Creator here.<p>I Refresh HN a few times a day, but the frontpage doesn't completely change in a few hours. Which means I see many stories multiple times.
But not all of them are interesting to me, and if one is, I put it in my reading list immediately.
That's why graying out the ones I already saw is helpful, so I don't have to look at them a second time.
Usually there are only ~5 new stories for me, and it's amazing to see immediately which ones those are.<p>It works by putting the ids of the stories into local storage and comparing them on load.
For the first load all of them will appear normally, if you refresh immediately they are all grayed out.