<div class="box">
  <iframe width="560" height="315" src="https://www.youtube.com/embed/wcEptQvKi8o" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</div>
.box {
  position: relative;
  width: 80%;
  height: 0;
  padding-bottom: 56.25%;
  //動画比率が16:9なので56.25%となります
  overflow: hidden;
  
  iframe {
    width: 100%;
    height: 80%;
    position: absolute;
    top: 0;
    left: 0;
  }
}

External CSS

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External JavaScript

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