Honestly, I don't know which term is technically correct here. My guess is that this is entirely subjective because the prototype chain has no physical analog. Here's my rationale for this phrasing: in my experience, the phrase "up the prototype chain," is used fairly often when people describe JavaScript's property access mechanism. That would make "down" the direction of base-to-instance.

I'm not satisfied using anecdotal evidence to decide on this, though. I want the documentation to make the most sense to the most people. Is there some more concrete reason to use the opposite wording?