HTML5.NET could be a real target platform. Multi-selectable for PCLs. You would use the MVVM pattern to share code across multiple native platforms (iOS, Android, WinStore, etc) and HTML5. Just like Xamarin native, but for the web as well.
Visit the HTML5.NET website for more details.
UPDATE: This project should actually be called MonoHTML5! And there is interesting overlap with Unity’s IL2PP project.
Kitsilano Software 