The original KML file (in ../kml/Marble writing.kml) was converted to GeoJSON format using https://geojson.io/; then attributes were edited by hand in that website to be the same as the original KML file.

The GeoJSON format is defined in RFC7946; the attributes follow the Simplestyle specification (https://github.com/mapbox/simplestyle-spec): all except the "name" attribute, which is commonly used elsewhere.