写一个div
<div class="triangle"></div>
1.向上
.triangle{
width:0;
height:0;
border-right:50px solid transparent;
border-left:50px solid transparent;
border-bottom:50px solid red;
}
2.向下
.triangle{
width:0;
height:0;
border-right:50px solid transparent;
border-left:50px solid transparent;
border-top:50px solid red;
}
3.向左
.triangle{
width:0;
height:0;
border-top:50px solid transparent;
border-bottom:50px solid transparent;
border-right:50px solid red;
}
4.向右
.triangle{
width:0;
height:0;
border-top:50px solid transparent;
border-bottom:50px solid transparent;
border-left:50px solid red;
}