You can add any document type you want, provided you include a manifest fallback (e.g., if you insert it in the spine or iframe) or an intrinsic one (e.g., nested objects) that is a core media type, such as xhtml.
The big issue, of course, is support. Rendering of non-core formats is not common, hence their being non-core. I've personally not heard of any reading systems that will render an excel sheet.
You can add any document type you want, provided you include a manifest fallback (e.g., if you insert it in the spine or iframe) or an intrinsic one (e.g., nested objects) that is a core media type, such as xhtml.
The big issue, of course, is support. Rendering of non-core formats is not common, hence their being non-core. I've personally not heard of any reading systems that will render an excel sheet.