Chrome-facilitated testing

As we prepare general availability for the relevance and measurement APIs, we intend to provide Chrome-facilitated testing that allows sites to meaningfully preview what it's like to operate in a world without third-party cookies. This will allow us to perform more effective API testing and grow confidence within the ecosystem as to its readiness for third-party cookie phase out.

We have worked with the CMA to ensure these testing modes align with the testing framework (and timeline) for third parties laid out in its note on Quantitative testing of Google's Privacy Sandbox technologies. As a result, the CMA anticipates that the results from testing in these modes can be used in its assessment of the Privacy Sandbox.

We plan to have two modes of Chrome-facilitated testing:

• Mode A: Ad techs can receive control and experiment labels on a portion of traffic and use these to conduct testing and experiments.
• Mode B: Chrome globally disables third-party cookies for some portion of all Chrome users.

These details are not final, and we'll publish further implementation guidance as we progress in Q3 2023. The current proposals are as follows.

# Mode A: Opt-in testing

Ad techs will be able to receive experiment labels for a portion of Chrome traffic. An ad tech can choose to coordinate with other ad techs, for example, to run Protected Audience auctions without third-party cookies for a consistent experiment group. Ad techs can also use these labels for their own independent experiments and testing.

Chrome will not modify the state of third-party cookies for users in Mode A. Chrome only provides the labels, as to ensure that ad techs can experiment with consistent control and experiment groups. This means that a publisher's site could still receive third-party cookie data for the publisher's own usage, even if their ad tech partners are participating in the experiment.

We expect this to allow for meaningful experimentation, where all involved sites and services can coordinate to ensure third-party cookies are not used at any point within the process. We anticipate providing labels for up to 10% of Chrome browsers via a new request header and low-entropy client hint. We encourage anyone interested in testing to provide feedback from the ecosystem on the method for accessing labels and the granularity of labels.

We plan to make the opt-in testing mode available starting in Q4 2023, and we'll continue this mode until third-party cookie deprecation.

# Mode B: 1% third-party cookie deprecation

Chrome will deprecate third-party cookies for up to 1% of browsers. There is no opt-in for this mode, as it will be applied globally. There is, of course, the possibility that some site features may be impacted if the site hasn't yet adopted an alternative solution, such as CHIPS or First-Party Sets.

We're working on mitigations to detect, address, and proactively alert site owners of issues that impact user experience during this phase.

Additionally, we plan to provide a small fraction of traffic within Mode B that has Privacy Sandbox relevance and measurement APIs disabled. Other APIs, like First-Party Sets, CHIPS, FedCM, and so on, will not be disabled. We anticipate that this combination will be helpful to establish a baseline of performance without third-party cookies, and we're seeking feedback on an appropriate fraction of traffic to devote to this subset of testing.

We plan to deprecate 1% of third party cookies in Q1 2024, and we'll work closely with the CMA before taking further steps to expand deprecation.

# Engage and share feedback

Feedback from a diverse set of stakeholders across the web ecosystem is critical to the Privacy Sandbox initiative. The dedicated feedback section provides an overview of the existing public channels, where you can follow or contribute to discussion, along with a feedback form to ensure you can always reach the Chrome team directly.

If you're a developer, you can ask questions and join discussions in the Privacy Sandbox Developer Support repository on GitHub.