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                <div class="warning">
  <p>⚠️ Your browser does not support <a href="">CSS <code>aspect-ratio</code></a>.</p>

<iframe src="" width="560" height="315" style="--aspect-ratio: 560 / 315"></iframe>


                iframe {
  aspect-ratio: var(--aspect-ratio); /* Use the CSS Variable defined aspect ratio */
  width: 100%;
  height: auto;

 * If supported, derive the aspect ratio from the height/width attributes.
 * This way iframes will behave like images (which have an intrinsic aspect ratio)
@supports (aspect-ratio: attr(width number) / 1) {
  iframe[width][height] {
    aspect-ratio: attr(width number) / attr(height number);

// Non-essential demo styles
* {
	display: border-box;

body {
  text-align: center;
  // limit width + center self
  width: 50vw;
  margin: 0 auto;
  // center contents
  display: flex;
  flex-direction: column;
  justify-content: center;
  min-height: 100vh;

#demo {
  // Abuse flexbox to lign the text in the center of the box
  display: flex;
  align-items: center;
  justify-content: center;

.warning {
	margin: 1em 0;
	padding: 1em;
	border: 1px solid #ccc;

.warning p {
	margin: 0;
	padding: 0;

@supports (aspect-ratio: 16 / 9) {
	.warning {
		display: none;