I agree that browsers should offer an API in order to get all the resources easily. It would make things much easier. However, for security reasons, this API would still be restricted to environments like Web Extensions or automation tools like Playwright. FYI, I also took another approach that might interest you, see https://github.com/gildas-lormeau/SingleFileZ. The main drawback is that the HTML produced by SingleFileZ is not valid (but tolerated by the HTML5 spec.).