画一个三角形

1、设置宽高，然后用border去画 

width: 0;
height: 0;
border-bottom: 50px solid yellow;border-top: 50px solid blue;border-left: 50px solid red;border-right: 50px solid green;

2、设置宽高，然后用border-width去画 

border-color: yellow blue red green;
border-style: solid;
border-width: 20px 20px 20px 20px;
height: 0;
width: 0;