/* ===== CÀI ĐẶT CHUNG ===== */
* {
    box-sizing: border-box;
  }
  
  body {
    font-family: Arial, sans-serif;
    margin: 0;
    padding-bottom: 200px;
  }
  
  section {
    padding: 60px;
    border-bottom: 2px dashed #ccc;
  }
  
  h2 {
    margin-bottom: 30px;
  }
  
  /* ===== BÀI 1: NÚT HOVER ĐỔI MÀU ===== */
  .btn {
    display: inline-block;
    padding: 12px 30px;
    margin: 10px;
    background: #3498db;
    color: white;
    border: none;
    border-radius: 25px;
    cursor: pointer;
    transition: background-color 0.5s ease, color 0.5s ease;
  }
  
  .btn:hover {
    background: orange;
    color: white;
  }
  
  /* ===== BÀI 2: GALLERY ẢNH HOVER ===== */
  .gallery {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 20px;
  }
  
  .gallery figure {
    overflow: hidden;
    border-radius: 10px;
  }
  
  .gallery img {
    width: 100%;
    height: 200px;
    object-fit: cover;
    transform: scale(1);
    transition: transform 0.4s ease, box-shadow 0.4s ease;
  }
  
  .gallery img:hover {
    transform: scale(1.1);
    box-shadow: 0 10px 20px rgba(0,0,0,0.3);
  }
  
  /* ===== BÀI 3: MENU UNDERLINE ===== */
  .nav {
    display: flex;
    gap: 30px;
    list-style: none;
    padding: 0;
  }
  
  .nav a {
    position: relative;
    text-decoration: none;
    color: #333;
    font-weight: bold;
  }
  
  .nav a::after {
    content: "";
    position: absolute;
    left: 0;
    bottom: -5px;
    height: 2px;
    width: 0;
    background: #3498db;
    transition: width 0.3s ease;
  }
  
  .nav a:hover::after {
    width: 100%;
  }
  
  /* ===== BÀI 4: CARD NỔI LÊN ===== */
  .card-container {
    display: flex;
    gap: 30px;
  }
  
  .card {
    width: 250px;
    border-radius: 12px;
    overflow: hidden;
    box-shadow: 0 4px 8px rgba(0,0,0,0.15);
    transition: transform 0.3s ease, box-shadow 0.3s ease;
  }
  
  .card:hover {
    transform: translateY(-10px);
    box-shadow: 0 12px 25px rgba(0,0,0,0.3);
  }
  
  .card img {
    width: 100%;
    height: 150px;
    object-fit: cover;
  }
  
  .card .content {
    padding: 15px;
  }
  
  /* ===== BÀI 5: BANNER FADE-IN ===== */
  .banner {
    height: 300px;
    background: linear-gradient(135deg, #3498db, #6dd5fa);
    display: flex;
    align-items: center;
    justify-content: center;
  }
  
  .hero-title {
    font-size: 36px;
    color: white;
    opacity: 0;
    animation: fadeIn 2s ease forwards;
  }
  
  @keyframes fadeIn {
    from { opacity: 0; }
    to   { opacity: 1; }
  }