As for DOCTYPE in epub3.
The spec for html5 clearly states:
Quote:
Definition and Usage
All HTML documents must start with a <!DOCTYPE> declaration.
The declaration is not an HTML tag. It is an "information" to the browser about what document type to expect.
In HTML 5, the declaration is simple:
<!DOCTYPE html>
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And the latest epub 3.2 spec states:
Quote:
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The XHTML profile defined by this specification inherits all definitions of semantics, structure and processing behaviors from [HTML] unless otherwise specified.
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And a search reveals no other mention of doctype.
The epub 3.2 spec further states:
Quote:
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For all document constructs used that are defined by [HTML], it MUST conform to the conformance criteria defined for those constructs in that specification, unless explicitly overridden in HTML Deviations and Constraints.
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And in that deviations section there is no mention of doctype.
The only other part of the specs that mentions doctype is for pure xml (not xhtml) files do not need a doctype thus it is not used on the opf or other pure xml document.
The xhtml1.1 spec interestingly enough says a doctype is required. And all discussion on future epub specs talks about accepting pure html [whatwg standard or html5] as a content document format and that requires a doctype.
So since it is required based on the exact language of the spec according to my reading, is required in epub2, is required in html5 and required in the html whatwg living spec, and required for xhtml 1.1 and only not required for pure xml files like the opf, Sigil will enforce that.
It is not something we are going to change.