css/css-position/position-sticky-stacking-context.html

<!DOCTYPE html>
<title>position: sticky should create a stacking context</title>
<link rel="match" href="position-sticky-stacking-context-ref.html" />
<link rel="help" href="https://www.w3.org/TR/css-position-3/#sticky-pos" />
<meta name="assert" content="position:sticky elements should create a stacking context" />

<style>
.indicator {
  position: absolute;
  background-color: green;
  z-index: 1;
}

.sticky {
  position: sticky;
  z-index: 0;
}

.child {
  position: relative;
  background-color: red;
  z-index: 2;
}

.box {
  width: 200px;
  height: 200px;
}
</style>

<div class="indicator box"></div>
<div class="sticky box">
  <!-- Because sticky forms a stacking context, this child remains on bottom
       even though it has a higher z-index than the indicator box. -->
  <div class="child box"></div>
</div>

<div>You should see a single green box above. No red should be visible.</div>