Common Language Runtime on Silverlight

Silverlight includes a mini-Common Language Runtime from the .NET framework, meaning that a subset of the full .NET platform that normally runs on desktops can be accessed from within the browser. Following the usual .NET model, Silverlight can be coded in a number of supported programming languages such as C#, JScript, VB, and the dynamical Phyton and Ruby .NET versions which were built by Microsoft and released under a shared source license in which developers can access and contribute to the base code.

The most important things about the CLR on Silverlight are its speed and its size. The Silverlight package including all the Silverlight technology features, the Common language Runtime, a Windows Presentation Foundation (which is the graphical subsystem feature of the .NET 3.0 framework, directly related to XAML), and .NET FX library API subset plus dynamic language support is around 4 MB and takes less than 20 seconds to install on a computer. In a demonstration displaying a game of chess routines written in .NET against native Javascript routines, the result obtained showed an enormous difference. For developers, this means that they can simply take their existing Javascript and copy it into Silverlight logic and have it perform multiple times faster than it does in the original browser environment. Furthermore, Silverlight applications can access and manipulate the browser in Document Object Model, which means they can reach outside and into the webpage itself. Since Silverlight runtime has become more common and known more and more developers for web applications have tapped into Silverlight technology for both performance increase and for better visual enhancement and user end experience.

Read more about it:

Multimedia in Silverlight
Silverlight Features - a Comparison with Adobe Flash
Development Premises
Common Language Runtime on Silverlight