You've been fed wrong information, Windows Store are
not just HTML applications, HTML (backed by JavaScript and CSS3) is just another API by Windows, using which you can develop Windows Runtime applications. You still get to develop applications using DirectX/C++ or XAML/C# and so on.
Secondly, if you're going to use HTML5 and JavaScript to create the application which would load the contents from your web app, you are going to use Visual Studio. Visual Studio is the only IDE that is going to allow you to perform such tasks, also Windows Store SDK is available in Visual Studio and many other factors ensure that
Visual Studio is a must!
If you want to learn more about developing apps, please
read this[
^]. Although .NET framework has been open sourced, but I don't see Windows Runtime would be open sourced any sooner. So instead of waiting, you can install Visual Studio (Express edition) and delete it once this
small app project is complete.