To begin any complex extensible programming project there is a need to study the basic research in this field. Because Silverlight is young enough in .Net programming the most similar technology is Composite UI Application Block (CAB) developed by Microsoft patterns&practices team. In this connection Silverlight Composite UI Application Block (SCAB) can be helpful. It can be obtained by simplifying CAB and adding Silverlight specifics. The SCAB project will have wide interest and should be divided into separate project.

The following requirements can be written for Silverlight Composite UI:

1. SCAB should allow to build applications composed of independent cooperating modules
2. Contain low size assemblies
3. Include Silverlight specific (Xaml, Downloader etc.)
4. Have open source content to allow source code be included into other projects
5. Provide Html and Jscript interaction within the page
6. Provide the interaction with server through web-services
7. Support SEO nature of Internet

This project has many unclear non researched details thus comments and advises from developers will be very helpful.