HTML Code

<!DOCTYPE html>
<html>
<head>
  <title>CSS :fullscreen Div Example</title>
  <style>
    body {
      font-family: system-ui, sans-serif;
      padding: 1.5rem;
    }

    #fullscreen-element {
      width: 300px;
      height: 200px;
      background: #93c5fd;
      color: #1e3a8a;
      display: flex;
      align-items: center;
      justify-content: center;
      border-radius: 0.5rem;
      font-weight: 600;
      margin-bottom: 1rem;
    }

    :fullscreen {
      width: 100vw;
      height: 100vh;
      background: #1e3a8a;
      color: #fff;
      font-size: 2em;
      border-radius: 0;
    }

    button {
      padding: 0.6rem 1.2rem;
      background: #2563eb;
      color: #fff;
      border: none;
      border-radius: 0.5rem;
      font-weight: 600;
      cursor: pointer;
      font-size: 1rem;
    }
  </style>
</head>
<body>
  <div id="fullscreen-element">Click to Go Fullscreen</div>
  <button id="fullscreen-btn" type="button">Enter Fullscreen</button>

  <script>
    const el = document.getElementById('fullscreen-element');
    document.getElementById('fullscreen-btn').addEventListener('click', () => {
      if (document.fullscreenElement) {
        document.exitFullscreen();
      } else {
        el.requestFullscreen();
      }
    });
  </script>
</body>
</html>

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CSS :fullscreen Selector - Div Example