How to add css to Twig templates

Support Themes & Styling
1

I would like to understand how should I include the following HTML syntax in a Twig template (custom theme):

< ! - - [if lt IE 9]> < script type=“text/javascript” src=“js/customFoo.js” > < / script > < ! [endif] - - >

2

many ways to add assets

in base.html.twig for example

See Antimatter base.html.twig

or with assets plugin