... This seems like a really cool and vastly under utilized feature of HTTP. It also seems particularly intriguing for web services. You could return JPEGs from that mapping service for the older client platforms, but also serve up SVG for the newer clients so they can scale and rotate their maps. What could possibly go wrong? ... The second problem that I ran across highlights the real core problem with conneg. I was trying to use the W3C XSLT service to do some transformations on my web pages. Now the server side software I use to run Well-Formed Web does conneg and can return either HTML or an RSS item fragment for each URI. At the time I was serving up XHTML 1.0, which is valid XML and thus good input into an XSLT service. So the way the XSLT service works is that you enter two URIs, one for the source content and the other for the XSLT sheet to apply to the source content. My transformation kept failing and it was because of the Accept headers that the XSLT service sent when it went to retrieve the source content. My server kept returning the RSS item fragment and not the XHTML. Now this would have been fine if I wanted to apply an XSLT sheet to my RSS item fragment, but in this case I wanted it to apply to the XHTML. Note that the problem could have been completely reversed, I could have been trying to apply the XSLT to the RSS item and not to the XHTML and my server could have returned the XHTML all the time. The crux of the problem is that when I gave the URI to the XSLT transformation service I have no way of specifying what mime-type to request. I get no chance to tweak the services Accept: header. Let's cover that again to clarify. If I hand you a URI only, and that URI supports conneg, then I get no control over which representation you retrieve. In the cases where you are passing a URI into a service that is later going to retrieve a represenation from that URI, you really have no idea which representation it's going to get. That could mean that you end up passing your RSS feed to the W3C HTML validator, or you end up passing XHTML instead of RSS into an XSLT translator service, or you end up passing a 12MB PNG to a handheld instead of that 20KB SVG file. You end up with a problem that is hard to debug and one that wouldn't exist if each URI had only one mime-type.