Microsoft has now released the source code for WinJS 3.0 and made the project open source. It’s now available on GitHub. You can get the WinJS source code for your programming tools, like Nuget as well, by downloading here.
Before making it open source, Microsoft also went back through the framework and made everything work across all major platforms. It now works in major browsers and hybrid app platforms like Cordova.
You can try different WinJS UI controls via the helpful Try WinJS webpage.