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To prevent looping, W3C validation services refuse to process requests coming from W3C servers and return an HTTP status code 403. When the mobileOK Checker runs tests on a page that contains a W3C CSS badge with a link to the CSS validation service, the request to the W3C CSS validation service returns a 403, and the mobileOK Checker reports an HTTP_RESPONSE-11. At the very least, the mobileOK Checker should know about these exceptions... or not report the error when we know it is not a real error. See: http://lists.w3.org/Archives/Public/public-bpwg/2009Aug/0030.html
HTTP_RESPONSE-11 failures ignored when the domain name ends with w3.org. That is a bit broad and will hide some errors in particular when checking real W3C web pages. but I think we can live with that.