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You are quickly redirected away from that page, so you don't see it (unless you look at Firebug's Net tab, sniff packets, or equiv).

Edit: acutally, it's just included as a stylesheet (with a HTTP link header)

It's not the redirect that hides it. View the page source before redirect and you will see there are no <link rel="stylesheet"> or <style> tags.

header('Link: </visited.php?css&uri='.$_GET["uri"].'>; rel="stylesheet"; type="text/css";');

It's in your source, look.

I meant the source the client-side sees. I know the `Link` header is sent in the application-end source.

And what do you think the client does with this? It downloads the file and implements the CSS rules.

Whether or not it is "hidden" from an average user, doesn't mean you are not using CSS.