<div class="this-element">
  This Element
</div>

<div class="slightly-behind">
  Slightly Behind
</div>
.this-element {
  z-index: 100;
}

.slightly-behind {
  z-index: calc(100 - 1);
}

/* Extra Styles */

.this-element {
  height: 10rem;
  width: 10rem;
  display: flex;
  justify-content: center;
  align-items: center;
  font-family: sans-serif;
  background-color: lime;
  position: absolute;
}

.slightly-behind {
  position: absolute;
  top: 6rem;
  left: 6rem;
  height: 10rem;
  width: 10rem;
  display: flex;
  justify-content: center;
  align-items: center;
  font-family: sans-serif;
  background-color: dodgerblue;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.