<n*>
  <ul class="n*bar">
    <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="#">服装服饰</a></li>
        <li><a href="#">家居用品</a></li>
      </ul>
    </li>
    <li><a href="#">关于我们</a></li>
    <li><a href="#">联系我们</a></li>
  </ul>
</n*>

.n*bar {
  list-style: none;
  margin: 0;
  padding: 0;
  background: #333;
  overflow: hidden;
}

.n*bar > li {
  float: left;
  position: relative;
}

.n*bar a {
  display: block;
  color: white;
  text-decoration: none;
  padding: 14px 20px;
}

.dropdown:hover .dropdown-menu {
  display: block;
}

.dropdown-menu {
  list-style: none;
  margin: 0;
  padding: 0;
  position: absolute;
  top: 100%;
  left: 0;
  background: #444;
  min-width: 160px;
  display: none;
  z-index: 1;
}

.dropdown-menu li a {
  padding: 12px 16px;
}

.dropdown-menu li a:hover {
  background: #555;
}

.dropdown-menu {
  opacity: 0;
  visibility: hidden;
  transition: opacity 0.3s ease, visibility 0.3s ease;
}

.dropdown:hover .dropdown-menu {
  display: block;
  opacity: 1;
  visibility: visible;
}