For systems like HTTP that use MIME types to indicate how to manipulate data, it would sometimes be useful to specify a number of types in order of preference. For instance, an RSS 1.0 feed can be dealt with by an RSS 1.0 parser; it's also a valid RDF file that any RDF toolchain can handle, a valid XML file which could be displayed in an XML editor, and text that can be displayed verbatim if all else fails. This could be specified by sending a list of Content-Types rather than just one, or by having a hierachy of Content-Types in a shared database somewhere.