They seem to be using this framework: https://github.com/AvaloniaUI/Avalonia https://github.com/AvaloniaUI/Avalonia Avalonia aims at compatibility with Windows

They seem to be using this framework: https://github.com/AvaloniaUI/Avalonia https://github.com/AvaloniaUI/Avalonia Avalonia aims at compatibility with Windows Presentation Framework (WPF) XAML. The web platform is explicitly supported: https://avaloniaui.net/platforms#wasm https://avaloniaui.net/platforms#wasm. The VB Form model must be implemented on top of this. This is for the UI part. Now for the code part, the dotnet toolchain has supported WebAssembly (wasm) generation for some time: https://devblogs.microsoft.com/dotnet/extending-web-assembly-to-the-cloud/ https://devblogs.microsoft.com/dotnet/extending-web-assembly... I suppose the C# code is translated to MSIL then wasm.