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http://jigsaw.w3.org/css-validator/validator?text=a{border-top:1px%20solid% 20lightblue} finds an error, but http://jigsaw.w3.org/css-validator/validator?text=a{border-top:1px%20solid% 20lightblue}&usermedium=all does not. I think the Validator should assume usermedium=all if the usermedium parameter is not specified, currently it seems to pass the null value around which apparently causes some undesirable behavior.