HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>fs class sync</title>
  <style>
    #fs { padding: 24px; border: 2px solid #333; background: #eee; }
    #fs.is-fullscreen { background: #1a1a2e; color: #fff; }
  </style>
</head>
<body>
  <div id="fs">
    <p>Class sync demo</p>
    <button type="button" id="toggle">Enter fullscreen</button>
  </div>
  <script>
    var el = document.getElementById("fs");
    var btn = document.getElementById("toggle");
    function syncClass() {
      el.classList.toggle("is-fullscreen", Boolean(document.fullscreenElement));
      btn.textContent = document.fullscreenElement
        ? "Exit fullscreen"
        : "Enter fullscreen";
    }
    el.addEventListener("fullscreenchange", syncClass);
    btn.addEventListener("click", function () {
      if (document.fullscreenElement) {
        document.exitFullscreen();
      } else {
        el.requestFullscreen();
      }
    });
  </script>
</body>
</html>

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Element fullscreenchange — CSS class sync (Try It)