Chapter 2 - XML
Today HTML is a standard language in which Web pages are written. This language relies on a set of
predefined tags, which control the appearance of a Web page (such as bold, italic, numbered and
unnumberred lists, line breaks etc.)
While XML also relies on tags for marking up Web content, it allows users to define their own tags.
In this sense, XML is a domain-independent markup meta-language (a language for defining a markup
language). The user-defined tags define the structure of a Web page, which becomes thus machine
processable. On the other hand, XML tags do not describe the appearance of Web pages, so XML separates
content from formatting, a nice property that is useful for deining different presentations and views
based on the sama data.
XML actually comes along with a family of languages that support various activities around the core
DTDs and XML Schema: two languages that allow the user to define his own vocabulary.
XPath: a language supporting access to parts of XML documents. Access is the necessary prerequisite
for querying XML documents.
XQuery: a query language for XML.
XSLT: a language defining transformations from XML to HTML, or between XML representations. Thus XSLT is
a key tool for the syntactic manipulation of XML documents.
In the desing of the Semantic Web, XML provides the basic layer for syntactic manipulation.