Michael wrote:
This sounds quite promising, I will try it out. Basically I could leave the default unchanged and assign the desired ID to natural=land in the polygons style?
yes, but bare in mind that natural=land is also a common OSM tag. If someone wants to render an island in a lake with natural=land and you render natural=land as background, the island disappears under the lake. On the other hand, the opposite is also possible, when you have an island as natural=land and a lake on this island with natural=water. If you want to put natural=land above natural=water then the lake disappears. This kind of tagging is obsolete, since it is more common now to use multipolygons to avoid those problems, so I think using a background polygon for natural=land should be fine.