If this shows itself to be highly useful as a concept, then I would perhaps avoid reinventing the wheel on the file format side of things, and just standardize what we already have:
- Come up with a file extension (.hmml)
- Decide on an entrypoint filename and format (index.html)
- Use an existing standard for combining resources into one file (tar + zstd)
Now you have something that is usable only using pre-existing tools.