Sometimes Search Engine Optimisation best practices get pushed aside in favour of fancy form. This can potentially frustrate the hell out of your audience.
Some users will be visiting your site for a very specific piece of information – and those users will want to be able to find that information as quickly as they possibly can. If, for some reason, they can’t just navigate straight to that information; if, for example, they have to use a search function to find it, then they will need the ability to tell which search results they have already looked at, otherwise they might end up looking at the same result over and over again, which will quickly frustrate them, and might even prompt them to go to a whole different website altogether.
The ‘visited’ attribute on a link provides this very functionality and it has been around since the beginning of the web, dating back to ancient times when websites were comprised of only links and text and nothing else! Example: The WWW Project