css/css-will-change/will-change-fixpos-cb-transform-style-1.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS will-change: 'will-change: transform-style' creates a containing block for fixed positioned elements</title>
<link rel="author" title="L. David Baron" href="https://dbaron.org/">
<link rel="author" title="Mozilla" href="http://www.mozilla.org/">
<link rel="help" href="https://drafts.csswg.org/css-will-change-1/#will-change">
<link rel="help" href="http://www.w3.org/TR/css3-transforms/#transform-style-property">
<link rel="match" href="green-square-100-by-100-offset-ref.html">
<meta name="assert" content="If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.">
<style>
html, body { margin: 0; padding: 0; }
div { width: 100px; height: 100px }
#wc { will-change: transform-style; margin: 100px 0 0 100px; background: red }
.child { top: 0; left: 0; width: 50px; background: green }
#fixpos { position: fixed }
#abspos { position: absolute; left: 50px }
</style>
<body>
  <div id="wc">
    <div class="child" id="fixpos">
    </div>
    <div class="child" id="abspos">
    </div>
  </div>
</body>