Markup Languages: More Than HTML5 - Web Standards: Mastering HTML5, CSS3, and XML - page 52

SGML Languages
Standard Generalized Markup Language (SGML) is a markup language family. It has been used since the mid-1980s.
One of the major features of SGML is flexibility.
The most important SGML language for web designers is HTML, which has been the core language of the World
Wide Web from the very beginning.

■ Despite the similarities in the markup syntax of earlier HTML versions and HTML5, the latest version is no
longer based on SGML; however, it is backward-compatible with conventional HTML parsing.

Caution

HTML
The idea behind Hypertext Markup Language (HTML) was born at the European Laboratory for High-Energy Physics
(CERN) in Geneva, Switzerland, as early as 1989 [8]. One year later, the World Wide Web project was also started there.
In the beginning, HTML was used to share information between scientists. The major elements of the language were
formed at that time, including headings, paragraphs, and hyperlinks, but the semantics of the first documents were
limited [9]. New elements and multimedia capabilities were added later that caused interoperability problems for
documents used across different platforms.
The relatively simple syntax and vocabulary of the language made rapid and wide distribution possible, but it is
also the reason for invalid markup found in more than 90 percent of HTML documents.

XML Languages
Extensible Markup Language (XML) is a universal format for structured documents and data on the World Wide Web.
Since XML has been extended from SGML, it can be considered as a restricted, machine-readable form of SGML that
removes those features of SGML that might result in loose source code.
In contrast to most computer languages, XML has no fixed, predefined set of tags. With XML, individual markup
formats can be defined. Some of the most well-known and frequently used XML formats and/or XML serializations on
the Web are XML, XHTML, SVG, MathML, RDF, and Atom.
Many markup languages were originally designed for textual content, later followed by other types of information
such as music and video, playlists, vector graphics, and news feeds. The big advantage of XML is that it is strict,
well-defined, and extendible. With XML you can combine multiple markup languages into single profiles, such
as XHTML + MathML + SVG or XHTML + SMIL. The additional vocabularies (elements, attributes, and further
components) of other XML applications can be used through the namespace mechanism declared by the xmlns
attribute. This is one of the major advantages of XHTML over HTML 2 .

XHTML
Extensible Hypertext Markup Language (XHTML) is an XML language family that can be used as an alternative
to HTML. XHTML is an application of XML and thus more restrictive than HTML. In contrast to HTML, XHTML
documents must be written without errors; otherwise, rendering engines give error messages instead of rendering the
content. XHTML documents require an XML parser rather than an SGML parser.

2 HTML5 provides additional features through its Application Programming Interfaces (APIs). Before the introduction of HTML5,
the XHTML modules and external vocabularies provided the only way to extend the core markup features, which was available
in XHTML documents only.