Escape hatch

What a template gets

<div class="slide-content">
  {{content}}
  <img src="{{image}}" alt="{{alt}}" />
  <div>{{right}}</div>
</div>

The <main> around it stays ours, so backgrounds, the aspect ratio and class: keep working whatever the template does. A placeholder that is not a thing a slide has is an error, not an empty space.