Authors might be able edit the text files within that container zip file using their chosen editor (this would be application specific), so making the files inside the container easy to read and write is important. We can probably do better than using plain CSS too. Perhaps a preprocessor could be used instead such as SASS (original, non-SCSS syntax) which uses indention instead of curly braces/semicolons - closer to Markdown’s philosophy.
Before getting caught up on details, it would be worth coming up with some use cases for a container specification to give it some real world context and scope. Perhaps another topic should be started for brainstorming this.