IE9 Detection and CSS Stylesheet Selection in WordPress

<?php
  $user_agent = $_SERVER['HTTP_USER_AGENT'];
  if (strpos($user_agent, 'MSIE 9') !== false || strpos($user_agent, 'Opera') !== false) {
?>
     <link rel="stylesheet" type="text/css" media="all" href="<?php bloginfo( 'template_url' ); ?>/opera.css" />
<?php } else if(strpos($user_agent, 'MSIE') !== false) { ?>
     <link rel="stylesheet" type="text/css" media="all" href="<?php bloginfo( 'template_url' ); ?>/ie.css" />
<?php } ?>

Browser Detection plugin in wordpress : http://wordpress.org/extend/plugins/php-browser-detection/