This function used to add css in document. It create as Script tag to attached css. See below example:
App::Html()->addCSS(SRC,[Media]);
SRC: Source path of the script. If we do not pass full path the system automatically add base URL.
echo App::Html()->addCSS("/css/print.css","print");
<link rel="stylesheet" type="text/css" href="http://www.example.com/css/pring.css" media="print" />
echo App::Html()->addCSS("http://www.abccom/mycss/css/style.css");
<link rel="stylesheet" type="text/css" href="http://www.abccom/mycss/css/style.css" media="all" />