Yeah @Ziriax Noesis is where it is at, ATM IMO (LOL). It has recently released v2.0 and has a pretty decent Xaml model. They have working markup extensions in their C++ version with managed C# support on the way. It is free for indie developers, as well.
Also, I think this is pretty big news, but it was recently announced that Mono is porting to WebAssembly. So that would essentially put everything back together again as far as a cross platform .NET solution goes. Since Noesis runs on Mono, then it would benefit from the port.
Additionally Avalonia continues to be in the mix and they will benefit from the Mono port to WebAssembly as well.