|| SGML ISO 8879-1986
|| HTML 2.0
|| Simplified and stripped
down SGML draft (dubbed XML)
|| HTML 3.2
|| XML working draft
|| XML 1.0 proposed
recommendation HTML 4.0 Recommendation
In 1986 the Standard Generalized Markup Language (SGML)
became an international standard for defining descriptions of the structure and content of
different types of electronic documents. SGML, the "mother tongue" of HTML and
XML, is used for describing thousands of different document types in many fields of human
activity, from transcription of ancient Sumerian tablets to the technical
documentation for stealbombers, and from patient's clinical records to musical notations.
SGML has withstood the test of time. Its popularity is
rapidly increasing among organizations with large amounts of document data to create,
manage, and distribute as in the Defense, Aerospace, Semiconductor and Publishing
industries. However, various barriers exist to delivering SGML over the Web. These
barriers include the lack of widely supported stylesheets, complex and unstable software
because of SGML’s broad and powerful options, and obstacles to interchange of SGML
data because of varying levels of SGML compliance among SGML software packages.
These difficulties have condemned SGML to being a
successful niche technique rather than a mainstream tool. Indeed some cynics have
renamed SGML in 'Sounds Good Maybe Later'.
HTML is a subset of SGML, the most frequently used document
type in the Web. It defines a single, fixed type of document with markup that lets you
describe a common class of simple office style report, with headings, paragraphs, lists,
illustrations, etc., and some provision for hypertext and multimedia.
HTML was defined to allow the transfer, display and linking
of documents over the internet and is the key enabling technology for the WWW. Prior to
the emerging of the internet, it was unusual in the word of computing to hear the word
"page" used to describe elements of data. But HTML web pages have amazing
similarities with paper in their role of information publishing. Both HTML and paper pages
- are optimized for visual clarity,
- focus on ultimate usability (but not on reusability),
- contain no contextual information, and
- have no document structure to enable
Today's web is created by Hand for Eyes only. HTML has too
low an "Information IQ" to enable many desirable applications. HTML was designed
as a markup language an with simple structures, strong emphasis on formatting
and was weak for encoding content. It was not designed to encode structure and
semantics needed for complex applications.
Because of the lack of SGML support in
mainstream Web browsers, most applications that deliver SGML information over the Web
convert the SGML to HTML. This down-translation removes much of the intelligence of the
original SGML information. That lost intelligence virtually eliminates information
flexibility and poses a significant barrier to reuse, interchange, and automation.
For this reason, XML (Extensible Markup
Language) was developed by the XML working group (known as the SGML Editorial Review
Board) formed under the auspices of the World Wide Web Consortium (W3C) in 1996. XML is a highly
functional subset of SGML. The purpose of XML is to specify an SGML subset that works very
well for delivering SGML information over the Web. When the mainstream Web browsers
support XML, it is believed that it’s going to be very easy to publish SGML
information on the Web. It's actually misnamed because XML is not a single Markup
Language. It is a metalanguage to let users design their own markup language.
XML is a public format and not a
proprietary format of any company. The v 1.0 specifications was accepted by the W3C as
Recommendation on February 10, 1998.
XML was conceived as a means of regaining
the power and flexibility of SGML without most of its complexity. While retaining the
beneficial features of SGML, XML removes many of the more complex features of SGML that
make the authoring and design of suitable software both difficult and costly. But XML also
lacks some important capabilities of SGML that primarily affect document creation, not
document delivery. That’s because XML was not designed to replace SGML in every respect.
The question that is open is not whether
XML will succeed as a widespread data format, but rather how fast, to what level of
success and with what products. The question of whether XML would enter the market was
answered when Microsoft, Adobe, Netscape and other big market players not only supported
the development of the new standard but began making sizable product investments to this
new format. The leading Web browser Products already support XML in their latest releases.
The momentum building behind the XML effort means that XML is inevitably destined to
become the mainstream technology for powering broadly functional and highly valuable
business applications on the Internet, intranets, and extranets.
XHTML is a working draft for there formulation of HTML 4.0
[HTML] as an application of XML 1.0 [XML]. It is the basis for a family of future document
types that extend and subset HTML.
There are two major reasons for content developers to adopt
XHTML. First, XHTML is designed to be extensible (Design you own tags). Second, XHTML is
designed for portability. There will be increasing use of non-desktop user agents to
access Internet documents. Some estimates indicate that by the year 2002, 75% of Internet
document viewing will be carried out on these alternate platforms. In most cases these
platforms will not have the computing power of a desktop platform, and will not be
designed to accommodate ill-formed HTML as current user agents tend to do. Indeed if these
user agents do not receive well-formed XHTML, they may simply not display the document.
For further information on XHTML read the XHTML working draft.