To preserve users' privacy, Firefox and other browsers will lie to web applications under certain circumstances:

• The window.getComputedStyle method, and similar functions such as element.querySelector, will always return values indicating that a user has never visited any of the links on a page.
• If you use a sibling selector such as :visited + span, the adjacent element (span in this example) will be styled as if the link were unvisited.
• In rare scenarios, if you're using nested link elements and the element being matched is different from the link whose presence in history is being tested, the element will be rendered as if the link were unvisited, as well.

You can style visited links, but there are limits to which styles you can use. Only the following styles can be applied to visited links:

• color
• background-color
• border-color (and its sub-properties)
• column-rule-color
• outline-color
• text-decoration-color
• text-emphasis-color
• The color parts of the fill and stroke attributes

In addition, even for the above styles, you won't be able to change the transparency between unvisited and visited links, as you otherwise would be able to using the alpha parameter to rgb() or hsl(), or the transparent keyword.

Here is an example of how to use styles with the aforementioned restrictions:

: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 */
}

Overall, these restrictions shouldn't affect web developers too significantly. They may, however, require the following changes to existing sites:

• Using background images to style links based on whether they've been visited will no longer work, since only colors can be used to style visited links.
• Colors that are otherwise transparent will fail to appear if styled in a :visited selector.

• privacy-related changes coming to CSS :visited on Mozilla Hacks
• Plugging the CSS History Leak on the Mozilla Security Blog
• Preventing attacks on a user's history through CSS :visited selectors