After struggling with various adoptions for getting collapsible fieldsets to work within nodes for Drupal 7 I came across a final solution.
Make adjustments to
template.php (or in a module if you prefer):
In D7 you must also make sure that your legend contains a span with class
fieldset-legend, and that your content is contained in a div with class
fieldset-description, and that you include
<fieldset id="fieldset-id" class="collapsible"> <legend><span class="fieldset-legend">Fieldset title</span></legend> <div class="fieldset-wrapper"> <div class="fieldset-description">Fieldset description</div> Fieldset content </div> </fieldset>