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RSS/Atom feeds can use them. How does it make sense to maintain two versions of the same data on the server?

Exactly. The Atom feed of my website declares an XSLT stylesheet which transforms it to HTML. That way it can be served directly to, and renders prettily by, a web browser (see https://paul.fragara.com/feed.xml). For the curious, the XLST can be found here: https://gitlab.com/PaulCapron/paul.fragara.com/-/blob/master...

Btw, you can also apply an XSLT sheet to an XML document using standard JavaScript: https://developer.mozilla.org/en-US/docs/Web/API/XSLTProcess...