<div class="tab">
      <div class="tab-item">
        <router-link to="/">商品</router-link>
      </div>
      <div class="tab-item">
        <router-link to="/ratings">评价</router-link>
      </div>
      <div class="tab-item">
        <router-link to="/seller">商家</router-link>
      </div>
    </div>

 .tab {
    display: flex;
    width: 100%;
    height: 40px;
    line-height: 40px;
    border:none;
    position:relative;
  }
  .tab:after{
    content: '';
    position: absolute;
    bottom:0;
    left: 0;
    background: rgba(7, 17, 27, 0.1);
    width: 100%;
    height: 1px;
    -webkit-transform: scaleY(0.5);
    transform: scaleY(0.5);
    -webkit-transform-origin: 0 0;
    transform-origin: 0 0;
  }