Comment by graemep

Thank you, that is quite informative.

The specific posts I see are from the Royal National Institute of Blind People who really ought to know.

What they do is add the image description at the end of each text post, even thought this matches the alt text.

This is the one about using alt text: https://www.facebook.com/rnibuk/posts/pfbid037RmtoSxfAJX82G4...

They do now have a comment on that one that explains their reasoning (I did not see it until just now).