<n*>
  <ul class="menu">
    <li><a href="#">首页</a></li>
    <li class="dropdown">
      <a href="#" class="dropbtn">产品</a>
      <ul class="dropdown-menu">
        <li><a href="#">电子产品</a></li>
        <li><a href="#">服装</a></li>
        <li><a href="#">图书</a></li>
      </ul>
    </li>
    <li><a href="#">关于我们</a></li>
  </ul>
</n*>

/* 清除默认样式 */
.menu {
  list-style: none;
  margin: 0;
  padding: 0;
  background-color: #333;
  display: flex;
}
<p>.menu > li {
position: relative;
}</p><p>.menu a {
display: block;
color: white;
text-decoration: none;
padding: 12px 16px;
}</p><p>.menu a:hover {
background-color: #555;
}</p><p>/<em> 隐藏下拉菜单 </em>/
.dropdown-menu {
list-style: none;
margin: 0;
padding: 0;
position: absolute;
top: 100%;
left: 0;
width: 200px;
background-color: #444;
box-shadow: 0 4px 6px rgba(0,0,0,0.1);
opacity: 0;
visibility: hidden;
transition: opacity 0.3s ease;
}</p><p>/<em> 悬停时显示下拉菜单 </em>/
.dropdown:hover .dropdown-menu {
opacity: 1;
visibility: visible;
}</p><p>/<em> 下拉项样式 </em>/
.dropdown-menu li a {
padding: 10px 16px;
}</p><p>.dropdown-menu li a:hover {
background-color: #666;
}</p>