I suggest to update the Simple 3D buildings specification to reflect gathered practical experience and statistics of usage.
Does it make sense to introduce the following additional rules of F4 maps to the specification?
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building:part not fully included in a building are ignored.
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building:part inherits every building* tags from their parent building if they’re not overrided on the part (color, roof shape, height etc).
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building:part polygons are removed from the outline polygon they intersects unless the outline is tag with building:part=yes.
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outline with building:part=no or building:parts=vertical/horizontal are ignored.
That basically means: building outline building=* is used in 3D rendering unless it’s tagged with building:part=no.