All I see is this main.js[0] and everything is drawn on a canvas! (probably explains why text is not as sharp as I remember back in the day)
The Avalonia framework is a third party UI toolkit.
I'm seeing a long load time in Firefox but in Chromium, loading takes maybe a second. Loading the entire .NET runtime isn't exactly lightweight, but it seems there's also some kind of browser engine problem in my Firefox setup that slows this page down artificially.
I must say I'm impressed that this approach even works with only 16MiB of data transferred over the line (though that 16MiB decompresses to about 50MiB).
Avalonia aims at compatibility with Windows Presentation Framework (WPF) XAML. The web platform is explicitly supported: 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... I suppose the C# code is translated to MSIL then wasm.
Something interesting is that the top bar text (File, Edit, etc.) is a little bit blurry, but when I click on one of them (e.g. File), the drop down text (e.g. New Project) is completely crisp with no antialiasing.