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.