Mediawiki syntax makes it possible to add classes to table rows (but not list items, for instance).
I would expect
{| class="mw-collapsible" |+ Caption |- | always visible |- class="mw-collapsible-content" | hidden when collapsed |}
and
{| class="mw-collapsible" ! Header |- | always visible |- class="mw-collapsible-content" | hidden when collapsed |}
to work much the same as
<div class="mw-collapsible"> always visible <div class="mw-collapsible-content"> hidden when collapsed </div> </div>