Add mark as seen buttons to your favorite websites.
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As Arturs said, it does that one thing and does it right. Though making it do it right is a bit hard when you're consuming tons of content, as you have to manually mark it as read by clicking on the little dot. A keyboard shortcut to toggle this on the current focused content, or the ability to automatically mark as read as it scrolls off the page (like many news reader / RSS / Reddit (RIP) apps do) would be much easier to make this to use. Currently I use this as a "bookmarking" tool of sorts on Twitter by marking a bunch as read when I'm done, then I can see the batch of darkened tweets as my placed bookmark.
Does one thing and does it right.
https://kiman.odesa.in
Nathan Adamec
https://sniklaus.com
Maxime Cousinou
Browser extensions
webagent
Dima PM
Matt0x5b
Unknown
Alexander Logach
nickells
edoreld