<div class="youtube__wrapper">
  <iframe src="https://www.youtube.com/embed/P00HMxdsVZI" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</div>
.youtube__wrapper{
  width:100%;
  height:500px;
  /*↑heightの数値に困るのよね*/
  iframe{
    width:100%;
    height:100%;
  }
}
View Compiled

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.