The Declarative Performance Observer proposes a reliable browser-resident telemetry system that reports of performance metrics from navigation initiation to page termination. By using a declarative HTTP response header, it ensures that data is captured even in scenarios where the request failed due to the network error or the renderer process is killed by the OS.
Currently, web developers face significant challenges in measuring the full end-to-end reliability of user journeys, particularly abandoned navigations that occur before JavaScript execution or after a session terminates. Existing web APIs are limited by their dependency on JavaScript, meaning that early failures like DNS timeouts or connection errors remain invisible to the site. Furthermore, capturing metrics during abrupt terminations—such as renderer crashes due to memory pressure or sudden tab closures—is unreliable, as beacons sent at these moments are often lost.
Explainers: https://github.com/explainers-by-googlers/declarative-performance-observer