如何使导航栏元素变暗，悬停列表项除外

<nav>
  <ul>
    <li><a href="#">首页</a></li>
    <li><a href="#">关于</a></li>
    <li><a href="#">服务</a></li>
    <li><a href="#">联系</a></li>
  </ul>
</nav>

nav ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
  background-color: #f1f1f1;
}

nav li {
  display: inline-block;
}

nav li a {
  display: block;
  padding: 10px 20px;
  text-decoration: none;
  color: #333333;
  transition: background-color 0.3s ease;
}

nav li a:hover {
  background-color: #999999;
  color: #ffffff;
}