protected function setBackgroundPositionXProperty( $value, $priority = self::PRIORITY_NORMAL )
    {
        $x = (string) $value;
        $y = (string) $this->getRawPropertyValue( 'background-position-y' );

        if ( '' !== $x && '' !== $y )
        {
            $this->setRawProperty(
                'background-position',
                $x . ' ' . $y,
                $priority ?: $this->getRawPropertyPriority(
                    'background-position-y'
                )
            );
        }

        return $this->setRawProperty( 'background-position-x', $x, $priority );
    }