• HTML Version 2
• HTML3 (1995)
• HTML 3.2 (1997)
• HTML4 (1998)
• HTML 4.01 (1999) -> XHTML
• HTML5 (2014)

In 2004, at a two day workshop in San Jose, California on Web Applications and Compound Documents, the World Wide Web Consortium staff advocated replacing that stack with non-backward-compatible XHTML2 + XForms + SVG + MathML + RDFa.

However, a poll taken at the end of the workshop showed that browser makers unanimously favored an incremental approach based on evolving HTML4/ XHTML1 + CSS + DOM.

In 2005, the  W3C  began work on HTML5 to advance regular HTML, and in 2009 it acknowledged that HTML5 would be the only next-generation version of HTML, although it would include both XML and non-XML serializations.

• Fixes to HTML4
• Richer Semantic Markup
• New Forms Capabilities
• Native Multimedia
• Programmable Vector Graphics
• Powerful APIs

All browsers need two modes: quirks mode for the old rules, strict mode for the standard.