Not sure but it may be TopoJSON: - https://github.com/topojson/topojson https://github.com/topojson/topojson - https://github.com/topojson/topojson-specificatio

Not sure but it may be TopoJSON: - https://github.com/topojson/topojson https://github.com/topojson/topojson - https://github.com/topojson/topojson-specification https://github.com/topojson/topojson-specification My guess is that Github just recognizes and supports previewing this particular spec.