From RDFa Working Group Wiki
Subject: [Comment response] To Gregg Kellogg
Thank you for your comment
on the RDFa 1.1 drafts.
The issue you raise on the case sensitiveness of the @prefix value was recorded as issue 22 for the Working Group, was then discussed in the group. The resolution says that
"For prefixes defined via xmlns: and @prefix, the prefix text should be converted to lowercase by the RDFa Processor."
and this text will appear in the next draft of the document.
I hope this resolution answers your concerns
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