SOURCE

<div class="card-container">
  <div class="main-card">
    <h3>VIP</h3>
    <p>购买VIP可享优惠...</p>

    <div class="sale-card">促销</div>
  </div>
</div>

.card-container {
    position: relative;
    width: 300px;
    margin: 50px;
}

.main-card {
    background: #ffdac7;
    border-radius: 8px;
    padding: 20px;
    box-shadow: 0 4px 8px rgba(0,0,0,0.1);
    min-height: 100px;
    position: relative;
    z-index: 0;
    /* 为了保证after伪元素在主元素的底层，需要将主元素转成3D，通过translateZ控制图层 */
    transform-style: preserve-3d;
}

.main-card::after {
    position: absolute;
    content: '';
    top: calc(50% - 30px);
    right: -6px;
    width: 0;
    height: 0;
    /* 想要两个直角边不等长需要用到scaleX变形一下 */
    transform: translateY(-50%) translateZ(-100px) scaleX(2);
    border-right: 10px solid transparent;
    border-top: 10px solid transparent;
    border-left: 10px solid #98491f;
    border-bottom: 10px solid #98491f;
}

.sale-card{
    position: absolute;
    top: 50%;
    right: -15px;
    background: #ff8645;
    padding: 0 20px;
    height: 40px;
    display: flex;
    align-items: center;
    border-radius: 8px 0 0 8px;
    transform: translateY(-50%);
}