HTTP P3P Header
P3P HTTP header is supposed to set P3P policy in the form of
P3P:CP="your_compact_policy". However, P3P did not take off. Most web browsers have never fully implemented it. A lot of websites set this header with fake policy text and that was enough to fool browsers of the existence of a P3P policy and grant permissions for third party cookies. Here is an example:
P3P: CP="This is not a P3P policy! See http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=151657 for more info."