H5 is a C# to JavaScript/TypeScript transpiler that enables C# developers to target web platforms. The system consists of a compiler service (h5-compiler dotnet tool), a translator core that converts C# AST to JavaScript/TypeScript using visitor patterns and attribute-driven rules, and runtime libraries providing .NET-like functionality in JavaScript. Key features include SDK-style project integration, extensive globalization support through locale definitions, and support for JavaScript interop via attributes like [H5.External] and [H5.Template]. The toolchain processes C# source files through lexical analysis (using NRefactory), semantic analysis, translation to target language syntax, and emits JavaScript/TypeScript output with optional source maps and HTML wrappers.