Very neat! Can someone explain the json is being rendered? I had no idea json could be rendered as an image?
https://github.com/billywojcicki/vexillologist/blob/master/w...
My guess is that Github just recognizes and supports previewing this particular spec.
Edit: I was wrong, it's topojson as per the other reply, based on the data-type attribute that's set on the div outside the map