HTML is the "language of the web," a data description language based on structural markup. The various "chunks" of text of various types are marked up using various tags that indicate such things as whether a piece of text represents a title, header, web link, as well as orienting it within the document "structure."
Official HTML definitions have been based on SGML (Standard Generalized Markup Language) document formatting definitions. Unfortunately, the two most notable vendors of HTML browsers, Netscape and Microsoft have long paid only lip-service to the idea that HTML is supposed to conform to standards. They have added a variety of extensions that have not conformed with the notion of structural markup.
The most recent "HTML 4.0" standard appears to return to structural markup, providing the separate Cascading Style Sheet mechanism to apply physical markup as a separate abstraction.
Rendering schemes such as DSSSL are in progress; this allows presentation style information to be attached via separate components that can be applied to a variety of similar documents.
A XML-based specification submitted by Marimba and Microsoft. It's an installation/distribution/update system. It has a lot of the same overall functionality as Linux package managers such as RPM or Debian's dpkg, providing "tags" to describe dependencies, versioning, and other such things.
Java is a common tool for client-side code
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