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When the CSS Validator encounters a fatal error such as an ill-formed XML document it does not generate a SOAP response but rather XHTML markup, for example http://jigsaw.w3.org/css-validator/validator? uri=http://www.websitedev.de/markup/validator/tests/white-space-preceding-xml- decl.html&output=soap12
Fixed now, it generates a SOAP Fault instead.