<link rel="stylesheet" type="text/css" href="<? echo base_url();?>css/all.css">

$config['base_url'] = 'http://webscarlets.com/ci/index.php';
$config['index_page'] = 'index.php';

<link rel="stylesheet" type="text/css" href="<? echo base_url();?>/css/all.css">

$config['base_url'] = 'http://webscarlets.com/ci/';

<?php echo link_tag('css/mystyles.css'); ?>

<link href="http://site.com/css/mystyles.css" rel="stylesheet" type="text/css" />

Options +FollowSymLinks
Options -Indexes
DirectoryIndex index.php
RewriteEngine on
RewriteCond $1 !^(index.php|images|styles|scripts|robots.txt|favicon.ico)
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?/$1 [L,QSA]

define("LAYOUT_URL","http://localhost/yoursite/css/");

<link rel="stylesheet" type="text/css" href="<?php echo LAYOUT_URL;?>all.css">

//config.php
    $config['base_url'] = 'http://webscarlets.com/ci/';
    $config['index_page'] = 'index.php';

$this->load->helper('url');

echo base_url()

<div id='content'> <?php echo $content; ?> </div> -> css is not display
     <div style='background-color:red;'> <?php echo $content; ?> </div> -> css OK