Instead of requiring images in the caching we could create base64 encoded data URIs and add those to the css file. I would be willing to do this as long as it's something you are looking for.
Thanks for the feedback. I've rewritten the codelab from scratch and believe I have fixed this issue. The code has been updated, and the new guide is now live.
If you find any issues with the new codelab, please let us know!
Instead of requiring images in the caching we could create base64 encoded data URIs and add those to the css file. I would be willing to do this as long as it's something you are looking for.