layouts/shortcodes/youtube-embed.html

<a href="#" data-reveal-id="youtube-{{ .Get "id" }}">{{ .Get "title" }}</a>
<div id="youtube-{{ .Get "id" }}" class="reveal-modal medium" data-reveal>
  <h2>{{ .Get "title" }}</h2>
  <div class="flex-video widescreen" style="display: block;">
    <iframe width="560" height="315" src="https://www.youtube.com/embed/{{ .Get "id" }}" frameborder="0" allowfullscreen></iframe>
  </div>
  <a class="close-reveal-modal">&#215;</a>
</div>