It looks like you're trying to diff a ReSpec document with the output from running ReSpec, is that on purpose? There's going to be a big difference between the two, but it'll tell you a *lot* more about what ReSpec does than about actual changes in the document since March. I don't know htmldiff, but is it quite possible that it doesn't like <section> elements?
htmldiff doesn't care about section elements. But it also doesn't know
how to deal with the pre-generated data. Generate the HTML (or XHTML)
output and then diff the output. Or.... use the in-built htmldiff
support in respec. It can generate the html and then compare that
output with a static html version.
..to a local copy of csp-specification.dev.html on my machine, then rendered it
in a browser (ReSpec seems to work fine), then Cntl-Alt-Shift-s, click
[Diffmark], and the diff generation begins, but then hangs with a blank page
and a message of "There is no oldfile and no oldcontent parameter."
Should this approach not work?
In terms of rendered html diffs, and the W3C Mercurial web app, I'd like to be
able to diff a rendered view of the latest version of a spec against some
identified prior version, or other referenceable file, as well as rendered-view
diffs between individual changesets, sorta like wikipedia can do.