Default Namespace Declaration |
    xmlns="namespace_URI"
When this attribute is included in an element start-tag, it and any unprefixed elements contained within it are automatically associated with the namespace URI given. If the xmlns attribute is set to the empty string, any effective default namespace is ignored, and unprefixed elements are not associated with any namespace.

An important caveat about default namespace declarations is that they do not affect unprefixed attributes. Unprefixed attributes are never explicitly named in any namespace, even if their containing element is.