[KevinH]

Well having a doctype was required for epub2 to allow for named entities. For epub3 the doctype is still required for the spec case and I believe that epubcheck has a long standing bug about not requiring it. Only html allows it to be skipped. Sigil will always treat missing doctype as a error for both epub2 and epub3. Sigil also requires the xml header as well. Neither of those are changing. Calibre simply cares less about doctypes and epubcheck and spec meeting than Sigil, as that has always been Kovids view and why calibre has its own check function.

I am travelling now but upon my return I will simply revert this change, and we will go back to how it has worked for years. Sigil will continue to require the doctype as the original epub3 spec clearly required it for xhtml no matter what epubcheck does. Only html said it was optional.