<div class="experiment experiment--1">
  <iframe width="560" height="315" src="https://www.youtube.com/embed/P00HMxdsVZI" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</div>
<p>Height</p>
<div class="experiment experiment--2">
  <iframe width="560" height="315" src="https://www.youtube.com/embed/P00HMxdsVZI" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</div>
<p>Padding-bottom</p>

* {
  margin: 0;
  box-sizing: border-box;
  font-family: sans-serif;
}

body{
  padding: 10vw;
}

.experiment {
  position: relative;
  display: inline-block;
  width: 100%;
  height: auto;
  outline: 2px solid hotpink;
  &--1 {
    height: 56.25%;    
    iframe {
      width: 100%;
      height: 56.25%;    
    }
  }
  &--2 {
    position: relative;
    padding-bottom: 56.25%; /* 16:9 */
    height: 0;
    iframe {
      position: absolute;
      top: 0;
      left: 0;
      width: 100%;
      height: 100%;
    }
  }
}

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