Standards, Versions, and Core Features
A W3C recommendation defines a set of features that software must support to claim compliance with the standard. It does not specify how the software must implement any given feature.
In 1994, the W3C issued a recommendation for HTML Version 2.0, the first version of HTML to achieve wide adoption. This version defined the core document structure, hyperlink anchors, formatting, and input form functionality.
Version 3.0 existed as a draft for some time, but the W3C HTML working group deemed that this version attempted to incorporate too many new features, and it was abandoned in favor of Version 3.2. Version 3.2 extended Version 2.0 in 1996 with tables, applets, and text flow around images.
In 1997, Version 4.0 added mechanisms for style sheets, scripting, frames, object embedding, and various improvements. The years since have seen a lot of emphasis on XML. The W3C released a recommendation early in 2000 for XHTML Version 1.0, a reformulation of HTML 4.0 as an XML application. This sets the foundation for future XML-compatible development.
XML vs. HTML
XML, the Extensible Markup Language, is a text-based language specification designed for defining and structuring data. XML is an open standard, published as a recommendation by the W3C in 1998.
Like HTML, XML is a selected subset of SGML and is based on tags and attribute name/value pairs. However, unlike HTML, XML uses tags only to delimit data elements and does not predefine the meaning of the tags, nor does it impose any preconceived notion about how the data delimited by the tag is displayed.