An early draft of Microsoft's XAML specification has been brought online under its "open specification promise." This fulfills a promise made last week at OSBC.
XAML stands (roughly) for eXtensible Application Markup Language, and enables the creation of object hierarchies for use online.
The Open Specification Promise is the company's pledge not to sue you for what's in the documentation, based on claimed patents covering the technology.
Microsoft's XAML isn't the only one out there of course. Open source programmers have long had access to MyXaml which went Version 1.0 in 2004. Its sponsor is Ironsuit Software.
Open source programmers are also familiar with United XAML and the Open XUL Alliance, both on Sourceforge.