747 votesAnonymous commented
I voted for this feature but now I believe that it will be incorporated in the best possible way by the XAML Standard project https://blogs.windows.com/buildingapps/2017/05/19/introducing-xaml-standard-net-standard-2-0/
And I think it's the best way possible because it's not a single binary implementation that works for all platforms, but a syntactic definition whose implementation can be adapted to each platform's features and performance requirements..
Noesys, for example, is a C++ library that works very well on several platforms, but it is mainly a WPF clon. And it is good for some purposes (e. g. full-screen unity games), but here we are asking for a modern UI with a different application architecture and controls specially designed for touch gestures and mobile use as UWP.