Hi Peter.I recommend you take a look at what Larry Masinter pointed out. It becomes clear that all of this has already been solved 9 years ago, but because it (apparently) isn't enabled in Apache by default, I and many others never even realised it existed:
Content negotiation for HTTP based on parameters beyond MIME types
was the focus of a significant amount of IETF work, including
RFC 2295 ("Transparent Content Negotiation in HTTP")
RFC 2296 ("HTTP Remote Variant Selection Algorithm -- RVSA/1.0")
and a set of 'media features' (including resolution, screen
size, pixel depth, etc.) in a Media Feature registry
(RFC 2506, BCP 31).
http://www.ietf.org/rfc/rfc2295Basically the client adds a "Negotiate: trans" header to the GET request, and initiates transparent (to the user) content negotiation (TCN). The server sends back a 300 response with a list of choices in an Alternates header, the format of which is defined in the RFC. The client then issues a new GET request for the one it wants, without having to send any environment information (dpi, &c.) to the server.
Clients can also send a few (small) Accept-* headers and ask the server to take an educated guess, but still return the list of Alternates along with it's guess, which could save round trips if the client decides that the server's guess was good enough ("the 150dpi would have been best for me, but I'll just use this here 200dpi image you've already sent"). If the resource is non-negotiable, the Negotiate header is ignored by the server and nothing changes.
In light of this, I suggest that WebKit developers (including me if I get the time) look into implementing this set of RFCs, and ignore all previous suggestions. It seems we've been in the classic chicken-and-egg situation since 1998, where clients don't implement it because servers don't have it turned on by default, and servers don't have it enabled because there are no client implementations to test against. Website owners are left none the wiser, and while a few server-side negotiation, most don't find it reliable enough to use, and have things like flag icons on their home page to choose your language.
Apple members should discuss with the IETF the addition of device- pixel-ratio to the Media Feature registry. This will allow TCN for <img> and <object> elements containing visual media, and anywhere else where CSS does not apply. I also suggest a "High-Res: Part III" blog post on webkit.org to draw attention to this feature and get more web developers aware of and using TCN, which I think is the biggest hurdle (implementing it won't be as hard).
- Nicholas.
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