2.1.14 [CSS-Level2-2009] Section 5.11.2, The link pseudo-classes: :link and :visited
The specification states:
User agents commonly display unvisited links differently from previously visited ones. CSS provides the pseudo-classes ':link' and ':visited' to distinguish them: The :link pseudo-class applies for links that have not yet been visited. The :visited pseudo-class applies once the link has been visited by the user. The two states are mutually exclusive.
Quirks Mode and IE7 Mode (All Versions)
The :link pseudo-class and the :visited pseudo-class are not mutually exclusive.