1)绝对
http://www.example.com/images/icons.png
http://www.example.com/images/icons.png
2)相对
../images/icons.png
3)???
/images/icons.png
4)???
//www.example.com/images/icons.png
解决方法
>绝对
http://www.example.com/images/icons.png
> Document-Relative ../images/icons.png
> Root-Relative /images/icons.png
> Protocol-Relative //www.example.com/images/icons.png
> Document-Relative ../images/icons.png
> Root-Relative /images/icons.png
> Protocol-Relative //www.example.com/images/icons.png
对于#4,我也常常称它们为“Protocol-Agnostic”