Design/Logic Flaw

The Cascading Style Sheets CSS implementation in WebKit in Apple Safari before 5.0 on Mac OS X 10.5 through 10.6 and Windows, and before 4.1 on Mac OS X 10.4, does not properly handle the :visited pseudo-class, which allows remote attackers to obtain sensitive information about visited web pages...

Mozilla Firefox information leakage

It's possible to retrieve information about visited URLs from the site...

Mozilla Firefox information leak

It's possible for script to check if given web page was visited by user...