Hello everyone, thanks for all the work you’ve been doing to limit harmful cross-site tracking and protect user privacy and security. It’s great to see how the browsers are putting an end to the systematic collection and sharing of personal data.

I want to bring back a question that has been mentioned in passing but was never properly considered. Once measures like Bounce Tracking Mitigation are in place, how will browsers ensure, from a technical perspective, that it is still possible for privacy-enhancing technologies (PETs) to operate? I’m hoping we can focus on the technical standardisation of the approach, on the understanding that innovation and competition are good for users and publishers so long as privacy, security and user experience are protected.

The BTM Explainer has a few short paragraphs on Block/Allow Lists but it’s unclear how this will work in practice. Let’s take the scenario of a technology that uses bounce tracking or an alternative cross-site tracking technology in a privacy-preserving way - not some sort of ad tech whitewash but something that meets the most stringent legal requirements on privacy (perhaps even on a path towards achieving zero trust status). For example, a PET that by design and default only tracks cross-site data with opt-in user consent, makes data deletion and portability easy, aggregates and processes the data in the browser, minimises the data accessible by the tracking company and does not share personal data with any third party. It is both possible and desirable for the market to produce software that delivers the benefits of cross-site data without compromising privacy and security, but the rules must be clear.

How will browsers provide technical stability for PETs? Mozilla has done a great job explaining their red lines at disconnect.me, although manually curated allow-lists are probably better for privacy than blocklists. Either way, the ground rules for PETs must be tightly defined for transparency and consistency.

Keen to hear the group’s thoughts.