<n* class="n*bar">
  <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*bar {
  display: flex;
  gap: 30px;
  padding: 20px;
  background: #fff;
}

.n*-link {
  text-decoration: none;
  color: #333;
  position: relative;
  padding: 10px 0;
}

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

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

.n*-link::after {
  left: 50%;
  transform: translateX(-50%) scaleX(0);
  transform-origin: center;
  transition: transform 0.3s ease;
}

.n*-link:hover::after {
  transform: translateX(-50%) scaleX(1);
}