css中多级导航栏的平滑悬停效果

<nav>
  <ul>
    <li><a href="#">菜单项1</a>
      <ul>
        <li><a href="#">子菜单项1</a></li>
        <li><a href="#">子菜单项2</a></li>
        <li><a href="#">子菜单项3</a></li>
      </ul>
    </li>
    <li><a href="#">菜单项2</a></li>
    <li><a href="#">菜单项3</a></li>
  </ul>
</nav>

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

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

nav ul li a {
  display: block;
  padding: 10px;
  text-decoration: none;
  color: #000;
  transition: background-color 0.3s ease;
}

nav ul li:hover > a {
  background-color: #f0f0f0;
}

nav ul ul {
  display: none;
  position: absolute;
  top: 100%;
  left: 0;
  background-color: #fff;
  padding: 0;
}

nav ul li:hover > ul {
  display: inherit;
}

nav ul ul li {
  width: 200px;
  float: none;
  display: list-item;
  position: relative;
}

nav ul ul ul li {
  position: relative;
  top: -60px;
  left: 200px;
}

nav ul ul li a {
  padding: 10px;
  color: #000;
  background-color: #fff;
  transition: background-color 0.3s ease;
}

nav ul ul li:hover > a {
  background-color: #f0f0f0;
}

nav ul ul ul {
  display: none;
  position: absolute;
  left: 100%;
  top: 0;
  background-color: #fff;
  padding: 0;
}

nav ul ul li:hover > ul {
  display: inherit;
}