You can't change text-decoration in :visited
. This is out of privacy concerns which are discussed at https://developer.mozilla.org/en/CSS/Privacy_and_the_:visited_selector. Basically changing computed properties can allow websites to determine which sites a user has visited.
The CSS properties that can be used to style visited links are color, background-color, border-*-color, outline-color and, column-rule-color and, when both the unvisited and visited styles are colors (not paint servers or none), the fill and stroke properties. For properties that are not permitted (and for the alpha components of the permitted properties, when rgba() or hsla() colors or transparent are used), the style for unvisited links is used instead.
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