Manual:$wgShowUpdatedMarker

Details
Show "Updated (since my last visit)" marker in Recent changes view, watchlist and history view for watched pages with new changes.

If $wgEmailAuthentication is false, this setting is ignored.

Adds a class to all entries of pages being watched that the user has not visited since the indicated change(s). The application of that class allows for css styling of watchlist entries (e.g. ).

A featured button above the watchlist allows (when clicked) that all entries can be marked as visited.