代码实现

.byy-edge{
	position: absolute;
    width: 0;
    height: 0;
    border-style: dashed;
    border-color: transparent;
    overflow: hidden;
}
.byy-edge.top{
	border-top: 5px solid #555;
}
.byy-edge.bottom{
	border-bottom: 5px solid #555;
}
.byy-edge.left{
	border-left: 5px solid #555;
}
.byy-edge.right{
	border-right: 5px solid #555;
}