Re: [whatwg/fetch] Purpose of the Content-DPR hint is not entirely clear. (#812)

I'll add one concrete example of this not being clear to server-implementers – my employer, Cloudinary, implemented (and still implements) `Content-DPR` incorrectly. We echo back the DPR of the client, no matter how the width of the returned resource matches up with the requested `Width`. The example in section 4 could use some fleshing-out!

Jeremey’s questions are all about describing how `Content-DPR` is actually used on the front end – I'll add that I think there should be more about how the server in the example calculated the `Content-DPR: 1` that it sends back. I assume that the resource it sent had a width of 160 pixels? That should be added to the example, along with a general formula that:

`Content-DPR` should = response resource width, in pixels ÷ ( request `Width` ÷ `request `DPR` )



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