Ahem, I thought wrong: Pandoc actually treats horizontal rules as text if there are attributes in the same line.
Thanks for all the suggestions!With the right settings, some horizontal rule syntaxes do indeed work:% echo '___{.pause}' | pandoc --from=commonmark+attributes<p><span class="pause">___</span></p>% echo '***{.pause}' | pandoc --from=commonmark+attributes<p><span class="pause">***</span></p>% echo '---{.pause}' | pandoc --from=commonmark+attributes<p>--<span class="pause">-</span></p>