chromium/third_party/blink/web_tests/external/wpt/css/css-will-change/will-change-stacking-context-opacity-2.html

<!doctype html>
<meta charset="utf-8">
<title>CSS Test: will-change: opacity should create a stacking context for inlines.</title>
<link rel="author" title="Philip Rogers" href="mailto:[email protected]">
<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1119651">
<link rel="help" href="https://drafts.csswg.org/css-will-change/#will-change">
<link rel="match" href="green-square-100-by-100-ref.html">
<style>
  html, body { margin: 0; }
  .indicator {
    position: absolute;
    width: 100px;
    height: 100px;
    background: green;
    z-index: 1;
  }
  .will-change-opacity {
    will-change: opacity;
    width: 100px;
    height: 100px;
  }
  .top {
    position: absolute;
    width: 100px;
    height: 100px;
    background: red;
    z-index: 3;
  }
</style>

<div class="indicator"></div>
<span class="will-change-opacity">
  <!-- Because will-change: opacity creates a stacking context, this child
       remains on bottom even though it has a higher z-index than the indicator
       box. -->
  <div class="top"></div>
</span>