Essentially the best option for drawing Polygons would be to determine 
their "resolution" based on size. So make large forests appearing at 
lower resolutions than small forests (well I think we all know that best 
would be if resolution of any element were adapted to map density, but I 
think that is even more complicated). I don't think this would be an 
easy task. Depending on the area or country (e.g. France with the Corine 
land cover import) putting forests at low resolutions really slows down 
map panning.

An "easy" compromise could be to put "stronger" douglas peucker filter 
on polygons than on roads (I currently use 5.4, because 10 seems to be 
too strong on roads). However for Polygons a value of 10 works pretty 
well. It would be great if there would be support for that (I don't know 
if the douglas peucker filter is applied after or before the style-file, 
if before of course the above will not be "easy"). The better mapped 
countries are getting, the more importance choosing the proper 
resolutions will have. The changes to the default style over the last 
weeks were already a good step forward, but outside of cities OSM rarely 
has more than 20-30% of ways and landuse covered (I dare people in 
future will not draw few big forests, but split up more and more 
detailed) so the solely based on tags approach will become more and more 
difficult... (well at least I hope the difference between countryside 
and cities will flatten out a bit).
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