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Well, the author can say it is anything they want, but that doesn't change what it actually is. It is literally not possible to send XHTML5 as text/html, because as soon as you label it as text/html, you are stating "it is HTML".

I used to think that too, but then I realised that in the real world it is different. Browsers sniff, and media types are hard-wired into software, rather than being an extension point. You have to row with the oars you have got. As I said, I send documents with media type text/html, not because they are necessarily HTML, but because I want them in the browser. I agree that the document gets *processed* as HTML, but the document doesn't magically change type just because it gets sent with a certain media type.