XML in a Nutshell, 3rd Edition

Namespaces and DTDs

Namespaces are completely independent of DTDs and can be used in both valid and invalid documents. A document can have a DTD but not use namespaces or use namespaces but not have a DTD. It can use both namespaces and DTDs or neither namespaces nor DTDs. Namespaces do not in any way change DTD syntax nor do they change the definition of validity. For instance, the DTD of a valid document that uses an element named dc:title must include an ELEMENT declaration properly specifying the content of the dc:title element. For example:

<!ELEMENT dc:title (#PCDATA)>

The name of the element in the document must exactly match the name of the element in the DTD, including the prefix. The DTD cannot omit the prefix and simply declare a title element. The same is true of prefixed attributes. For instance, if an element used in the document has xlink:type and xlink:href attributes, then the DTD must declare the xlink:type and xlink:href attributes, not simply type and href.

Conversely, if an element uses an xmlns attribute to set the default namespace and does not attach prefixes to elements, then the names of the elements must be declared without prefixes in the DTD. The validator neither knows nor cares about the existence of namespaces. All it sees is that some element and attribute names happen to contain colons; as far as it’s concerned, such names are perfectly valid as long as they’re declared.