Web Design in a Nutshell, 3rd Edition

Namespace

An XML namespace is a collection of element and attribute names as defined by the DTD of a particular markup language. In XML documents, you must explicitly identify the namespace so the client (in this case, the browser) knows that you intend the q element in your document to be a “quote” and not a “question” from some other (theoretical) XML language for exams.

The namespace is specified using the xmlns attribute in the html root element. The value is the location of an online documentation of that namespace. The namespace identifier for XHTML 1.0 and 1.1 is xmlns="http://www.w3.org/1999/xhtml".