after与before应用

.box li:hover {outline: 1px solid #0cf}
.box li:after {
position: absolute;
display: block;
border-top: 28px solid #4390df;  //三角形设置
border-left: 28px solid transparent;
right: 0;  //位置设置。记得li要设置相对定位
top: 0;
content: "";
z-index: 101;
}
.box li:hover:before {  //打钩实现，当这里设置为after时，则上面要换成before
position: absolute;
display: block;
content: "√";
color: #fff;
font-size: 20px;
right: 0;top: 0;
z-index: 102;
}