(Yes, it's really "vistited", not "visited" above.)
It appears most, or maybe all, browsers adhere to the prohibition of only
allowing a limited subset of CSS actions in "a:visited" for security
reasons, even though it is legitimate CSS, but it took me a while to
figure this out.
I wonder if perhaps your CSS validator might better serve the public if it
checked for and issued a warning when encountering such CSS constructs.