Rather than read through links by hand, it was more convenient to concatenate the HTML and run
to get a list of links, then sort and uniquify these in Emacs, and run
wget --force-html -i FILE
to try downloading these things. Because the SocialMedia Classroom blocks non-registered users, those links show up as errors.
M-x occur in Emacs is useful for collecting “dead links”, which I posted to http://metameso.org/~joe/link-errors.html?