大致思路给伪类做成小三角之后旋转45度 给伪类加阴影：
.detail_container::after{
width: 0;
height: 0;
border-style: solid;
border-width: 5px;
border-color: #fff #fff transparent transparent;
transform: rotate(45deg);
box-shadow: 1px -1px 0.5px rgba(155, 155, 155, 0.5);
// box-shadow: 0 2px 5px 0 rgba(155, 155, 155, 0.5);
content: "";
position: absolute;
top: 14px;
right: -5px;
}