The present situation is that rivers implicitly render below highways in all common renderings. That's not necessarily bad. With some formality to the layering arrangement, it sure would save a lot of tagging hassle and maintenance.
The new cloud tag for example, is clearly to be rendered after everything but the celestial tags. Ahem. April 1st aside: the number of important implicit assumptions is relatively small. Rivers under, power lines over, closed ways under except if they're tagged building, etc. Currently this type of layering is implicit in various bits rendering software, but it could be formalized at the tag definition level to help meet certain mapper expectations. --- In the case of the river/highway layer warning: if the warning had never existed, chances are the various workaround schemes would never have come up. Rivers would run under roadways, and tagging would be needed only in the rare case of a ford or an arroyo with no culvert.
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