xmlns

xmlns is a feature of the XML Namespaces specification that enables disambiguation of element and attribute names when combining XML vocabularies. It works by declaring a mapping between a short prefix and a namespace URI, using either xmlns:prefix="URI" or xmlns="URI" for a default namespace. The declared mappings apply to the element on which the declaration appears and to all of its descendants, unless they are overridden by later declarations.

For example, a document might start with: <library xmlns="http://example.org/books" xmlns:dc="http://purl.org/dc/elements/1.1/">

<title>The Great Book</title>

<dc:title>Catalog Title</dc:title>

</library>. In this snippet, unprefixed elements like title belong to the default namespace http://example.org/books, while the prefixed dc:title belongs to the Dublin Core namespace.

Important notes: default namespaces affect element names but not attributes; to place an attribute in a namespace you must qualify it with a prefix (for example dc:author="..."). Namespace declarations have scope determined by the element tree and may be redefined in inner elements.

Namespaces help prevent name collisions and enable mixing of vocabularies in XML-based standards such as XHTML, SVG, SOAP, and RSS. The namespace URI is a string that serves as a unique identifier; it is not required to be dereferenced, though it may resolve to documentation.

Common pitfalls include assuming the default namespace applies to attributes, or assuming that the actual URL must be accessible at the time of parsing. Prefix choices are local to the document; different documents may use different prefixes for the same URI.