A new draft of the W3C Attribution API published in July 2026 has moved into wider industry review, and the argument that followed is not really about the specification. It is about who gets to hold the measurement layer of the open web once cookies are gone.
01What it is and who is building it
The Attribution API is a browser-mediated way to measure advertising performance without cross-site tracking. Rather than an ad tech vendor stitching a user’s journey together across sites, the browser itself records that an ad was seen and a conversion happened, and reports back an aggregated, privacy-protected result.
It is being developed in the W3C’s Private Advertising Technology (PAT) working group, with participation from Apple, Google, Microsoft and Mozilla — every major browser engine. Notably, PAT is co-chaired by Aram Zucker-Scharff of the Washington Post, who has stressed that the work predates Privacy Sandbox and operates in the open. A publisher holding a co-chair seat on the group defining post-cookie measurement is not a small detail.
The timeline matters: the concept predates Privacy Sandbox, Google scaled Privacy Sandbox back in mid-2025, and the latest draft landed in July 2026 into the vacuum that retreat created.
02The objections
Concentration. James Rosewell, co-founder of Movement for an Open Web, warns that browser-mediated attribution “could concentrate too much influence over measurement within browser platforms.” The companies that would run the measurement layer are the same ones that sell advertising against their own properties.
Architectural familiarity. Consultant Don Marti has questioned how closely the design resembles Google’s earlier Attribution Reporting API — the concern being that a rejected proposal returns wearing a standards body’s badge.
Governance. There is an open debate over whether advertisers, publishers, agencies and independent measurement providers have adequate say, or whether browser vendors set the terms and everyone else implements.
Consultant Scott Messer makes the counter-case bluntly: attribution on the open web is good, because it is what drives advertising investment toward it. A web where advertisers cannot measure results is a web advertisers underfund.
The cookie's replacement is being designed right now, in public, by four browser vendors and whoever else shows up.
03Why marketers should care
| This specification decides what you will be able to measure | Whoever defines a conversion, the aggregation threshold and the reporting delay defines the shape of every post-cookie performance report you will read. It is being written now, in public, by a group you can join. |
|---|---|
| Aggregation floors will make some of your media unreportable | Privacy- preserving aggregation suppresses results below a minimum count. Smaller campaigns, tighter targeting and long-tail placements can return no reportable conversions — not poor performance, but no data. On a plan those look identical, and the budget goes to whichever one has numbers. |
| Reporting delay changes how you can optimise | A measurement layer that reports in days rather than minutes rules out intraday optimisation on the open web while walled gardens keep it. That is a structural advantage handed to platforms by a technical default. |
04What marketers should do
05The bottom line
The cookie’s replacement is being designed right now, in public, by four browser vendors and whoever else shows up. Publishers spent the Privacy Sandbox years complaining they had no seat at the table. This time there is a publisher co-chairing the working group and a public draft open for comment. The measurement layer will be built either way — the only question is whether publishers helped specify it or spent the next five years discovering what it does to their yield.