"iOS and Android from day one, with one shared codebase in UX Markup and JavaScript. You can also access all native features when needed by adding Objective-C, Swift or Java code directly to your project." (from their official site)

I guess their actual compiler for "UX Markup" could be written in .NET but Javascript is definitely and unfortunately invited.

Bent from Fuse here — The UX Markup compiler is actually written in Uno, a lightweight C# dialect that compiles to native C++ for iOS and Android. JavaScript is just used for business logic and runs on a separate thread from the UI engine.

Will Uno ever be open sourced?

Markup looks a lot like XAML