This is just my own understanding, but doesn't a webpage consist of a bunch of nodes, which can be combined in any way. And an html document is supposed to be a complete set of nodes, so a combination of those won't be a single document anymore.
Nodes can be addressed individually, but a document is the proportion for transmission containing also metadata. You can combined nodes as you like, but you can't really combined two already packed and annotated documents of nodes.
So I would say it is more due a semantic meaning. I think there was also the idea of requesting arbitrary sets of nodes, but that was never developed and with the shift away from a semantic document, it didn't make sense anymore.
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.
I wasn't talking about the nodes in the DOM. I meant the minimal annotated information snippets, that the WWW is supposed to consist of, as opposed to the minimum addressable units.
Nodes can be addressed individually, but a document is the proportion for transmission containing also metadata. You can combined nodes as you like, but you can't really combined two already packed and annotated documents of nodes.
So I would say it is more due a semantic meaning. I think there was also the idea of requesting arbitrary sets of nodes, but that was never developed and with the shift away from a semantic document, it didn't make sense anymore.