You can't do that with CSS alone. You'll need a server-side language like PHP to do it.
With PHP:
Put this on the very top of your page (even before <html>)
PHP Code:
<?php
$id = sha1(time());
if (!isset($_COOKIE['links'])) {
setcookie('links',$id);
}
?>
And on every link, add this to the url: ?id=<?php echo $id; ?>
Example:
HTML Code:
<a href="mypage.php?id=<?php echo $id; ?>">My Link!</a>
...and if you already have a ? in the URL, put &id=<?php echo $id; ?>
Example:
HTML Code:
<a href="mypage.php?subscribe=true&id=<?php echo $id; ?>">My Link with 2 variables!</a>
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