Originally Posted by crankydave

Correct me if I'm wrong but couldn't a site opt not to use P3P at any time?

Originally Posted by w3C

P3P uses machine readable descriptions to describe the collection and use of data. Sites implementing such policies make their practises explicit and thus open them to public scrutiny. Browsers can help the user to understand those privacy practises with smart interfaces. Most importantly, Browsers can this way develop a predictable behavior when blocking content like cookies thus giving a real incentive to eCommerce sites to behave in a privacy friendly way. This avoids the current scattering of cookie-blocking behaviors based on individual heuristics imagined by the implementer of the blocking tool which will make the creation of stateful services on the web a pain because the state-retrievel will be unpredictable.