You can delete this, or notify me to delete this if this is out of the scope of this forum. It might be but I can't help but feel you will have the answer.
It's very much out of scope for the language itself; that's something for the HTML spec to decide.
The language says that an import evaluates JS code, and when it can, and what a Module Namespace object must look like - iow, on the importing side, you shouldn't necessarily be able to determine what kind of module you're importing.
Worth checking out: GitHub - tc39/proposal-import-assertions: Proposal for syntax to import ES modules with assertions
That links to CSS and HTML module proposals (not as part of ECMAScript) at the bottom of the motivation section.
Okay, my apologies for the post delete if you like.