Codeigniter - Creating a custom library

css.php

<?php

class Css {

	var $files = array();

	public function add($file) {
		$this->files[] = $file;
	}

	public function generate() {

		$cssString = "";
		$CI = &get_instance();
		$CI->load->helper('html');

		foreach($this->files as $file) {

			$cssString .= link_tag('css/' . $file) . "\n";
		}

		return $cssString;
	}
}

hello.php

<?php 

class Hello extends MY_Controller {

	public function index() {

		$this->load->library('css');
		$this->css->add('main.css');
		$this->css->add('print.css');

		echo $this->css->generate();
	}
}