Language: PHP
Label: safe
CWE: CWE-79 \u2014 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

Code:
    public function validate($string, $config, $context)
    {
        static $allowed_values = array(
            'line-through' => true,
            'overline' => true,
            'underline' => true,
        );

        $string = strtolower($this->parseCDATA($string));

        if ($string === 'none') {
            return $string;
        }

        $parts = explode(' ', $string);
        $final = '';
        foreach ($parts as $part) {
            if (isset($allowed_values[$part])) {
                $final .= $part . ' ';
            }
        }
        $final = rtrim($final);
        if ($final === '') {
            return false;
        }
        return $final;
    }