- The HTML
- The JS
- The CSS
In some sort of a structured way that's easy to load and distribute as a library. I don't like that to use webcomponents the right way, you have to load the logic from an import and then inline the html aspects of it.
There are userland single-file component projects, like my Heximal[2] project. Though Hexiaml specifically doesn't tackle the module format - it requires some kind of HTML Module polyfill.
I don't think the current situation is so bad though. The container format should mostly be irrelevant to consumers of a component. You need to import the component defintion whether that definition is in a .js file or .html file. Once you import it you use it the same.
There should be very few cases where the file format matters. Maybe for use when JS is turned off if HTML-based web components are considered safe to evaluate (their templates might expressive enough to potentially allow attacks).
[1]: https://github.com/WICG/webcomponents/issues/645
[2]: https://heximal.dev/ (Sorry the site is messed up on mobile. I need to fix it!)