Re: [whatwg/fetch] Accept headers in spec cause confusion (#274)

valenting left a comment (whatwg/fetch#274)

Hi folks,

We recently changed Firefox to  [Remove image mime types from document accept header](https://bugzilla.mozilla.org/show_bug.cgi?id=1917177) in order to match the fetch spec.
After doing so, we got some push back:
- Sites using the document Accept header to decide which image formats to include in the document could no longer do so.
- Navigating to an image URL can't negotiate content type, because image formats are no longer present in the document accept header. [see bug 1950012](https://bugzilla.mozilla.org/show_bug.cgi?id=1950012)

I agree with @annevk that the document accept header shouldn't be used to negotiate image content types - since there's no guarantee that the same content-types will be present in the image request's Accept header.

I think the second issue is trickier. Since the browser doesn't know whether a document load is to an image or a HTML file ahead of time, it will always send the same document accept header. In order to enable content type negotiation this would suggest that image mime types **should** be part of the document accept header. But this might be an avenue for future bloat, because you can make the same argument for audio, video, and other types of content. Should we include all of them in the document accept header? 🤷

Thoughts?

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