3、用CSS画三角形

一篇技巧理解
https://www.jianshu.com/p/fe2074729251

1、有一条横竖边（上下左右）的设置为border-方向
原原点在横竖边对着的顶点上。
1.1 Triangle Up

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

1.2 Triangle Down

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

1.3 Triangle Left

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

1.4 Triangle Right

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

2、有两个横竖边（上下左右）的设置，若斜边是在三角形的右边
原原点在底边的任意一个顶点
2.1 Triangle Top Left

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

2.2 Triangle Top Right

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

2.3 Triangle Bottom Left

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

2.4 Triangle Bottom Right

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

3、 其它图形

3.1 圆形

#circle {
  width: 100px;
  height: 100px;
  background: red;
  border-radius: 50%
}

3.2 半圆

.semi-circle{
  width:100px;
  height:50px; 
  background-color:#cb18f8;
  border-radius:50px 50px 0 0; /* 左上、右上、右下、左下 */
}

3.3 椭圆

#oval {
  width: 200px;
  height: 100px;
  background: red;
  border-radius: 100px / 50px;
}

3.4 梯形

#trapezoid {
  border-bottom: 100px solid red;
  border-left: 25px solid transparent;
  border-right: 25px solid transparent;
  height: 0;
  width: 100px;
}