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The XML Family
of Technologies
It is important to remember that, unlike HTML, XML is not a solution in itself. XML
defines a framework that can be used to create solutions, but in isolation it does very little
apart from produce highly readable and organised documents. XML is at its most power-
ful as a syntax for defining other related technologies, the most relevant of which are
covered below.
The XML Family of Technologies
There are four main categories of XML technologies.The first of these consists of dif-
ferent versions of the XML language (or metalanguage) itself.The XML specifications
describe the concrete syntax of XML documents, and partially the behaviour of an XML
processor, i.e. a software module that is used to read XML documents and to render their
content and structure. Four slightly different abstract models for describing the informa-
tion available in XML documents have been introduced at W3C:
- The XML Information Set specification provides definitions for other specifications
that refer to the representation of information in a well-formed XML document.
- The XPath Data Model is the specification for addressing parts of an XML document.
XPath allows users to select elements from a hierarchical XML structure, a process
similar to the way SQL retrieves information from a database.
In addition to support for XML Web documents, Mozilla uses XML internally too.
A number of features exploit this technology:
This is the codename for Mozilla's internal datastore, which uses XML in
the form of RDF, the Resource Description Framework.This incorporates:
Bookmarks, history of visited pages, search results, information on file systems,
FTP locations, and Web site maps.
A concept incorporated into the latest versions of Mozilla and
Netscape whereby users type keywords into the location bar and receive additional
information on Web pages from a dedicated remote server.The client/server com-
munication is encoded for these purposes in XML.
XUL (XML-based User Interface Language):
This is an XML-based language
for describing the contents of windows and dialogues, incorporating support for
typical dialogue controls as well as numerous widgets. XUL describes the contents of
an entire window, which could contain multiple individual HTML documents. It is
a helpful tool for cross-platform information exchange since it allows one user inter-
face described in XUL to be rendered by a platform-specific implementation of the
XUL engine on a different OS or hardware architecture.
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