Out of all of these metaphors, the two most enduring are paper and physical space.
Browser implementations of Sol LeWitt’s conceptual and minimal art, many of which only exist as instructions like this:
Vertical lines, not straight, not touching, covering the wall evenly.
Hit this URL to give yourself a design constraint (or obstruction). Kind of like Brian Eno’s oblique strategies but with different categories of constraints: formal, methodological, and conceptual.