三角形实现原理：宽度width为0；height为0；（1）有一条横竖边（上下左右）的设置为border-方向：长度 solid red，这个画的就是底部的直线。其他边使用border-方向：长度 solid transparent。（2）有两个横竖边（上下左右）的设置，若斜边是在三角形的右边，这时候设置top或bottom的直线，和右边的斜线。若斜边是在三角形的左边，这时候设置top或bottom的直线，和左边的斜线。
二、实现
2.1 Triangle Up
#triangle-up {
width:
0;
height: 0;
border-left: 50px solid transparent;
border-right: 50px solid transparent;
border-bottom: 100px solid red;
}
2.2 Triangle Down
#triangle-down {
width:
0;
height: 0;
border-left: 50px solid transparent;
border-right: 50px solid transparent;
border-top: 100px solid red;
}
2.3 Triangle Left
#triangle-left {
width:
0;
height: 0;
border-top: 50px solid transparent;
border-right: 100px solid red;
border-bottom: 50px solid transparent;
}
2.4 Triangle Right
#triangle-right {
width:
0;
height: 0;
border-top: 50px solid transparent;
border-left: 100px solid red;
border-bottom: 50px solid transparent;
}
2.5 Triangle Top Left
#triangle-topleft {
width:
0;
height: 0;
border-top: 100px solid red;
border-right: 100px solid transparent;
}
2.6 Triangle Top Right
#triangle-topright {
width:
0;
height: 0;
border-top: 100px solid red;
border-left: 100px solid transparent;
}
2.7 Triangle Bottom Left
#triangle-bottomleft {
width:
0;
height: 0;
border-bottom: 100px solid red;
border-right: 100px solid transparent;
}
2.8 Triangle Bottom Right
#triangle-bottomright {
width:
0;
height: 0;
border-bottom: 100px solid red;
border-left: 100px solid transparent;
}
