JSFiddle - React, Tailwind, and code Playground

HTML

<div class="auto-resizable-iframe">
  <div>
    <iframe
     frameborder="0"
     allowfullscreen=""
     src="http://www.youtube.com/embed/_OBlgSz8sSM">
     </iframe>
  </div>
</div>

CSS

.auto-resizable-iframe {
  max-width: 420px;
  margin: 0px auto;
}

.auto-resizable-iframe > div {
  position: relative;
  padding-bottom: 75%;
  height: 0px;
}

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