CodePen Embed - Adaptive block

<div class="container">
  <div class="child"></div>
</div>

.container {
  width: 200px;
  margin: 0 auto;
  border: 5px solid gray;
  background-color: lightblue;
}
.child {
  width: 75%;
  padding-top: 75%;
  background-color: lightcoral;
}

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