<div class="my-element">
  <div class="child">I am behind my parent</div>
</div>

.my-element {
	background: rgb(232 240 254 / 0.5);
  border: 1px solid lightblue;
}

.my-element .child {
	position: relative;
	z-index: -1;
  
  background: pink;
  border: 1px solid hotpink;
  padding: 1rem;
  width: 275px;
}

/* Decorative styles */
.my-element {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  width: 250px;
  height: 250px;
}

.child {
  font-weight: bold;
}