CSS能够生成各种形状。正方形和矩形很容易，因为它们是 web 的自然形状。添加宽度和高度，就得到了所需的精确大小的矩形。添加边框半径，你就可以把这个形状变成圆形，足够多的边框半径，你就可以把这些矩形变成圆形和椭圆形。

上三角

#triangle-up {
width: 0;
height: 0;
border-left: 50px solid transparent;
border-right: 50px solid transparent;
border-bottom: 100px solid red;
}

#triangle-down {
width: 0;
height: 0;
border-left: 50px solid transparent;
border-right: 50px solid transparent;
border-top: 100px solid red;
}

#triangle-left {
width: 0;
height: 0;
border-top: 50px solid transparent;
border-right: 100px solid red;
border-bottom: 50px solid transparent;
}

#triangle-right {
width: 0;
height: 0;
border-top: 50px solid transparent;
border-left: 100px solid red;
border-bottom: 50px solid transparent;
}

#triangle-left {
  width: 0;
  height: 0;
  border-top: 50px solid transparent;
  border-right: 100px solid red;
  border-bottom: 50px solid transparent;
}

#triangle-topright {
width: 0;
height: 0;
border-top: 100px solid red;
border-left: 100px solid transparent;
}

#triangle-bottomleft {
width: 0;
height: 0;
border-bottom: 100px solid red;
border-right: 100px solid transparent;
}

#triangle-bottomright {
width: 0;
height: 0;
border-bottom: 100px solid red;
border-left: 100px solid transparent;
}