Detaching from the above:
From the previous quote of the specs, I would say that the <guide> is optional, but it cannot be empty.
EDIT: the schema (
http://www.idpf.org/epub/20/spec/OPF...m#AppendixTOCA ) seems to confirm:
Quote:
<define name="OPF20.guide-element">
<element name="guide">
<ref name="OPF20.optional-id-attribute"/>
<ref name="OPF20.guide-content"/>
</element>
</define>
<define name="OPF20.guide-content">
<oneOrMore>
<ref name="OPF20.reference-element"/>
</oneOrMore>
</define>
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Interestingly, epubcheck complains about the empty <guide></guide> if you validate an EPUB 3, but not if you validate an EPUB 2, I have just tried.