1.向下的三角形
0;
height: 0;
border-left: 10px solid transparet;
border-right: 10px solid transparet;
border-top: 10px solid red;
2.向上的三角形
0;
height: 0;
border-left: 10px solid transparet;
border-right: 10px solid transparet;
border-bottom: 10px solid red;
3.向左的三角形
0;
height: 0;
border-top: 10px solid transparet;
border-bottom: 10px solid transparet;
border-right: 10px solid red;
4.向右的三角形
0;
height: 0;
border-top: 10px solid transparet;
border-bottom: 10px solid transparet;
border-left: 10px solid red;