The XML family of standards is made up of standards that  work together to provide the infrastructure for the Web.  The basis of most of these standards can be found in ISO 8879, SGML.  This standard for the abstract encoding of textual data provides the syntax for both the current version of HTML as well as the current XML 1.0.

First, let us consider HTML.  HTML adopted the tag syntax of SGML but instead of allowing for user-defined tags, HTML is, itself, a common set of tags to be used for Web publishing.  HTML was the first of the family of standards related to XML and, in fact, preceded XML by about 7 years.

As the use of HTML spread and moved from the academic to commercial arena and came under the supervision of the W3C, users began to demand new functionality in two areas.  First, designers wanted more control over formatting style to be added to HTML.  And others who wanted to use HTML for business transactions wanted HTML extended to allow for the coding of content within Web pages.  In the end, two new standards were undertaken by W3C.  The first was CSS (stylesheets for HTML) which provided for more extensive format control.  And the second was XML (to extend HTML to handle content).  XML was built on the syntax of SGML just as HTML was.  But CSS was designed with a syntax independent of SGML and tags.

Following the development of XML, a variety of other standards were built using XML as a basis and syntax.  Notably are XHTML, XSL, XSLT, XPointer, and XLink.  The XML Schema Definition Language (XSD) was written in XML Syntax and provided a new mechanism for specifying XML applications and XML application standards.

In addition, the Resource Definition Framework uses XML as a syntax.  RDF, like XML, has other standards built using its syntax and framework.  These standards include PICS and P3P.

While the DOM is all about both HTML and XML, it is not written using the XML sytax.  So both DOM and CSS are highlighted as being related, yet having their own unique syntax and specification.

In addition to core technology standards, developed by the W3C, XML has served as the basis for a host of standards that provide for a more specific application or business function. Typically, application standards are developed by industry groups that have a common business interest.  Among these XML-based standards we find ICE, SVG, MathML, OFX, XTM, and ebXML.  Each of these standards relies upon XML and often other core standards.