XML in a Nutshell, 3rd Edition

Namespace URIs

Many XML applications have customary prefixes. For example, SVG elements often use the prefix svg, and RDF elements often have the prefix rdf. However, these prefixes are simply conventions and can be changed based on necessity, convenience, or whim. Before a prefix can be used, it must be bound to a URI like http://www.w3.org/2000/svg or http://www.w3.org/1999/02/22-rdf-syntax-ns#. It is these URIs that are standardized, not the prefixes. The prefix can change as long as the URI stays the same. An RDF processor looks for the RDF URI, not any particular prefix. As long as nobody outside the w3.org domain uses namespace URIs in the w3.org domain, and as long as the W3C keeps a careful eye on what its people are using for namespaces, all conflicts can be avoided.

Namespace URIs do not necessarily point to any actual document or page. In fact, they don’t have to use the http scheme. They might even use some other protocol like mailto in which URIs don’t even point to documents. However, if you’re defining your own namespace using an http URI, it would not be a bad idea to place some documentation for the XML application at the namespace URI. The W3C got tired of receiving broken-link reports for the namespace URIs in their specifications, so they added some simple pages at their namespace URIs. For more formal purposes that offer some hope of automated resolution and other features, you can place a Resource Directory Description Language (RDDL) document at the namespace ...