1.18 shouldn't be failing anymore since reedy put a symlink in place for it.
Ideally people would code things like that (aka stuff that isn't actually MediaWiki) shouldn't be doing it to be dependant on MW skin folders, eg: build their own css files, Because like this is pointing out, things can change (not just directories, but the actual files as well).
I believe "//bits.wikimedia.org/skins/{monobook/main.css}" is setup to always point to the latest version so that should be alright, but someone else will need to confirm.