向上的三角形
#triangle-up{
width:0;
height:0;
border-left:50px solid transparent;
border-right:50px solid transparent;
border-bottom:100px solid black;
}
向下的三角形
#triangle-down{
width:0;
height:0;
border-left:50px solid transparent;
border-right:50px solid transparent;
border-top:100px solid black;
}
向左的三角形
#triangle-left{
width:0;
height:0;
border-top:50px solid transparent;
border-bottom:50px solid transparent;
border-right:100px solid black;
}
向右的三角形
#triangle-right{
width:0;
height:0;
border-top:50px solid transparent;
border-bottom:50px solid transparent;
border-left:100px solid black;
}