<section class="container">
  <div class="elem-before">最初に出てきた要素</div>
  <div class="elem-after">後から出てきた要素</div>
</section>

※ どちらもz-indexの指定なし
.container {
  position: relative;
  padding: 1rem 1rem 60px;
}
.elem-before, .elem-after {
  padding: 20px;
  width: 150px;
  text-align: center;
}

.elem-before {
  background: skyblue;
}

.elem-after {
  position: absolute;
  left: 50px;
  top: 55px;
  background: orange;
}

External CSS

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External JavaScript

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