Sid Stamm wrote on 1/12/2009 3:19 PM: 
> It seems to me that unless client browsers *never* send CSP-related
> data to the server then the server can ultimately determine which
> clients are using CSP.

I agree, without the client advertising CSP-support, sites will test for CSP 
just as they test for JavaScript, cookies, etc.  You could probably test for 
CSP by using policy-uri, if the browser requests it from your server, then it 
supports CSP.  To prevent an attacker from causing a browser to load it ala 
CSRF, you could even add a nonce to the request:

        X-Content-Security-Policy: policy-uri /policy.csp?nonce=ABC123



- Bil

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