protected function parseRuleString($ruleString)
    {
        foreach ($this->getSelectorStrings($ruleString) as $selectorString)
        {
            // Check for invalid selector (e.g. if starting with a comma, like in this example from
            // the spec ",all { body { background:lime } }")
            if ($selectorString === "") {
                $this->setIsValid(false);
                $this->addValidationError("Invalid selector at '$ruleString'.");
                break;
            }

            $this->addSelector(new StyleSelector($selectorString, $this->getStyleSheet()));
        }
    }