<n* class="n*-menu">
  <a href="#" class="n*-link">首页</a>
  <a href="#" class="n*-link">关于</a>
  <a href="#" class="n*-link">服务</a>
  <a href="#" class="n*-link">联系</a>
</n*>

.n*-link {
  text-decoration: none;
  color: #333;
  display: inline-block;
  position: relative;
  padding: 10px 15px;
}

.n*-link::after {
  content: '';
  position: absolute;
  bottom: 0;
  left: 0;
  width: 0;
  height: 2px;
  background-color: #007acc;
  transition: width 0.3s ease;
}

@keyframes underline-slide {
  from {
    width: 0;
  }
  to {
    width: 100%;
  }
}

.n*-link:hover::after {
  animation: underline-slide 0.4s ease forwards;
}

.n*-link::after {
  content: '';
  position: absolute;
  bottom: 0;
  left: 0;
  width: 0;
  height: 2px;
  background-color: #007acc;
  transition: width 0.3s ease;
}

.n*-link:hover::after {
  width: 100%;
}