I encounter, as I think, an improper usage of layer=* tag.
The user apm-wa tags parks, school territories, airports with negative layer value even if there are no viaducts/tunnels around that would require such tag.
- amenity=school area http://www.openstreetmap.org/way/696953991
- airport https://www.openstreetmap.org/relation/6332536
- park https://www.openstreetmap.org/way/90493853/history
You can read his reasoning in discussion to changeset 71224337. Citation: “… buildings are by default set to layer=0, if you set grounds at layer=-1, the buildings and grounds are rendered correctly in the standard layer. In other words, the buildings are rendered ABOVE the ground, not below the ground”. Also note the comment to the changeset 41016067: “adjusted layer of airport boundary to -2 to get it under the aprons, which are set at -1”.
In my opinion, the user mixes up the layer tag in OSM and object layer precedence (z-index) in a render. If so, please admonish him in some way, e.g. on changeset 71224337 page.