Visited
About
The :visited pseudo-class matches hyperlinks that you visited before. For privacy reasons, the selector has many limitations. You can only style a limited set of properties, like color. Reading the color value of the element via JavaScript will return the value of an unvisited link. Querying visited links via JavaScript is not possible. In some browsers the styles may even be limited to hyperlinks of the same domain.
Hyperlinks
The playground will not match any link because of the privacy limitations. However, links that are visited will have a different color.