Is xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance” a special case in XML?

Question

When we use a namespace, we should also indicate where its associated XSD is located at, as can be seen in the following example:

Answer 1

If you look to content of: http://www.w3.org/2001/XMLSchema-instance you found that there is text:

...

  
   
    
XML Schema instance namespace
    
See the XML Schema
       Recommendation for an introduction
...

Because xmlns="http://www.w3.org/1999/xhtml" you see above link as HTML document.

According to http://www.w3.org/TR/xmlschema-1/#schema-loc (Schema Representation Constraint: Schema Document Location Strategy) schema-aware processors may implement any combination of the following strategies, in any order:

• Attempt to resolve the namespace name to locate such a resource.

So http://www.w3.org/2001/XMLSchema-instance point to itself (yea, recursion!!).

See similar answer: Where is the XSD file for "http://www.w3.org/2001/XMLSchema-instance"?

PS. Any specialized XML processor can treat http://www.w3.org/2001/XMLSchema-instance as known thing without actually download this definition (and most do that).

PPS. Semantic of http://www.w3.org/2001/XMLSchema-instance defined by XML Schema standard http://www.w3.org/TR/xmlschema-1/