I inject this into YT

    // make VISITED links distinct
    //subscriptions/lists
    css += 'a#video-title {color: #167ac6!important;}';
    css += 'a:visited#video-title {color: #141761!important;}';
    //commenter channel name
    css += 'a#author-text {color: #167ac6!important;}';
    css += 'a:visited#author-text {color: #141761!important;}';
    //sidebar
    css += 'a #video-title {color: #167ac6!important;}';
    css += 'a:visited #video-title {color: #141761!important;}';
    
    css += '#content-text a, #description a {color: #167ac6!important;}';
    css += '#content-text a:visited, #description a:visited{color: #141761!important;}';

makes pretty clear which links are visited