With regard to "street_polygons", what does "mapped as polygons" mean?

[SomeoneElseOSM]

https://shortbread-tiles.org/schema/1.0/#layer-street_polygons says:

Layer “street_polygons”
Holds polygons geometries of certain streets mapped as polygons

OSM doesn't have an explicit area type, so there are a couple of ways that things can be thought of as polygons:

1. It's a closed way, and all objects with particular tag/value combinations can be thought of as areas
2. It's got an explicit area=yes tag

The highway tag isn't explicitly an area tag - a residential road that forms a circle is just a circular road, not a large area. However, a highway=pedestrian arguably is. Some mappers use area=yes on these but some do not, meaning that some pedestrian areas aren't detected as such - see https://pnorman.github.io/tilekiln-shortbread-demo/#18/53.959634/-1.081985 and https://pnorman.github.io/tilekiln-shortbread-demo/#18/53.960158/-1.084093 .

After a quick look, I'm unaware of any mapping of genuinely circular (i.e. not area) highway=pedestrian ways, so it would make sense to me to say "if a highway=pedestrian is a closed way, assume it is an area". That code change would need to be implemented by whoever implements Shortbread-schema tiles, of course. That's what I've done when I've consumed OSM data elsewhere.

However, separately to that, it'd be good if the documentation here could clarify that either "mapped as polygons" means "a highway with an area=yes tag, or that certain sorts of closed way highway values such as pedestrian are assumed to be areas.