<h3>לא רספונסיבי</h3>
<iframe width="560" height="315" src="https://www.youtube.com/embed/5QGQzGQMd5E?rel=0" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<h3>רספונסיבי</h3>
<div class="container">
  <div class="iframe_wrap">
    <iframe width="560" height="315" src="https://www.youtube.com/embed/5QGQzGQMd5E?rel=0" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</div>

.iframe_wrap{
  position: relative;
  padding-top: 57%;
}
.iframe_wrap iframe{
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
}
.container{
  max-width: 560px;
}
iframe{
  max-width: 100%;
}