XML Companion, The, Third Edition

General extensions

There are a number of standards that do not apply to a specific usage of XML, but are applicable to most or all possible applications of this standard (they are introduced here, then discussed in detail in the following chapters).

Namespaces

When an XML document conforms to a specific DTD, all the elements in this document can be said to be in a single 'namespace'. The element and attribute names are assigned by the same authority, and are either unique, or contextually unique, so there is no ambiguity about their purpose.

But it is often desirable for an XML document to contain elements from multiple DTDs, or 'namespaces'. This could confuse software (and people too), especially when two DTDs define an element or attribute ...