the W3C has launched a couple of new working groups for developing
standard APIs and formats for RIA development.

http://www.w3.org/2006/webapi/
http://www.w3.org/2006/appformats/

"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."