$def with (caption, additional_classes='')

<div class="hidden loadingIndicator $additional_classes">
  <figure>
  <img src="/images/ajax-loader-bar.gif" alt="$_('Loading indicator')" loading="lazy">
  <figcaption style="
      justify-content: center;
      display: flex;
  ">$caption</figcaption>
  </figure>
</div>