css/css-transforms/perspective-containing-block-dynamic-1b.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS transforms: Creating 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-transforms-2/#perspective-property">
<link rel="match" href="containing-block-dynamic-1-ref.html">
<meta name="assert" content="It also establishes a containing block (somewhat similar to position: relative), just like the transform property does.">
<meta name="flags" content="dom">
<style>
  html, body { margin: 0; padding: 0 }
  #changeperspective {
    position: absolute;
    top: 100px;
    left: 100px;
  }

  #abscovered {
    position: absolute;
    top: 50px;
    left: 50px;
    background: red;
    height: 100px;
    width: 100px;
  }

  #fixedmoves {
    position: fixed;
    top: 50px;
    left: 50px;
    background: green;
    height: 100px;
    width: 100px;
  }
</style>
<body>
  <div id="changeperspective">
    <div id="abscovered"></div>
    <div id="fixedmoves"></div>
  </div>
  <script>
    var changeperspective = document.getElementById("changeperspective");
    var fixedmoves = document.getElementById("fixedmoves");
    var causeFlush = fixedmoves.offsetTop;
    changeperspective.style.perspective = "1000px";
  </script>
</body>