Some browser-vendors have decided/realised that separately styling a:visited hyperlinks represent a security/privacy threat to the user. Therefore some, though not all, have removed the ability to style a:visited differently.
I suspect that this is true of Chrome.
References:
- http://djtechnocrat.blogspot.com/2010/12/browser-privacy-css-history-sniffing-in.html
- http://blog.mozilla.com/security/2010/03/31/plugging-the-css-history-leak/
- See also this Stackoverflow question: Google chrome a:visited background image not working
