XAML editing in SlickEdit
Good question on the slickedit community forum about the best way to configure it to edit XAML. In Visual Studio we chose to build a XAML Language Service to provide the editing support for XAML files, not reuse the XML Editor.
The way we did that was that the Language Service learned the rules of the language and how to get a Xaml Schema from a CLR Type. [MS-XAML] describes the rules of XAML...we don't have a public spec or API yet on how to get XAML Schema from a clr type, but if you have questions, please contact me...
XSD for XAML History
Although for a few years (until we started shipping the XAML language service) we provided intellisense support for early WPF developers by providing an XSD. That XSD had a few flaws, see a few previous posts on the XSD for XAML topic: