Pure CSS solution for embed videos with an aspect ratio of 16:9

responsive-video.css

.video-container {
  position: relative;
  padding-bottom: 56.25%; /*16:9*/
  padding-top: 30px; 
  height: 0; 
  overflow: hidden;
}
 
.video-container iframe,
.video-container object,
.video-container embed {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}

responsive-video.html

<div class="video-container">
  <iframe width="640" height="360" src="//www.youtube.com/embed/ukKd8W3Bvo0" frameborder="0" allowfullscreen></iframe>
</div>

Thanks for sharing this!

Thanks to CSS3:
https://developer.mozilla.org/en-US/docs/Web/CSS/@media/aspect-ratio
https://web.dev/aspect-ratio/

You can simply use this new property:

iframe{
aspect-ratio: 16 / 9;
width: 100%;
}

*Warning it has bad support :(
https://caniuse.com/mdn-css_properties_aspect-ratio