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This used to be possible, but browsers put into place various mitigations: restricting which properties a :visited selector can affect, always computing both the visited and non-visited style to avoid timing attacks, etc. http://dbaron.org/mozilla/visited-privacy has a writeup describing the issues and the solutions that were adopted.

You used to be able to do that, but it was fixed in 2010: https://blog.mozilla.org/security/2010/03/31/plugging-the-cs...

Yes, this attack was demoed to work several years ago but has been patched as far as I know:

https://developer.mozilla.org/en-US/docs/Web/CSS/Privacy_and...

"The first change is that Gecko will lie to web applications under certain circumstances. In particular, getComputedStyle() and similar functions such as element.querySelector() always return values indicating that a user has never visited any of the links on a page."