"Browsers keep track of the pages that a user has visited, and they use this information to style anchor elements on a page differently if a user has visited that link before. Most browsers give visited links a different color by default; some web developers rely on the :visited CSS selector to style visited links according to their own preferences.
It is well-known that styling visited links differently from unvisited links opens the door to side-channel attacks that leak the user’s browsing history. One notable attack used window.getComputedStyle and the methods that return a NodeList of HTMLCollection of anchor elements (e.g. document.querySelectorAll, document.getElementsByTagName, etc.) to inspect the styles of each link that was rendered on the page. Once attackers had the style of each link, it was possible to determine whether each link had been visited, leaking sensitive information that should have only been known to the user.
In 2010, browsers implemented a mitigation for this attack: (1) when sites queried link styling, the browser always returned the “unvisited” style, and (2) developers were now limited in what styles could be applied to links. However, these mitigations were complicated for both browsers to implement and web developers to adjust to, and there are proponents of removing these mitigations altogether." https://github.com/explainers-by-googlers/Partitioning-visited-links-history
@kvadd Yes, but data brokers can buy that information for ad targeting and marketing purposes. These usages should be specifically outlawed, according to the GDPR.