css导航栏下划线跟随效果

body {
    margin: 0;
    padding: 0;
  }

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

ul {
  height: 50px;
  line-height: 50px;
  background-color: #f5f5f5;
  border-radius: 8px;
}

ul li {
  display: inline-block;
  padding: 0 20px;
  position: relative;
  cursor: pointer;
}

li::before {
  content: "";
  position: absolute;
  top: 0;
  left: 100%;
  width: 0%;
  height: 100%;
  border-bottom: 2px solid #000;
  transition: all 0.2s linear;
}

li:hover::before {
  left: 0;
  width: 100%;
}

li:hover~li::before {
  left: 0;
}

li:active {
  background: #000;
  color: #fff;
}

<ul>
  <li>不可思议的CSS</li>
  <li>导航栏</li>
  <li>光标小下划线跟随</li>
  <li>PURE CSS</li>
  <li>Nav Underline</li>
</ul>