chromium/third_party/blink/renderer/platform/loader/fetch/README.md

Low-level fetching code.

Fetching/loading code is divided into:
- core/fetch: Fetch API
- core/loader: high-level fetching
- platform/loader/fetch: low-level fetching

### Request Prioritization

The [request priority](https://fetch.spec.whatwg.org/#request-internal-priority)
used for any given fetch is a browser-specific internal attribute that is
generated by considering the
[destination](https://fetch.spec.whatwg.org/#concept-request-destination)
of the resource, any explicit
[priority](https://fetch.spec.whatwg.org/#request-priority) hints as well as
browser-specific heuristics. In Chrome's case, it includes:
* Position in the document so that late-body scripts are not given as high of
a priority as early render-blocking scripts.
* If a fetch is generated by a preload or if it is for the underlying resource
directly.

There is a web.dev article describing the priority scheme used by Chrome
[here](https://web.dev/priority-hints/#browser-priority-and-fetchpriority).

Determining request priority is spread across 3 different locations in Chrome:
- [platform/loader/fetch/resource_fetcher.cc](resource_fetcher.cc): The core
prioritization logic handling most resource requests.
- [/content/browser/loader/navigation_early_hints_manager.cc](../../../../../../content/browser/loader/navigation_early_hints_manager.cc):
Prioritization for `Link:` preload response headers in
[HTTP 103](https://httpwg.org/specs/rfc8297.html) early-hint responses.
- [/chrome/browser/predictors/loading_predictor_tab_helper.cc](../../../../../../chrome/browser/predictors/loading_predictor_tab_helper.cc):
Prioritization for resources that are likely going to be needed.