css二级导航代码

<nav class="main-nav">
  <ul>
    <li><a href="#">首页</a></li>
    <li>
      <a href="#">产品</a>
      <ul class="sub-menu">
        <li><a href="#">产品1</a></li>
        <li><a href="#">产品2</a></li>
        <li><a href="#">产品3</a></li>
      </ul>
    </li>
    <li><a href="#">关于我们</a></li>
    <li><a href="#">联系我们</a></li>
  </ul>
</nav>

.main-nav ul {
  list-style: none;
  padding: 0;
  margin: 0;
}

.main-nav li {
  display: inline-block;
  position: relative;
}

.main-nav a {
  display: block;
  padding: 10px 20px;
  text-decoration: none;
  color: #333;
}

.sub-menu {
  display: none;
  position: absolute;
  top: 100%;
  left: 0;
  background-color: #fff;
  border: 1px solid #ccc;
  box-shadow: 0 2px 5px rgba(0,0,0,0.2);
}

.sub-menu li {
  display: block;
  width: 200px;
}

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