validating www.alexanderworks.org

My pages validate as xhtml 1.0

When I try to use the css validator on my html pages, I get this error 
message:

Target: http://www.alexanderworks.org/
org.xml.sax.SAXException: Please, fix your system identifier (URI) in 
the DOCTYPE rule.

What's going on?

Thanks,




--
Philip Pawley
Liverpool, UK
http://www.alexanderworks.org/
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