That is a very strange question. Why would you ask it? Content-Type is the canonically proper use of an http header field, as it is metadata about the content, rather than content itself.
One could argue that for some content, how to style it is just a suggestion anyway. Ie you could think of “our style guide for text content can be found at this address” as metadata for semantic HTML documents
That breaks down when you try to think like a content creator, nowadays everyone talks about UX, I'd say the styling has at least equal weight in terms of communicating ideas. (net negative if the document is shown without it applied, really pissed artists etc)
