<div class="aspectratio"> 16:9</div>

@import url("https://fonts.googleapis.com/css2?family=Exo:wght@600&display=swap");

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  width: 100vw;
  min-height: 100vh;
  font-family: "Exo", Arial, sans-serif;
  background-color: #222;
  color: #fff;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.5rem;
  gap: 10px;
}

.aspectratio {
  width: 50vh;
  background-color: #09f;
  aspect-ratio: 16 / 9;

  display: flex;
  justify-content: center;
  align-items: center;
  animation: change 15s linear alternate infinite;
}

@keyframes change {
  0% {
    width: 30vh;
  }
  100% {
    width: 60vh;
  }
}