zen-8.x-7.0-alpha15/STARTERKIT/components/components/responsive-video/responsive-video.twig

<div class="responsive-video {{modifier_class}}">
  <iframe class="responsive-video__embed" width="560" height="315" src="https://www.youtube.com/embed/8N_tupPBtWQ" frameborder="0" allowfullscreen></iframe>
</div>