.responsive-iframe {
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    width: 100%;
    height: 100%;
  }
  
<div {% if class %}class="{{class}}"{% endif %}>
    <iframe
        class="responsive-iframe"
        src="https://www.youtube.com/embed/{{id}}{% if autoplay %}?autoplay=1{% endif %}"
        webkitallowfullscreen
        mozallowfullscreen
        allowfullscreen>
    </iframe>
</div>