用纯 CSS 创建一个三角形的原理是什么？

span {
  width: 0;
  height: 0;
  border-top: 40px solid transparent;
  border-left: 40px solid transparent;
  border-right: 40px solid transparent;
  border-bottom: 40px solid #ff0000;
}
column1 | column2 | column3  
------- | ------- | -------  
column1 | column2 | column3  
column1 | column2 | column3  
column1 | column2 | column3  

![image.png](https://ucc.alicdn.com/pic/developer-ecology/a509187ad1b0467b80ca775c96fc20b8.png)