<div class="container">
<iframe width="560" height="315" src="https://www.youtube.com/embed/4iB2w6f-Kdo" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</div>
/* aspect-ratio 対応ブラウザ用のスタイル */
.container iframe {
  aspect-ratio: 16 / 9;
  width: 100%;
  height: auto;
}

/* aspect-ratio 非対応ブラウザ用のスタイル */
@supports not (aspect-ratio: auto) {
  .container {
    padding-top: 56.25%;
    position: relative;
  }

  .container iframe {
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0px;
    left: 0px;
  }
}

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External JavaScript

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