note: As I understand things nothing that can be covered by Copyright related laws starts life automatically as Public Domain, the creator would need to actively choose to make it such.
The contents of a generated Story HTML file can basically be broken down into two main parts:
1. The Story Format template.
2. The Passage Meta-data.
The content within this area can be futhur broken down into three sub-parts:
2a. Content / code written by the project's Author.
This content is generally automatically covered by Copyright laws, how the Author chooses to licence it is left up to them.
2b. Third-party code written by someone else which project's Author has included.
The Copyright licence(s) used to cover such content is controlled by the original Developers of said third-party code, however such licences are generally fairly permissive.