(X)HTML 5.0 is the 5th major revision of the core language of the World Wide Web: the Hypertext Markup Language (HTML). In this version, new features are introduced to help Web application authors, new elements are introduced based on research into prevailing authoring practices, and special attention has been given to defining clear conformance criteria for user agents in an effort to improve interoperability.
Specification: http://www.w3.org/TR/html5/ (W3C)
SVG 1.1 is a language for describing two-dimensional vector graphics in XML, combined with raster graphics and multimedia.
Specification: http://www.w3.org/TR/SVG/ (W3C)
XUL (XML User Interface Language) is Mozilla's XML-based language that lets you build feature-rich cross platform applications that can run connected or disconnected from the Internet.
Specification: http://developer.mozilla.org/en/XUL (MDC, Mozilla Foundation)
Charts is a custom markup language that lets you build awesome charts from markup.
Specification: http://www.amplesdk.com/reference/#/ample/chart/chart.xmlL (Clientside OY)
XForms 1.0 (in development)
XForms is an XML application that represents the next generation of forms for the Web. By splitting traditional XHTML forms into three parts—XForms model, instance data, and user interface—it separates presentation from content, allows reuse, gives strong typing—reducing the number of round-trips to the server, as well as offering device independence and a reduced need for scripting.
Specification: http://www.w3.org/TR/xforms/ (W3C)