The Visual WebGui framework was originally designed as an infrastructure that would allow a single server-side environment with the option for multiple types of client-sides.
This framework then allowed for users to create their own different presentation layers.
The default Visual WebGui presentation layer is DHTML, and most of the work done on the system as of now is done on this presentation layer.
As a Proof-Of-Concept, the Microsoft Silverlight presentation layer was introduced in Februrary 2008.

Microsoft Silverlight is a pure client-side application.
Originally, the common way for a Silverlight application to communicate with other elements on the page and with server-side, was through a JavaScript layer.
The Visual WebGui MS Silverlight presentation layer, for the very first time for MS Silverlight, provided a built in line of communication between the MS Silverlight client-side and the Visual WebGui server-side.
This line of communication is the same XML-HTTP technology that Visual WebGui provides also for it's DHTML client.
The MS Silverlight controls built for this presentation layer were able to render themselves based on the XML contributed by server-side, and update the server, using that same XML request mechanism.
This allowed for a much richer graphical user interface using the same controls on server-side.
Developers could open the XAML files of the controls and modify them in MS Expression Blend for example to give the controls a whole new look or new behavior.

The MS Silverlight Visual WebGui presentation layer POC is officially supported up to version Visual WebGui v6.3.

Ever since we have started implementing Silverlight presentation layer back in early 2008 (Initially with Silverlight 1.1), the Silverlight API and basic infrastructures have proved to be moving targets, and we found ourselves investing large efforts relatively in order to base Visual WebGui client kernel all over again on each version, minor and even in between early drops...

You should know that we have not abandoned the Silverlight presentation layer and we are even considering to implement other presentation layers such as Flex/Flash in the days to come.

Having said that, we are temporarily freezing the work towards support for Silverlight based presentation, until 2 major events have occurred:

• DHTML 6.4 version reaches rock stable release stage.
• Silverlight API becomes more mature and reaches a stage where we can count on a certain level of backwards compatibility.
  

(Official sources said that the SL 4.0 developer preview version might change about 15%-20% of its basic API until it reaches release stages)

Looking forward we strongly believe in the value of Visual WebGui running graphically rich presentation layers, lending its architectural benefits to Silverlight, Flash and HTML5 and we stay fully committed to implement them once they mature.

• Visual WebGui Extensions in Deployment

• Silverlight roadmap

• Silverlight Prism overview