<!DOCTYPE html>
<!--
 | Like all positioned elements, relatively positioned elements should
 | participate in stacking contexts, and thus be eligible to set z-index.
 |   https://www.w3.org/TR/CSS21/visuren.html#relative-positioning
 -->
<html>
<head>
  <style>
    .box {
      width: 50px;
      height: 50px;

      background-color: rgb(100, 100, 255);
    }

    .position-absolute-1 {
      position: absolute;
      left: 25px;
      top: 35px;
      width: 90px;
      height: 90px;
      background-color: rgb(100, 255, 100);

      z-index: 1;
    }
    .position-absolute-2 {
      position: absolute;
      left: 45px;
      top: 35px;
      width: 90px;
      height: 90px;
      background-color: rgb(255, 100, 255);
      z-index: 3;
    }

    .position-relative {
      position: relative;
      left: 25px;
      top: 25px;
      background-color: rgb(255, 100, 100);
      z-index: 2;
    }
  </style>
</head>

<body>
  <div class="box"></div>
  <div class="position-absolute-1"></div>
  <div class="position-absolute-2"></div>
  <div class="box position-relative"></div>
  <div class="box"></div>
</body>
</html>