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HTML 

              
                <!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Critical CSS Demo</title>
  <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;600&display=swap" rel="stylesheet">

    <!-- Critical CSS (inline) -->
  <style>
    body {
      margin: 0;
      font-family: 'Inter', sans-serif;
      background-color: #fff;
      color: #111;
    }

    header {
      display: flex;
      justify-content: space-between;
      align-items: center;
      padding: 1.5rem;
      background: #fafafa;
      box-shadow: 0 2px 5px rgba(0,0,0,0.05);
    }

    .logo {
      font-weight: 600;
      font-size: 1.2rem;
    }

    nav a {
      margin-left: 1rem;
      text-decoration: none;
      color: #000;
      font-weight: 500;
    }

    .hero {
      padding: 3rem 1.5rem;
      background-color: #f5f5f5;
      text-align: center;
      border-bottom: 1px solid #ddd;
    }

    .hero h1 {
      font-size: 2.5rem;
      margin-bottom: 1rem;
    }

    .hero button {
      padding: 0.75rem 1.5rem;
      font-size: 1rem;
      border: none;
      background-color: #000;
      color: #fff;
      cursor: pointer;
      border-radius: 4px;
    }
  </style>

</head>
<body>

  <header>
    <div class="logo">MySite</div>
    <nav>
      <a href="#">Home</a>
      <a href="#">Blog</a>
      <a href="#">Contact</a>
    </nav>
  </header>

  <section class="hero">
    <h1>Fast. Clean. Critical CSS.</h1>
    <button id="toggleStyles">Load full styles</button>
  </section>

  <section class="extras" id="extras">
    <div style="padding: 2rem; text-align: center;">
      <p>This content gets prettier once full CSS is loaded.</p>
    </div>
    <div class="product-grid">
      <div class="product"></div>
      <div class="product"></div>
      <div class="product"></div>
      <div class="product"></div>
    </div>
  </section>


</body>
</html>
!

CSS 

              
                    body {
      margin: 0;
      font-family: 'Inter', sans-serif;
      background-color: #fff;
      color: #111;
      transition: all 0.3s ease;
    }

    header {
      display: flex;
      justify-content: space-between;
      align-items: center;
      padding: 1.5rem;
      background: #fafafa;
      box-shadow: 0 2px 5px rgba(0,0,0,0.05);
    }

    .logo {
      font-weight: 600;
      font-size: 1.2rem;
    }

    nav a {
      margin-left: 1rem;
      text-decoration: none;
      color: #000;
      font-weight: 500;
    }

    .hero {
      padding: 3rem 1.5rem;
      background-color: #f5f5f5;
      text-align: center;
      border-bottom: 1px solid #ddd;
    }

    .hero h1 {
      font-size: 2.5rem;
      margin-bottom: 1rem;
    }

    .hero button {
      padding: 0.75rem 1.5rem;
      font-size: 1rem;
      border: none;
      background-color: #000;
      color: #fff;
      cursor: pointer;
      border-radius: 4px;
      transition: background 0.2s ease;
    }

    .hero button:hover {
      background-color: #333;
    }

    .extras {
      opacity: 1;
      transform: translateY(0);
      transition: all 0.4s ease;
    }

    .product-grid {
      display: grid;
      grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
      gap: 1.5rem;
      padding: 2rem;
      justify-content: center;
      max-width: 880px;       
  margin: 0 auto;
    }

    .product {
      background: none;
      border: 2px dashed #ccc;
      border-radius: 8px;
      padding: 1rem;
      height: 220px;
      display: flex;
      flex-direction: column;
      justify-content: center;
      align-items: center;
      overflow: hidden;
    }

    .product.loaded {
      background: #fff;
      border: none;
      color: #111;
      font-style: normal;
      box-shadow: 0 2px 8px rgba(0,0,0,0.1);
    }

    .product.loaded img {
      width: 100%;
      border-radius: 6px;
      margin-bottom: 0.5rem;
      filter: blur(10px);
      opacity: 0.5;
      transition: filter 0.8s ease, opacity 0.8s ease;
    }

    .product.loaded img.reveal {
      filter: blur(0);
      opacity: 1;
    }

    .product.loaded h3 {
      margin: 0.5rem 0 0.25rem;
      opacity: 0;
      transform: translateY(10px);
      transition: all 0.4s ease 0.3s;
    }

    .product.loaded h3.reveal,
    .product.loaded p.reveal {
      opacity: 1;
      transform: translateY(0);
    }

    .product.loaded p {
      opacity: 0;
      transform: translateY(10px);
      transition: all 0.4s ease 0.4s;
    }
!

JS 

              
                  const toggleBtn = document.getElementById('toggleStyles');
    let fullStylesLoaded = false;
    let styleLink = null;
    const originalProducts = document.querySelectorAll('.product');

    const images = [237, 238, 239, 240];

    function loadFullStyles() {
      styleLink = document.createElement('link');
      styleLink.rel = 'stylesheet';
      styleLink.href = 'https://unpkg.com/@fontsource/roboto/400.css';
      document.head.appendChild(styleLink);

      originalProducts.forEach((product, index) => {
        setTimeout(() => {
          product.classList.add('loaded');
          product.innerHTML = `
            <img src="https://picsum.photos/id/${images[index]}/200/120" alt="Product image">
            <h3>Product ${index + 1}</h3>
            <p>This is a great product that just got styled.</p>
          `;

          const img = product.querySelector('img');
          const h3 = product.querySelector('h3');
          const p = product.querySelector('p');
          setTimeout(() => {
            img.classList.add('reveal');
            h3.classList.add('reveal');
            p.classList.add('reveal');
          }, 50);

        }, 500 + index * 400);
      });

      toggleBtn.textContent = 'Undo full styles';
      fullStylesLoaded = true;
    }

    function undoFullStyles() {
      if (styleLink) {
        document.head.removeChild(styleLink);
        styleLink = null;
      }

      originalProducts.forEach((product) => {
        product.className = 'product';
        product.innerHTML = '';
      });

      toggleBtn.textContent = 'Load full styles';
      fullStylesLoaded = false;
    }

    toggleBtn.addEventListener('click', () => {
      if (!fullStylesLoaded) {
        loadFullStyles();
      } else {
        undoFullStyles();
      }
    });
!
999px

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