Only for RES users when navigating the front page, /r/all, or any subreddit..
Sets the link as 'visited' when using the 'Expand' button on text-posts, twitter links, i.redd.it and maybe more I haven't noticed.
Not working with some external links like youtube or i.reddituploads.com
Put it on the console or in your scripts in Tampermonkey or other extensions like that.
Tested only in Chrome.