private function openInternal($self_closing = false)
    {
        echo '<', $this->tag_name;

        foreach ($this->attributes as $attribute => $value) {
            if ($value !== null) {
                echo ' ',
                    $attribute,
                    '="',
                    SwatString::minimizeEntities($value),
                    '"';
            }
        }

        if ($self_closing) {
            echo ' />';
        } else {
            echo '>';
        }
    }