XHTML is a family of current and future document types and modules that reproduce,
subset, and extend HTML 4 [HTML4]. XHTML family document types are XML based, and
ultimately are designed to work in conjunction with XML-based user agents.
XHTML 1.0 is the first document type in the XHTML family. It is a reformulation
of the three HTML 4 document types as applications of XML 1.0. It is intended to
be used as a language for content that is both XML-conforming and, if some simple
guidelines are followed, operates in HTML 4 conforming user agents. Developers who
migrate their content to XHTML 1.0 will realize the following benefits:
- XHTML documents are XML conforming. As such, they are readily
viewed, edited, and validated with standard XML tools.
- XHTML documents can be written to operate as well or better
than they did before in existing HTML 4-conforming user agents as well as in new,
XHTML 1.0 conforming user agents.
- XHTML documents can utilize applications (e.g. scripts and
applets) that rely upon either the HTML Document Object Model or the XML Document
Object Model.
- As the XHTML family evolves, documents conforming to XHTML
1.0 will be more likely to interoperate within and among various XHTML environments.
What is HTML 4 ?
HTML 4 [HTML4] is an SGML (Standard Generalized Markup Language) application conforming
to International Standard ISO 8879, and is widely regarded as the standard publishing
language of the World Wide Web.
SGML is a language for describing markup languages, particularly those used in electronic
document exchange, document management, and document publishing. HTML is an example
of a language defined in SGML.
SGML has been around since the middle 1980's and has remained quite stable. Much
of this stability stems from the fact that the language is both feature-rich and
flexible. This flexibility, however, comes at a price, and that price is a level
of complexity that has inhibited its adoption in a diversity of environments, including
the World Wide Web.
HTML, as originally conceived, was to be a language for the exchange of scientific
and other technical documents, suitable for use by non-document specialists. HTML
addressed the problem of SGML complexity by specifying a small set of structural
and semantic tags suitable for authoring relatively simple documents. In addition
to simplifying the document structure, HTML added support for hypertext. Multimedia
capabilities were added later.
In a remarkably short space of time, HTML became wildly popular and rapidly outgrew
its original purpose. Since HTML's inception, there has been rapid invention of
new elements for use within HTML (as a standard) and for adapting HTML to vertical,
highly specialized, markets. This plethora of new elements has led to interoperability
problems for documents across different platforms.
What is XML ?
XML™ is the shorthand name for Extensible Markup Language [XML].
XML was conceived as a means of regaining the power and flexibility of SGML without
most of its complexity. Although a restricted form of SGML, XML nonetheless preserves
most of SGML's power and richness, and yet still retains all of SGML's commonly
used features.
While retaining these beneficial features, XML removes many of the more complex
features of SGML that make the authoring and design of suitable software both difficult
and costly.
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