Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<head>
<style>
a:visited
{
background-color:yellow;
}
</style>
</head>
<body>

<a href="http://www.w3schools.com">W3Sschools</a>
<a href="http://www.google.com">Google</a>
<a href="http://www.wikipedia.org">Wikipedia</a>

<p><b>Note:</b> The :visited selector style links to pages you have already visited.</p>

</body>
</html>

Result:

W3Schools.com - Try it yourself

Edit the code above and click "Submit Code" to see the result.