Properly adding conditional styles ( IE 8 , IE 9 etc...) and scripts to your wordpress theme

theme_name_scripts.php

<?php
	/**
	 * Wordpress is @ 4.5.3
	 * https://developer.wordpress.org/reference/functions/wp_style_add_data/
	 * https://developer.wordpress.org/reference/functions/wp_script_add_data/
	 * no need to use global $wp_data
	 * add to your theme's functions.php file
	 * */
	 
	define('THEME_TEMPLATE_PATH', get_template_directory_uri());

	function theme_name_scripts () {
		
		// add conditional IE 9
		wp_enqueue_style('theme-ie-9', THEME_TEMPLATE_PATH . '/assets/css/ie9.css');
		wp_style_add_data('theme-ie-9', 'conditional', 'lt IE 9');

		// add conditional IE 8
		wp_enqueue_style('theme-ie-8', THEME_TEMPLATE_PATH . '/assets/css/ie8.css');
		wp_style_add_data('theme-ie-8', 'conditional', 'lt IE 8');
		
		// add respond.min.js if lte IE8
		wp_enqueue_script('respond', THEME_TEMPLATE_PATH . '/assets/js/ie/respond.min.js');
		wp_script_add_data('respond', 'conditional', 'lte IE 8');
	}
	
	add_action('wp_enqueue_scripts', 'theme_name_scripts');
?>