Hi Lambertus,
The default style doesn’t have a land-tag like natural=background or natural=land. And therefore i didnt add it in the mapnik.typ file as well, the typ file does however have a tag for natural=sea. The tag natural=background I have invented myself for the OFM because natural=land is already used to tag islands in a lake or something. If you leave that land-tag out I dont know what the consequences are for the rendering. I dont use a parameter ‘polygons’.

Another option is to adapt the default style and use your own style (you can add a line like natural=land or background, throw away buildings etc and those ugly gsm masts, border lines etc)