SOURCE

<div class="container">
  <nav class="nav">
    <ul>
      <li class="nav-item">
        <a href="#" class="nav-item-a">前端样式</a>
        <div class="nav-item-underline"></div>
      </li>
    </ul>
  </nav>
  <button id="btn" class="btn">切换样式</button>
</div>

* {
  margin: 0;
  padding: 0;
}

li {
  list-style: none;
}

a {
  text-decoration: none;
}

body {
  display: flex;
}

.container {
  display: flex;
  flex-direction: column;
  width: 480px;
  margin: 0 auto;
  align-items: center;
  background: rgba(0,0,0,.1);
}

.nav {
  height: 80px;
  line-height: 80px;
}

.nav-item {
  position: relative;
  width: 80px;
  height: 60px;
  text-align: center;
}

.nav-item .nav-item-underline {
  position: absolute;
  margin: 0 auto;
  width: 0;
  height: 2px;
  /* letf: 0; */
  bottom: 0;
  background: blue;
  transition: .5s all;
}

.nav-item>a {
  display: block;
  color: #000;
  width: 100%;
  height: 100%;
  transition: .5s all;
}

.nav-item>a:hover {
  color: blue;
}

.nav-item:hover>.nav-item-underline {
  width: 100%;
}