HTML Code

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

    .slide {
      width: 280px;
      height: 160px;
      background: #e2e8f0;
      border-radius: 0.5rem;
      display: flex;
      align-items: center;
      justify-content: center;
      font-weight: 600;
      color: #334155;
      margin-bottom: 1rem;
    }

    .slide:fullscreen {
      width: 100vw;
      height: 100vh;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      background: linear-gradient(135deg, #1e3a8a, #312e81);
      color: #fff;
      font-size: clamp(1.5rem, 4vw, 3rem);
      border-radius: 0;
    }

    .slide:fullscreen p {
      font-size: 0.5em;
      opacity: 0.85;
      margin-top: 0.5rem;
    }

    button {
      padding: 0.6rem 1.2rem;
      background: #2563eb;
      color: #fff;
      border: none;
      border-radius: 0.5rem;
      font-weight: 600;
      cursor: pointer;
    }
  </style>
</head>
<body>
  <div class="slide" id="slide">
    Presentation Slide
    <p>Subtitle text here</p>
  </div>
  <button type="button" id="go-fs">Enter Fullscreen</button>

  <script>
    const slide = document.getElementById('slide');
    document.getElementById('go-fs').addEventListener('click', () => {
      if (document.fullscreenElement) {
        document.exitFullscreen();
      } else {
        slide.requestFullscreen();
      }
    });
  </script>
</body>
</html>

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CSS :fullscreen Selector - Flex Centering