纯CSS实现响应式导航栏的下拉子菜单效果的实现步骤

<nav class="navbar">
  <ul class="navbar-nav">
    <li class="nav-item">
      <a href="#" class="nav-link">首页</a>
    </li>
    <li class="nav-item">
      <a href="#" class="nav-link">产品</a>
      <ul class="sub-menu">
        <li><a href="#">产品一</a></li>
        <li><a href="#">产品二</a></li>
        <li><a href="#">产品三</a></li>
      </ul>
    </li>
    <li class="nav-item">
      <a href="#" class="nav-link">关于我们</a>
    </li>
    <li class="nav-item">
      <a href="#" class="nav-link">联系我们</a>
    </li>
  </ul>
</nav>

.navbar {
  background-color: #333;
  height: 60px;
  padding: 0 20px;
}

.navbar-nav {
  list-style-type: none;
  margin: 0;
  padding: 0;
  display: flex;
  align-items: center;
}

.nav-item {
  position: relative;
}

.nav-link {
  color: #fff;
  text-decoration: none;
  padding: 10px;
}

.sub-menu {
  display: none;
  position: absolute;
  top: 100%;
  left: 0;
  background-color: #333;
  list-style-type: none;
  width: 200px;
  padding: 0;
  margin: 0;
}

.nav-item:hover .sub-menu {
  display: block;
}

.sub-menu li {
  padding: 10px;
}

.sub-menu li a {
  color: #fff;
  text-decoration: none;
}

@media screen and (max-width: 768px) {
  .navbar-nav {
    flex-wrap: wrap;
  }
  
  .nav-link {
    padding: 10px 0;
  }
  
  .sub-menu {
    position: static;
    display: block;
    background-color: transparent;
    width: 100%;
  }
  
  .sub-menu li {
    padding: 10px;
    border-top: 1px solid #fff;
  }
}