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.