When you say you can't open it in anything but Windows 8.1...why would that be the case? There are project files for sublime and vs2013 on github...it's just javascript.
I'm aware I can open the .js files in a text editor, but that's like saying I could work on a C# project by just editing the .cs files in a text editor. Sure that's possible... but I would rather use the IDE.
You can't use WinJS with Windows 7.
They should also publish a sample website demo project for the rest of the planet that isn't using 8.1.