der.js

:link {
  outline: 1px dotted blue;
  background-color: white;
  /* The default value of background-color is `transparent`. You need to
     specify a different value, otherwise changes on :visited won't apply. */
}

:visited {
  outline-color: orange;    /* Visited links have an orange outline */
  background-color: green;  /* Visited links have a green background */
  color: yellow;            /* Visited links have yellow colored text */
}