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At the beginning of an otherwise perfectly valid document: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.w3.org/MarkUp/SCHEMA/xhtml11.xsd" xml:lang="en" dir="ltr"> will cause a validation error Line 2, Column 116: there is no attribute "xsi:schemaLocation" . You have used the attribute named above in your document, but the document type you are using does not support that attribute for this element. even though schemaLocation is a valid attribute in a root element of a document according to http://www.w3.org/TR/xmlschema-1/#xsi_schemaLocation, a W3C recommendation.
*** Bug 5642 has been marked as a duplicate of this bug. ***
I forwarded the feedback to the group maintaining the machine-readable schemas for XHTML 1.1: http://lists.w3.org/Archives/Public/www-html-editor/2008AprJun/0011.html