XML Namespaces
An XML document may use tags that come from different XML applications or vocabularies. For example, you might have an XHTML document that also contains some math expressions written using MathML. But in this case, the parser needs to differentiate between an a element coming from XHTML (an anchor) and an a element that might come from MathML (an absolute value).
The W3C anticipated such conflicts and responded by creating the namespace convention (see the Recommendation at http://www.w3.org/TR/REC-xml-names). A namespace provides a name for a particular XML vocabulary, the group of element and attribute names used in an XML application. This allows several XML vocabularies to be used in a single XML document.
When you reference elements and attributes in your document, the browser looks them up in the namespace to find out how they should be used. Namespaces have names that look just like URLs (they are not links to actual documents, however) to ensure uniqueness and provide information about the organization that maintains the namespace.
Namespaces are declared in an XML document using the xmlns attribute. You can establish the namespace for a whole document or an individual element. Typically, the value of the xmlns attribute is a reference to the URL-like namespace. This example establishes the default namespace for the document to be transitional XHTML:
<html xmlns="http://www.w3.org/1999/xhtml">
If you need to include math
markup, you can apply the xmlns attribute ...
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