How does this help accessibility, though?
(Maybe I don't know enough about HTML. Is the usual way to produce a descending enumeration to write the elements in reverse order and then re-reverse them in CSS or so, which makes them be read wrong by screen readers?)
Note also that reversing content via CSS is generally unwise, because things like screen readers, text selection and tab indexing operate on DOM tree order.
That's a good point. There's no obvious way to mark up a HTML list as being reversed in CSS alone, at least not without a bunch of Flexbox styles and stuff. So the likelihood that someone would use an unsemantic alternative for this is basically non existent.