For all TCP traffic or just HTTP ?
Do you want the CSS to redirect the traffic to a cache or proxy ???
We can only spoof the L7 traffic ... so HTTP or FTP.
So, for http, you will need to configure a content rule to catch
protocol tcp
port 80
And you always need to send the traffic somewhere.
So you will need to at least configure a service for your default gateway .
Make the service transparent and add it to your catch-all content rule.
Gilles.