid down in a Document Type Definition (DTD). A DTD defines
the relationships between legal tagged ‘elements’ or building blocks of an
XML document. XML documents need to be ‘validated’ using parsing
software, which checks them against the DTD to ensure that the element
tags are used correctly and they are ‘well formed’. A newer standard to
emerge as a W3C recommendation is the XML Schema. Schemas are like
DTDs but are written using XML syntax and allow greater flexibility.
Anybody can create their own mark-up vocabulary, so the structure is
intelligible to anyone else who consults the DTD or Schema.
XML and DTD are increasingly being used in the publishing and
library communities. For example, they are being used by the National
Library of Medicine which has created a free digital archive of
biomedical and life sciences journal literature known as PubMed Central
(http://www.pubmedcentral.nih.gov/). PubMed Central was launched in
2000 and provides free or open access to full-text journal articles,
sometimes on a delayed basis. Journal publishers are encouraged to
submit their material to PubMed Central as it provides a permanent and
freely accessible archive at almost no cost to the publisher. The National
Center for Biotechnology Information (NCBI), a centre of the National
Library of Medicine (NLM), created the Journal Publishing DTD with
the intention of providing a common format for the creation of journal
content in XML. Publishers are encouraged to submit their material in
either SGML format or XML format, or to use the DTD to define the
incoming data. The DTD was created from the Journal Archiving and
Interchange DTD Suite. The DTD Suite is a series of XML modules that
define elements and attributes that describe the textual and graphical
content of journal articles. In addition, the Journal Archiving and
Interchange DTD Suite describes some non-article material such as
letters, editorials and book reviews.
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Style sheets are an important development in web technology and they
can be used with both HTML and XML. Rather than using HTML to
determine the appearance of a web publication, the rules governing
presentation can be encoded into a style sheet. Applying a new style sheet
can change the appearance of a publication dramatically, even though
the information remains the same. XML style sheets use eXtensible Style
Language (XSL), which is used to create XSLTs that transform XML
documents into other formats like XHTML. Style sheets used with
HTML documents are typically in Cascading Style Sheet (CSS) format.
An excellent guide to XML for librarians has recently been published
by Gilmour (2003), which provides an introduction to the topic and
explains the relevance of XML to librarianship. In areas such as
cataloguing and classification, XML is becoming increasingly important,
because it provides a way to share and exchange information. Some
practical projects using XML in libraries are described by Banerjee
(2002). For example, in 1993, the library at the University of
California–Berkeley began developing a method for encoding archival
materials in XML. The project led to the development of the Encoded
Archival Description (EAD) standard, which is now maintained by the
Library of Congress. Banerjee also describes how Washington Research
Library Consortium is using XML to provide access to electronic
resources through a system known as ALADIN (Access to Library and
Database Information Network). Finally he describes how the Library of
Congress, in spring 2002, produced an official specification for
representing MARC data in XML format. He argues that XML will have
a greater impact on the work of libraries in the future and that the:
… simplicity and flexibility of XML make it possible to integrate
services and resources in ways that would have been impossible just
a few years ago. Vendors, libraries, and open source programmers are
all interested in finding ways to search many kinds of resources with
a single query, and XML represents a major step forward in making
this goal a reality. (Banerjee, 2002, p. 35)
Another term you may hear used is Standard Generalised Mark-up
Language (SGML). This is the oldest of the mark-up languages and
became an international standard in 1986 (ISO 8879, 1986). XML is
derived from SGML, and also provides a set of rules in a DTD to
describe the structure of an electronic document.
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Standards, specifications and access management
Metadata and standards
Following the proliferation of the World Wide Web in the mid-1990s
everyone suddenly started talking about metadata. Metadata has been
described as ‘data about data’, or the electronic equivalent of the library
catalogue record. However, Chowdhury (2003, p. 138) argued that this
definition does not say anything about the purpose of metadata and
suggested three definitions from other authors, including:
■
‘Data which describes attributes of a resource’ (Dempsey and Heery,
1997);
■
‘Meaningful data describing another discrete data object’ (Gill, 1998,
p. 9);
■
‘Data associated with objects which relieves their potential users of
having to have full advance knowledge of their existence or
characteristics’ (Dempsey and Heery, 1998, p. 149).
Metadata is used to facilitate the use of digital objects in a networked
environment.
Dublin Core
Dublin Core is a term that you are probably familiar with as it has been
around for a while now. The key thing to remember here is that it’s
Dublin, Ohio, not Dublin, Ireland! Dublin Core is a metadata standard
devised in 1995 by a group of librarians, digital library researchers and
technical experts and coordinated by OCLC. The organisation is known
as the Dublin Core Metadata Initiative (DCMI) and everything you
might want to know can be found from its website.
1
It has a large
number of working groups and members include the Library of
Congress, JISC, the national libraries of Germany, Canada and
Australia, to name but a few. Its mission is defined as follows.
… to make it easier to find resources using the Internet through the
following activities: 
(1) Developing metadata standards for discovery across domains, 
(2) Defining frameworks for the interoperation of metadata sets,
and, 
(3) Facilitating the development of community- or disciplinary-
specific metadata sets that are consistent with items 1 and 2.
2
What exactly is Dublin Core and what does it mean to everyday
librarians? Dublin Core is a set of metadata standards used to describe
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electronic resources. It has 15 elements covering a document’s title,
subject and coverage, intellectual property (e.g. creator and the
associated rights) and instantiation, such as the date, format or language.
You can check if a website has Dublin Core metadata by viewing the
HTML source. Surveys suggest that few websites currently use Dublin
Core. While librarians have been talking about the importance of
metadata standards, there is a very real issue of who creates metadata,
and whether web authors are the right people to be ‘cataloguing’ their
resources. Caplan (2003) has provided a detailed overview of the
metadata landscape, written specifically for librarians.
The Semantic Web
The Semantic Web is the current project of Tim Berners-Lee, which aims
to develop standards and tools that allow meaning to be added to the
content of web pages. It is a collaborative effort led by the W3C, with