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                <div class="wrapper">
   <div class="frame"></div>


                * { box-sizing: border-box; }

body {
  margin: 0;
  padding: 0;
  display: flex;
  flex-direction: column;
  justify-content: space-around;
  min-height: 100vh;

.wrapper {
  --frame-max-height: 30rem;
  --frame-ratio-w: 16;
  --frame-ratio-h: 9;
  background-color: grey;
  width: 100%;
  height: 100%;
  max-width: 90em;
  max-height: var(--frame-max-height);
  margin: 0 auto;

.frame {
  /* Padding is width/height, since % padding is based off the element's width */
  --ratio: calc(var(--frame-ratio-h, 1) / var(--frame-ratio-w, 1) * 100%);
   * The height of the frame is either the calculated padding value, or a maximum
   * passed in (using --frame-max-height). This effectively clamps the height.
  --frame-height: min(var(--ratio), var(--frame-max-height));
  position: relative;
  padding-bottom: var(--frame-height);
   * The width should be 100% where possible, but should maintain aspect ratio
   * first and foremost. In order to do so we can take the calculated height
   * and reverse engineer the width.
  width: min(calc(var(--frame-height, 0) * (var(--frame-ratio-w) / var(--frame-ratio-h))), 100%);
  height: 0;
   * Irrelevant
  background-color: blue;
  margin: 0 auto;