OpenSilver is an open-source framework designed to facilitate the development of rich internet applications (RIAs) using C# and XAML. It was developed as a successor to Microsoft Silverlight, enabling developers to migrate existing Silverlight applications to the web without rewriting their codebase. This framework is built on current web standards, including HTML5, CSS3, and WebAssembly, ensuring broad compatibility across modern web browsers regardless of the operating system without requiring plugins.