The default output directories of the javascript resources is "/js" and for the css file "/css". These can be changed by configuring the extension.
Given our spec reads like this and the javascript output directory is set to "/myCustomJsDirectory" and the css output directory is set to "/myCustomCSSDirectory":
<div id="id" class="collapsible">This should be collapsible.</div>
When run run the document contains the following resource links:
myCustomJsDirectory/collapse-expand.js
myCustomCSSDirectory/collapse-expand.css