css width same as height

.my-container {
  width: 100%;
  height: 150px;
  background: black;
  padding: 20px
}

.my-element {
  background: #fff;
  aspect-ratio: 1 / 1;
  height: 100%;
}

<div class="my-container">
  <div class="my-element">Height = Width</div>
</div>