private static function fixBackgroundPosition($css) {
		$replaced = preg_replace_callback(
			self::$patterns['bg_horizontal_percentage'],
			array('self', 'calculateNewBackgroundPosition'),
			$css
		);
		if ($replaced !== null) {
			// preg_replace_callback() sometimes returns null
			$css = $replaced;
		}
		$replaced = preg_replace_callback(
			self::$patterns['bg_horizontal_percentage_x'],
			array('self', 'calculateNewBackgroundPosition'),
			$css
		);
		if ($replaced !== null) {
			$css = $replaced;
		}

		return $css;
	}