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The error description contains a link to the CSS validator. That's a good thing. However, the mobileOK Checker runs CSS tests against the CSS Level 1 grammar whereas the CSS validator checks pages against CSS Level 2.1. This can create confusion when parse errors are reported by the mobileOK Checker that are due to incompatibilities between CSS Level 1 and CSS Level 2.1. The CSS validator also returns as errors syntax problems that are ignored by the mobileOK Checker in accordance with the forward-compatible parsing rules: http://www.w3.org/TR/REC-CSS1/#forward-compatible-parsing That should be explained as well, as this would have the opposite effect: no error in the mobileOK Checker and errors in the CSS validator!
Message completed to: [[ Check the style sheets with the W3C CSS Validator to find and correct CSS syntax errors. Support for CSS varies from one device to the other. It is relatively safe to assume that mobile devices that support CSS support the CSS Level 1 grammar along with the @media attribute. The CSS Validator checks style sheets against CSS Level 2.1 by default. CSS Level 1 may be selected prior to running the CSS validator in its "More options" menu. Unknown selectors, combinators, properties and values for the CSS Level 1 profile are reported as a separate warning by the mobileOK Checker (they are reported as errors by the CSS validator) and do not account for this message. ]]