[Spec] How fenced frames behaves for language negotiation as part of reducing accept language?

[Tanych]

As in this proposal, the browser should only send the user's most preferred language in the Accept-Language header instead of sending all languages.

For cross-origin iframe subresource requests, it will inherit the Accept-Language header from their parent frame. However, due to fenced frame's privacy guarantee of not allowing communication between the embedding frame and the fenced frame, it indicates that the fenced frame should not get the knowledge of the language.

For the initial prototype, we are going to not share the negotiation language to the fenced frame inner page, which will use the primary language (the first language in user's accept-language list) as default, to monitor how it works.

[shivanigithub]

@Tanych Thanks for helping document this!

[csreis]

For context, I was discussing this with @Tanych and @domfarolino in https://chromium-review.googlesource.com/c/chromium/src/+/3687391/comments/4f3b997c_bec153e8. It seems ok to start the prototype with a version that loads all Fenced Frames and their subframes with only the primary language, but that does seem like a regression in user experience. Dom and I seem to think it should be safe for the Fenced Frame's main frame to negotiate a language that is applied consistently throughout all frames of its page, without taking anything from the outer frame into consideration. That's worth considering as a later improvement to the prototype.