Both the formal documentation and the new proposal:
https://wiki.openstreetmap.org/wiki/Talk:Key:trail_visibility#New_text_proposal
and
https://wiki.openstreetmap.org/wiki/Key:trail_visibility
Trail visibility of horrible is “often pathless” - what does that mean? Presumably less than 50%, given that it isn’t “mostly pathless”. If there doesn’t have to be any trail surface present (like there is in the example photo) then a few cairns here and there can constitute a horrible path.
Obvious and visible are not synonyms in terms of intermediate visibility, is something “invisible” or just “not obviously visible”. The idea of an invisible path is sort of amusing. I’d argue that “faint” or “hard to see at times” would be more appropriate than “invisible” for intermediate unless it’s for very short periods.
What is the difference between “Path mostly but not always obvious/visible” and “Path sometimes invisible, mapped way partly pathless”? They seem to be describing the same thing to me - if a path is sometimes not visible, then it is by definition invisible and partly pathless.