Thanks for your informative reply. --- 
While it's good to resolve data quality issues, JOSM is a tool and the key is to understand it. Warnings simply indicate possible probelms but the program is not omnicient i.e. it cannot always know with certainty if there is in fact a problem, what it's cause is, and whether you are able to resovle it. JOSM does not prevent uploads contating features with errors or warnings for good reasons e.g. its checks are not perfect and you may have simply edited a feature which already had a problem uploaded by someone else like a tagging warning but you wanted to improve/update geometry. Blindly resolving warnings in the ID editor for example leads to contributors adding invalid layer tags instead of addressing poor geometry which caused overlapping data. If you do not know how or cannot resolve an error I recommend leaving it so that any potential issues are more visible to others, making them easier to spot and address. --- Sometimes warnings appear becuase new tagging has been adopted and tools like JOSM haven't been updated to reflect that.

As a first port of call consult the OSM wiki map features page for tagging releated queires https://wiki.openstreetmap.org/wiki/Map_features --- Use ctrl+f then search for key words and the contents page to find what you're looking for.
