HTML/CSS在边菜单右侧添加下拉菜单

<ul class="menu">
  <li><a href="#">菜单项1</a></li>
  <li><a href="#">菜单项2</a></li>
  <li class="dropdown">
    <a href="#">菜单项3</a>
    <ul class="dropdown-menu">
      <li><a href="#">下拉菜单项1</a></li>
      <li><a href="#">下拉菜单项2</a></li>
      <li><a href="#">下拉菜单项3</a></li>
    </ul>
  </li>
  <li><a href="#">菜单项4</a></li>
</ul>

.menu {
  list-style-type: none;
  margin: 0;
  padding: 0;
}

.menu li {
  display: inline-block;
}

.menu li a {
  display: block;
  padding: 10px;
  text-decoration: none;
  color: #333;
}

.dropdown-menu {
  display: none;
  position: absolute;
  background-color: #fff;
  padding: 10px;
}

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