因此作为URI，http://www.w3.org/1999/xhtml和
http://www.w3.org/1999/xhtml/index.htm
没有任何联系，即标识不同的资源。

但在浏览器中访问这两个URI时，他们的“含义”（即打开的页面）有可能相同：
比如http://www.w3.org/所在的服务器设定默认文档为index.htm

12.4) Can I resolve the URI used as an XML namespace name?
Yes.

You can also eat a tractor, but that doesn't mean it's a good idea. The URIs used as XML namespace names are not guaranteed to point to anything, so there is generally no reason to resolve them. Furthermore, there is nothing in the processing of XML namespaces that requires you to resolve these URIs. Finally, as was noted in the previous question, many types of namespace URIs are unresolvable on the Web.

That said, some people have advocated placing RDDL (Resource Directory Description Language, pronounced "riddle") documents at the locations pointed to by namespace URIs. A RDDL document "provides a text description of some class of resources and of individual resources related to that class", including such things as a human-readable description of what the namespace describes and links to resources such as schemas (in a variety of languages), stylesheets, and code. RDDL documents apparently are being used, although it is not clear how widely.

i believe it is a good web for sb. who is in the same trouble as me
http://www.rpbourret.com/xml/NamespacesFAQ.htm

Unfortunately, this answer seems to be one that developers are unable or unwilling to hear. With few exceptions, namespace URIs are URLs, Uniform Resource Locators, and it's not unreasonable for users to expect a URL to locate a resource. However, namespace URLs are identifiers, not locators. Thus, the error logs of the W3C and other purveyors of XML specifications have been filling up with requests for nonexistent pages at the end of namespace URLs, and XML mailing lists are besieged with questions about whether one can run an XML parser on a machine that isn't connected to the Internet and is thus unable to resolve the namespace URL.

After addressing this issue again and again on various mailing lists, Tim Bray and Jonathan Borden decided to do something about it. If they couldn't convince developers that there wasn't anything at the end of a namespace URL, then maybe it would be a good idea to invent something to put there. What they came up with was the Resource Directory Description Language, RDDL for short. RDDL is an extensible XML application for documents that live at namespace URLs. RDDL is designed to allow both human readers and software robots to find any sort of resource associated with a particular namespace. Instead of putting one thing at the end of a namespace URI, RDDL puts a document there that lists all the machine-processable documents that might be available, including, but not limited to: