4.28

<style>
.dropdown {
  position: relative;
  display: inline-block;
}
.dropdown-content {
  display: none;
  position: absolute;
  background-color: #f9f9f9;
  min-width: 160px;
  box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
  padding: 12px 16px;
}
.dropdown:hover .dropdown-content {
  display: block;
}
</style>
<div class="dropdown">
  <span>鼠标移动到我这！</span>
  <div class="dropdown-content">
    <p>菜鸟教程</p>
    <p>www.runoob.com</p>
  </div>
</div>