lib/Tokenizer.php
protected function _checkRule($words, $extra, $ignoreErrors) {
        $pairs = $this->_findRulePairs($words);

        if (is_bool($pairs)) {
            $value = $this->_mergeWords($words) . $extra;
            if (!strlen($value)) {
                return false;
            }

            $index = $words[0]['index'];
            if ($pairs === false && !$this->ignoreErrors && !$ignoreErrors) {
                $this->errors[] = new ParseError('Invalid css rule', $this->_css, $index);
            }
            return [
                'token' => 'code',
                'code'  => $value,
                'index' => $index,
            ];
        }
        return $pairs;
    }
protected function _checkRule($words, $extra, $ignoreErrors) {
        $pairs = $this->_findRulePairs($words);

        if (is_bool($pairs)) {
            $value = $this->_mergeWords($words) . $extra;
            if (!strlen($value)) {
                return false;
            }

            $index = $words[0]['index'];
            if ($pairs === false && !$this->ignoreErrors && !$ignoreErrors) {
                $this->errors[] = new ParseError('Invalid css rule', $this->_css, $index);
            }
            return [
                'token' => 'code',
                'code'  => $value,
                'index' => $index,
            ];
        }
        return $pairs;
    }
Check for valid css rule, return either code or rule token

@param array $words Array of words
@param string $extra Additional text to add if returning code
@param bool $ignoreErrors True if errors should be ignored
@return array|bool