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The message's label is an informational message that says: [[ The CSS Style contains at-rules, properties, or values that may not be supported ]] ... which should be clear enough. However, the description claims: [[ The linked resource uses a character encoding other than UTF-8. ]] That is wrong and confusing as the test only applies at the HTTP level, and the linked resource may well define the UTF-8 character encoding in the XML declaration and/or in a meta element.
Message description updated to: [[ The mobileOK Checker checked the HTTP Content-Type header returned in response to the linked resource and found out that: - the character encoding of the resource was not specified at that level. In this case, please note that the mobileOK Checker did not check whether the character encoding was specified at the content level (in the XML declaration or in a <code>meta</code> element) for performance reasons. Please check each resource individually to determine whether its character encoding is properly specified. - or that a character encoding different from UTF-8 was specified at the HTTP level. Given the number of character encodings used across the world, sending UTF-8 encoded content to browsers that requests it increases significantly the chances of properly displaying a page. ]] New message will get deployed in next release.