I think the quickest way to say it is that there is only one head on a page, and every HTML file needs a head. So if you include one into the other, you either have two heads, or the inner document didn't have a head.
See DofumentFragment - sounds a lot like this: https://developer.mozilla.org/en-US/docs/Web/API/DocumentFra...
They can just be html chunks. No need to make sense on their own.
Maybe a single tag that points at an url to load if someone attempts to load the chunk directly.