<div class="box">
  <iframe width="560" height="315" src="https://www.youtube.com/embed/L61M2YW8u70" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</div>
.box {
  width: 80vw;
  height: calc(80vw * 0.5625);
  //動画比率が16:9なので56.25%となります
  
  iframe {
    width: 100%;
    height: 100%;
  }
}

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