I wasnʼt thinking about multiparagraph summaries. But actually, I
realize that your suggestion in response would work with only one small
change to a variable in ox-html.
Assuming there are no objections, we could make this tweak to
‘org-html-html5-elements’ in org core.
I think “syntactic sugar” like
your original proposal could be implemented as a before-parsing-hook
that transforms the --- into the above syntax.
PS Once upon a time, I created ox-extras in contrib as a home for little
snippets like this hypothetical before-parsing-hook. It never really
took off, but maybe itʼs just biding its time...
Kaushal Modi