Companies like CSHTML5 (http://cshtml5.com) and DuoCode (http://duoco.de) are already doing this, but it would be great to see this effort officially adopted into the Microsoft Universal Windows Platform.
A mechanism that operates like PhoneGap/Cordova would have to be implemented so that access to external hardware APIs can be achieved.
That way, there is truly *ONE* WUP application codebase, and it can truly work *anywhere*.
There would be no reason to ever learn another language/platform paradigm. Microsoft wins. :)
Ironically, this would truly make UWP "Universal."
Please consider this idea!!!
This vote is part of a bigger vision that impacts a large part of Microsoft. You can see the Visual Studio vote for this vision here: http://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/10027638-create-a-ubiquitous-net-client-application-develo
And I would also suggest that instead of rendering to HTML Canvas, to render to WebGL to get the hardware accelerated goodness.