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It seems to be impossible to validate a XHTML 1.1 document. If the "lang" attribute is not defined for the "html" element, the appc-checker test fails with the following message: "<example xml:lang='en'> shall be written as <example lang='en' xml:lang='en'>". But using the "lang" attribute makes the markup-validator test fail as it is not a valid XHTML 1.1 attribute.
There was a bug with the (experimental) appendix C checker which unicorn uses. The checker was performing observations on document types it is supposed to ignore (HTML compatibility guidelines applu only to XHTML 1.0 documents served as text/html).
Fixed and tested.