EPUB 3 uses XHTML5 for content documents, so a doctype isn't required. Adding the HTML5 doctype isn't harmful, either:
<!DOCTYPE html>
It has the benefit of triggering the correct validation in some XML editors, and I suppose could also avoid your content being rendered in quirks mode if it's incorrectly served up as text/html.
As HTML is no longer defined by DTDs, EPUB 3 removed the ability to reference external entities from the doctype. If you include an XHTML 1.1 doctype, you're going to get this error when you validate, as that's what it's attempting to do:
External entities are not allowed in XML. External entity declaration found: %OEBEntities
EPUB 3 uses XHTML5 for content documents, so a doctype isn't required. Adding the HTML5 doctype isn't harmful, either:
It has the benefit of triggering the correct validation in some XML editors, and I suppose could also avoid your content being rendered in quirks mode if it's incorrectly served up as text/html.
As HTML is no longer defined by DTDs, EPUB 3 removed the ability to reference external entities from the doctype. If you include an XHTML 1.1 doctype, you're going to get this error when you validate, as that's what it's attempting to do: