The W3C had a working group within the "Rich Web Client Activity" named "Declarative Formats for Applications and User Interfaces". Unfortunately the group stopped working in 2007, http://www.w3.org/TR/dfaui/
The goals: "This WG's Charter includes a deliverable named Specification of a declarative format for applications and user interfaces (called DFAUI in this document) and it is defined as follows: This deliverable should be based on an existing application/UI format, such as Mozilla's XUL, Microsoft's XAML, Macromedia's MXML or Laszlo Systems' LZX, provided the owners of the format are willing to contribute. The format should allow embedded program code. This format, combined with the deliverables below and existing technologies including XHTML, CSS, XForms, SVG and SMIL, should provide a strong basis for rich client application development." Neither Mozilla, nor Microsoft, Macromedia (now Adobe) or Laszlo provided input, as it seems. And I read somewhere that the project was stopped due to inactivity. I doubt that there was a common interest in creating an open standard for a unified Flex/Laszlo/Silverlight XML+JavaScript based declarative language. - Raju
