From the Mozilla Developer Network:

Historically, the CSS :visited selector has been a way for sites to query the user’s history, by using getComputedStyle() or other techniques to walk through the user’s history to figure out what sites the user has visited. This can be done quickly, and makes it possible not only to determine where the user has been on the web, but can also be used to guess a lot of information about a user’s identity.

In the old days, this used to be a clever hack, but it’s no longer achievable in a modern browser.