Media diagnostics · Primary-source explanation; examples are illustrative. Instructions are based on publisher guidance; a UK IPTV account was not tested on this platform.
Check whether the measurement is exposed
MDN documents more than one reason for a Resource Timing transferSize of zero. A resource can have come from a local cache without contacting the server, but a cross origin resource which fails the Timing Allow Origin check can also report zero. A performance report which labels every zero as a cache hit can therefore misclassify remotely fetched artwork or media. Identify the request origin and exposure rules before interpreting the number.
Compare two known cases
For an original developer exercise, use a small resource the developer controls and compare it with a permitted remote resource whose timing detail is not exposed. Keep the actual network observations alongside the JavaScript values. If both show zero, that equality alone does not establish identical transfer behaviour. Do not estimate saved mobile data by adding every zero sized entry to a cache success count; that calculation would silently convert unavailable measurements into known results.
Keep sizes and permissions distinct
The documentation also describes a fixed header size representation rather than a literal measurement of all header bytes. Preserve the field definition and browser version when doing byte accounting. Timing Allow Origin is a timing exposure mechanism, not a copyright licence or account authorisation. This article explains a measurement ambiguity and does not claim a tested cache hit ratio, actual UK IPTV transfer sizes or permission to inspect protected response data.