Dollchan

You haven't mentioned wich imageboard you're talking about.
I think it's 4chan.

Higlighting visited threads in catalog there ― it's completely possible to achieve with just CSS.

The page at StackOverflow explains
https://stackoverflow.com/questions/56418220/css-selectors-avisited-childs
https://css-tricks.com/almanac/selectors/v/visited/
> You can only use :visited to change those properties if the link already has them in the “unvisited” or :link state. You can’t use it to add properties that aren’t already present on the link.

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