Page Validates as XHTML1.1 but Won't got Through CSS Calidator

I receive the error:

" Target: http://www.gamehideout.co.uk
Please, validate your XML document first!

Line 1

Column 3

The markup declarations contained or pointed to by the document type declaration must be well-formed. "

even though my site (www.gamehideout.co.uk) validates as XHTML 1.1 Strict.


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