OpenStreetMap's standard tile layer, from tile.openstreetmap.org.
These servers are run by volunteers under a
tile usage policy that asks for
an identifying User-Agent, caching, and moving heavy use elsewhere. This source makes
at most two requests to them at once, across every render in the node. Set
config :staticmaps, contact: "…" so they can reach you; without it, the first fetch
logs a warning.