There is no reason to use Atom. RSS has in practice some specifics like content:encoded and description that are strange at first look, but by now are solved in every reader. So why not just use the standard that works?
There’s pretty much no documentation of anything in the feed world past 2007. Everything interesting happened between 2002 and 2007, and then everyone forgot about it because it was good enough¹ and we could just keep copying the code we wrote last time.
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¹ Except podcasts. Have I mentioned them? They’re still frozen in 2004.
