HTTP URIs should be dereferenceable, i. e. HTTP clients can look up the URI using the HTTP protocol and retrieve a description of the resource that is identified by the URI. The Web is intended to be an information space that may be used by humans as well as by machines. Both should be able to retrieve representations of resources in a suitable form, e.g. HTML for humans and RDF for machines, using an HTTP mechanism called content negotiation. The basic idea of content negotiation is that HTTP clients send HTTP headers with each request to indicate what kinds of documents they prefer. Servers can inspect these headers and select an appropriate response. If the headers indicate that the client prefers HTML, then the server will respond by sending an HTML document. If the client prefers RDF, then the server will send the client an RDF document.

Cool URIs for the Semantic Web describes and motivates two different strategies used to make URIs that identify real-world objects dereferenceable - 303 URIs and hash URIs. Both strategies ensure that objects and the documents that describe them are not confused, and that humans as well as machines can retrieve appropriate representations.