Information Technology Reference
In-Depth Information
HTML can define links using absolute and relative URIs (see Section 2.2.1
above) using the document location as the base URI. Using the above-
mentioned fragment identifier, links to different parts of a document can also
be defined. Links are created using an anchor tag
<a>
which specifies the loca-
tion of the link in the
href
attribute, for example,
<a href="http://www.wsmo.
org/">link</a>
in Fig. 2.3.
This simple mechanism of linking from one HTML page to another has
been one of the success factors of the current Web. Such links enable crawlers
to find and index pages, an essential prerequisite for e
cient searching. More-
over a link also contains semantic information: if page
A
links to page
B
this
means that page
B
supposedly has some valuable information. Search engines
such as Google use this information to rank their search results.
The ease of use and the availability of many WYSIWYG editors for HTML
and cascading style sheets (CSS) have made it possible for almost anyone to
publish information on the Web. This active, widespread participation has
contributed significantly to today's success of the Web. Apart from standard-
ization of basic features by the W3C, HTML has been extended to include
support for scripting languages, such as JavaScript, developed by Sun Mi-
crosystems, multimedia formats, such as Macromedia Flash, developed and
distributed by Adobe Systems (formerly by Macromedia), and other vendor-
specific extensions which are widely supported by modern browsers, either na-
tively or via plug-in mechanisms. Nevertheless, despite many vendor-specific
extensions and widely adopted features, the W3C is trying to keep standard
HTML within controlled bounds in order to enable widest possible accessi-
bility and interoperability. To this end, the W3C has published Web Content
Accessibility Guidelines,
8
which provide several levels of conformance in order
to guarantee accessability of Web content.
From its first standardized version 2.0 described in RFC 1866 of Novem-
ber 1995, HTML evolved to the last version of “classic” HTML, version 4.1,
published as a W3C Recommendation on December 24, 1999. From that time
on, the further evolution of classic HTML was frozen in favor of XHTML, the
XML version of HTML to be discussed in the next section.
The need for separation of content and layout, just as in templates in usual
word processors, led to extension of the basic HTML standard by cascading
style sheets, where the word “cascading” indicates that one style sheet can in-
herit from another, permitting the combination of several stylistic preferences
in one document.
2.3 From HTML to XML
The freedom of HTML has its drawbacks. First, although CSS take the first
step towards separation of content and layout, the tags and attributes in
8
http://www.w3.org/TR/WAI-WEBCONTENT
.
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