Bordered or Shape in/as background

shape as background, shape in background, bordered background -> passepartout

I only just remembered the actual tag a minute ago. There aren’t many who will know this tag by name.

Should they, though? Passepartout presently says in its description “square border” (including rectangles, presumably) - and other “shapes” might not be that, plus a few other “shapes” have their own tags, like circle background

After looking up the definition, it doesn’t even seem to match what it’s used for.
But as it stands, that’s what I thought the tag was intended for. “Simple shaped borders” that are used as the background.