When creating a quality Joomla extension you will place your styles and javascript codes on seperate files.
Then here is the way to add those files in to the joomla extension the proper way.
$document = JFactory::getDocument();
$document->addStyleSheet(JURI::root().’mod_name/css/stylesheet.css’);
$document->addScript(JURI::root(). ‘mod_name/js/script.js’);
$document->addStyleSheet(JURI::root().’mod_name/css/stylesheet.css’);
$document->addScript(JURI::root(). ‘mod_name/js/script.js’);