Microsoft Silverlight is a web application framework that provides functionalities similar to those in Adobe Flash, integrating multimedia, graphics, animations and interactivity into a single runtime environment. Initially released as a video streaming plugin, later versions brought additional interactivity features and support for CLI languages and development tools. The current version 4 was released to developers on April 12, 2010, and the end-user runtime was released on April 15, 2010.
Silverlight provides a retained mode graphics system similar to Windows Presentation Foundation, and integrates multimedia, graphics, animations and interactivity into a single runtime environment. In Silverlight applications, user interfaces are declared in Extensible Application Markup Language (XAML) and programmed using a subset of the .NET Framework. XAML can be used for marking up the vector graphics and animations. Silverlight can also be used to create Windows Sidebar gadgets for Windows Vista.
Strictly speaking, there is no such thing as Silverlight SEO since Silverlight sites or regions of sites are not searchable and indexable by search engines. Developers must create and maintain duplicate HTML content to be sent to search engine robots if site content is to be indexed. This same HTML is also sent to users without Silverlight installed. This is because there is no Silverlight version of the Flash Headless Player for Search Engines which enables search engines like Google and Yahoo to index Flash content. Silverlight's lack of support for SEO explains why it is widely seen as a tool for developing Intranet or business to business Extranet applications rather than public facing Internet sites. An exception would be the case of high value video sites such as the Netflix, recent Olympics, and Sunday Night Football sites where Silverlight's strong DRM and adaptive streaming justify the extra work involved in maintaining duplicate HTML content for search engines. According to the comments of one of Microsoft's senior RIA architects at Mix 2010 there may be a Silverlight Player for Search Engines in the as yet undetermined near future.