Browser triggered reportEvent() call on creative load for reliable impression reporting

[warrrrren]

Context

The current fenced frame reporting API allows the buyer's creative to decide which entities to send events to.
This denies publishers and sell-side entities a guaranteed mechanism to log the impressions rendered by a buyer. In situations today wherein the buyer creative has errors and cannot render, publishers depend on sell-side entities to accurately record impressions and bill buyers accordingly.

Proposal

Add a new reserved event(automatic beacon perhaps) fired by the browser once a frame with an ad creative is initialized.
suggested name: reserved.rendered

The browser will call the URL associated with the event registered by the component and top-level sellers when the iframe/fenced-frame initializes the supplied URN/config.

[rdgordon-index]

#826 is related

[warrrrren]

Thanks, @rdgordon-index. Was it brought up in a prior weekly sync call?

[rdgordon-index]

Yes, cf.

turtledove/meetings/2023-10-25-FLEDGE-call-minutes.md

Line 104 in d43b8c8

## Additional browser event beacons - https://github.com/WICG/turtledove/issues/826

[warrrrren]

Thank you.

So if I understand from the conversation, the sendReportTo() function inside reportResult() is called only when an ad renders, making it sufficient to know that an ad was rendered. I didn't see this documented, must have missed it.

Is there a similar mechanism then to detect instances only where a bid was submitted to the top-level auction from the component auction?

[MattMenke2]

Note that sendReportTo() is actually called right after the auction, however, the report is only sent when navigation to the ad starts (Or when deprecatedUrnToURL is invoked with sendReports=true, though that method is, of course, slated for removal at some point). It's only requested at most once per action, even if the winning ad is used multiple times.