If you are shown a product ad whilst browsing searchengine.example and then later look up the product at reviews.example, then end up making a purchase at shop.example, your Mozilla browser will send all of these events to one or more aggregation services that allows shop.example to understand (at least in aggregate, assuming you trust the cartels running the aggregation services) that you were exposed to their product at searchengine.example and further exposed to their product at reviews.example.
Where previously an ad tech company was ultimately able to track users based on source IP address (even if cookies had been disabled by a user), IPA now allows these companies to track users across multiple IP addresses, and regardless of the user's cookie settings, via a unique tracking identifier. It is also proposed that the operating system provides the unique tracking identifier which can then be used by all applications or browsers on a device, allowing different devices behind a single IP address to be distinguished.
Most of these private attribution systems are specifically designed so that the people running the ad can count how many people clicked their ads, but not who clicked them or what other things they did. Safari had a proposal in which you could only have a certain number of campaigns running per domain, so you couldn't set up a separate """campaign""" for each user and fingerprint them all at once. I don't know how the Mozilla proposal differs.
Whether or not user-agents should care about this sort of thing is an orthogonal question.
[0] https://www.theregister.com/2023/06/29/google_trueview_skept...
[1] Remarketing in particular is responsible for the "feeling of being seen" from modern ads where you search for one thing and get 10,000 ads for the thing for the next week
What prevents any of the following solutions from providing assurance to advertising clients, without also destroying the Internet and general purpose computing:
1. Building trust with advertising clients be treating them with respect, honesty and transparency. If your clients don't trust your advertising network and were demanding assurances in the form of WEI and similar proposals, surely it's obvious there are bigger problems. The advertising client would likely have dropped the advertising network long ago but can't due to monopolies existing.
2. Advertising networks undergo independent audits (results available to clients) and become more transparent to clients in how their advertising spend is being used.
3. Advertising clients survey users at checkout to ask whether they found the product/service from an advert, or whether they recall seeing an advertising campaign and where they remember seeing it.
4. Advertising clients host advertisements at ads.company.example (in a few highly restricted formats) so they can keep track of impressions themselves.
5. (still a bad idea, based on user surveys, but one which Google et al should have considered for minimisation of data collection and privacy impact) Browsers collect advertising metrics during use and when a user makes a transaction at an online store, the online store asks the user (via the browser) for permission to obtain those saved advertising metrics to provide only to the online store. Users can review the entirety of information sent to the online store before it is sent. Advertising networks don't have a need to access browsing history for everyone on the Internet in real time.
6. Online stores and similar continue to rotate their marketing spend through various advertising networks and marketing campaigns, checking their own metrics to see if advertising campaigns have been having an impact. Campaigns could include marketing using the Internet but outside the reach of Google et al such as use of campaign-specific coupons and products marketed through product review websites, referral schemes, influencers, etc.