The spec references "format" field without declaring it

[nyurik]

The vector_layers field references format field, but it is not declared in the spec. https://github.com/mapbox/tilejson-spec/tree/master/3.0.0#33-vector_layers

Note: When describinng a set of raster tiles or other tile format that does not have a "layers" concept (i.e. "format": "jpeg"), the vector_layers key is not required.