Scale iFrame css width 100% like an image

Question

I want to scale an iFrame through CSS to width: 100%, and the height should scale proportionally to the width.

With an tag this

Answer 1

You could use viewport units here instead of %. Like this:

iframe {
    max-width: 100vw;
    max-height: 56.25vw; /* height/width ratio = 315/560 = .5625 */
}

DEMO (Resize to see the effect)

body {
  margin: 0;
}
.a {
  max-width: 560px;
  background: grey;
}
img {
  width: 100%;
  height: auto
}
iframe {
  max-width: 100vw;
  max-height: 56.25vw;
  /* 315/560 = .5625 */
}