/* 初始状态：透明 + 下移20px */
  .fade-in-section {
    opacity: 0;
    transform: translateY(20px);
    transition: opacity 0.6s ease-out, transform 0.6s ease-out;
    will-change: opacity, transform;
  }
  
  /* 进入视口后的状态 */
  .fade-in-section.is-visible {
    opacity: 1;
    transform: translateY(0);
  }
  
  /* 可选：给不同元素添加延迟，产生错落感 */
  .fade-in-section.delay-1 { transition-delay: 0.1s; }
  .fade-in-section.delay-2 { transition-delay: 0.2s; }
  .fade-in-section.delay-3 { transition-delay: 0.3s; }