XML in a Nutshell, 3rd Edition

What’s at the End of a Namespace URL?

The people who wrote the namespaces specification couldn’t agree on what should be put at the end of a namespace URL. Should it be a DTD, a schema, a specification document, a stylesheet, software for processing the application, or something else? All of these are possible, but none of them are required for any particular XML application. Some applications have DTDs; some don’t. Some applications have schemas; some don’t. Some applications have stylesheets; some don’t. Thus, for the most part, namespaces have been purely formal identifiers. They do not actually locate or identify anything.

“Namespaces in XML” specifically states that “The namespace name, to serve its intended purpose, should have the characteristics of uniqueness and persistence. It is not a goal that it be directly usable for retrieval of a schema (if any exists).” That is, it is not required that there be anything in particular, such as a DTD or a schema, at the end of the namespace URL. Indeed, it’s not even required that the namespace name be potentially resolvable. It might be an irresolvable URN such as urn:isbn:1565922247. On the other hand, this doesn’t say that there can’t be anything at the end of a namespace URL, just that there doesn’t have to be.

Nonetheless, this hasn’t stopped numerous developers from typing namespace URLs into their web browser location bars and filling the error logs at the W3C and elsewhere with 404 Not Found errors. It hasn’t stopped weekly ...