Johann,
the idea to printout a warning sounds good. But there are some restrictions within mkgmap that makes it unfeasable.
Yes, unfortunatelly I know the internas somewhat :-)
When mkgmap reads the OSM data all tags which keys are not referenced in the style files are dropped. This improves the performance and reduces the memory footprint *enormously*. The style file processing is done at a later stage.
So why not printout a warning at this stage, where the key is dropped? Or maybe not printout directly, but fill a statistics class with it. So the user can see, there are x (thousand) occurences of a key he has not handled in his style file.
So in case your style file only contains a rule for natural=water you would get a warning for natural=land but not for landuse=residential.
I don't think that's what you want?
I personally don't need this feature at all. It was meant only as a suggestion in response to the original poster. I will not implement it. ;-) If I will find some time for the mkgmap project, I have other preferences. Johann