It's not immediately clear what this does do your HTML/CSS files. It's sort of hinted at in the logo image, but the (many) examples would be much more helpful if some sample input and output text were provided.

I'm not convinced the use case of sticking CDNs in front of URLs is broad or complex enough to justify bringing in another tool (e.g. I'm pretty sure Rails does this out of the box). That said, I could see this growing into a general-purpose URL transformer.