将幻灯片向下添加到悬停时的导航栏

<nav>
  <ul>
    <li><a href="#">首页</a></li>
    <li><a href="#">关于我们</a></li>
    <li class="dropdown">
      <a href="#">幻灯片</a>
      <div class="dropdown-content">
        <!-- 幻灯片内容 -->
      </div>
    </li>
    <li><a href="#">联系我们</a></li>
  </ul>
</nav>

nav {
  background-color: #f1f1f1;
  padding: 10px;
}

ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
  overflow: hidden;
}

li {
  float: left;
}

li a {
  display: block;
  color: #333;
  text-align: center;
  padding: 14px 16px;
  text-decoration: none;
}

li.dropdown {
  position: relative;
}

.dropdown-content {
  display: none;
  position: absolute;
  background-color: #f9f9f9;
  min-width: 160px;
  box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
  z-index: 1;
}

.dropdown-content a {
  color: #333;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
}

li.dropdown:hover .dropdown-content {
  display: block;
}

var dropdown = document.getElementsByClassName("dropdown")[0];
dropdown.addEventListener("mouseover", function() {
  var dropdownContent = this.getElementsByClassName("dropdown-content")[0];
  dropdownContent.style.display = "block";
});

dropdown.addEventListener("mouseout", function() {
  var dropdownContent = this.getElementsByClassName("dropdown-content")[0];
  dropdownContent.style.display = "none";
});