Currently categories must be rendered as a chunk of HTML with no ability to customize the output e.g. to provide different handling when there are no categories to display.
Use case: rendering a section like so is not possible:
<h2>Categories</h2>
{{^has-categories}}No categories found.{{/^has-categories}}
{{#categories}}{{{html}}}{{/categories}We should give more flexibility to skin developers, possibly making it consistent with other menu code such as sidebar/footer.
See also https://www.mediawiki.org/wiki/Topic:Wbk8a97qbn5fxczs