Hi,
At the moment you can make your app responsive by registering the "Windows.Current.SizeChanged" event and adapting the layout according to the size of the window (Window.Current.Bounds.Width and Window.Current.Bounds.Height).
You can see an example by opening the Showcase application (in Visual Studio, click File => New Project => C#/XAML for HTML5 => Sample Showcase).
In the future, we are going to support the UWP (Universal Windows Platform) way of handling responsivity in XAML, which is to use the AdaptiveTrigger, which you can see in action at:
http://mikaelkoskinen.net/post/uwp-xaml-responsive-layout-using-grid-and-adaptivetriggerRegards,
JS-Support