css/css-contain/contain-layout-dynamic-005.html

<!DOCTYPE html>
<html class="reftest-wait">
<meta charset="utf-8">
<title>Dynamic change to layout containment</title>
<link rel="help" href="https://drafts.csswg.org/css-contain/#contain-property">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1874826">
<meta name="assert" content="Verify floats are properly displayed after dynamic change to layout containment.">
<link rel="match" href="contain-layout-dynamic-005-ref.html">

<link rel="stylesheet" type="text/css" href="/fonts/ahem.css" />
<script src="/common/reftest-wait.js"></script>
<script src="/common/rendering-utils.js"></script>

<style>
  div {
      font: 25px/1 Ahem;
  }
  #wrapper {
      width: 15em;
      background: lightgray;
  }
  .containment {
      contain: layout;
  }
  .float {
      box-sizing: border-box;
      border: 1px solid black;
  }
  .left {
      float: left;
      width: 1em;
      height: 2em;
  }
  .right {
      float: right;
      width: 1em;
      height: 3em;
  }
</style>

<body>
  <p>PASS if the boxes with dark colors are floating next to the boxes with
    light colors.</p>
  <div id="wrapper">
    <div>
      <div id="container" class="containment">
        <div><div class="float left" style="background: lightblue"></div></div>
        <div class="float right" style="background: lightyellow"></div>
        <div>XX XXX XXXX X</div>
      </div>
      <div class="float left" style="background: blue"></div>
      <div>
        <div>
          <div><div class="float right" style="background: yellow"></div></div>
          <div>XXX XXXXXX XXXX XX XX</div>
        </div>
        <div>XXXX XXXXX XX</div>
      </div>
      <div>XXX XXX XX XXXX</div>
    </div>
  </div>
  <script>
    window.addEventListener("load", async () => {
        container.classList.remove('containment');
        await waitForAtLeastOneFrame();
        takeScreenshot();
    });
  </script>
</body>
</html>