The way service workers work is that only headers set at the API-level will be exposed to service workers. E.g. Host is not exposed as the network-level takes care of that (and you can simply use request.url for it).
Per https://github.com/slightlyoff/ServiceWorker/issues/303 https://github.com/slightlyoff/ServiceWorker/issues/348 it would be desirable to have Accept / Accept-Language be set by APIs, such as <img>. XMLHttpRequest already does this (unless a developer added those headers), see http://xhr.spec.whatwg.org/ Any idea how to best define this Ian? If we are eventually going to expose something like a "Fetch" object for each API that can issue a fetch it would seem best if these details were defined at the API-level. I guess for now I'll add some notes to the network-level bits of Fetch to indicate Accept / Accept-Language cannot be set at that point by the user agent. -- http://annevankesteren.nl/