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http://fetch.spec.whatwg.org/#basic-fetch [[ If request's omit credentials mode is always ]] ..then you include cookies? Either the "omit credentials" mode is misnamed or this logic is the wrong way around, it seems. Or I'm just more confused than I've ever been by a spec..
Sounds like you have not read many specs ;-) https://github.com/whatwg/fetch/commit/85b2762776b507184f139d0d8702bb8abd9e55f9