Board Thread:Support Requests - Getting Technical/@comment-6330346-20130416221731/@comment-10688548-20130417001848

"Collapsible" and "collapsed" styles are for the old system of collapsing: JavaScript that had to be added by users to the scripts of the wiki before it would work.

Newer versions of MediaWiki come with collapse support built-in, and don't need any special effort. Now, the class names are "mw-collapsible" and "mw-collapsed". Check out Echoblast53's link for more information.