Attr.namespaceURI

The Attr.namespaceURI read-only property returns the namespace URI of the attribute, or null if the element is not in a namespace.

namespace = attribute.namespaceURI

In this snippet, an attribute is being examined for its localName and its namespaceURI. If the namespaceURI returns the XUL namespace and the localName returns "browser", then the node is understood to be a XUL <browser/>.

if (attribute.localName == "value" &&
    attribute.namespaceURI == "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul") {
  // this is a XUL value
}

This is not a computed value that is the result of a namespace lookup based on an examination of the namespace declarations in scope. The namespace URI of an attribute is frozen at the attribute creation time.

In Firefox 3.5 and earlier, the namespace URI for HTML attributes in HTML documents is null. In later versions, in compliance with HTML5, it is https://www.w3.org/1999/xhtml as in XHTML.

You can create an attribute with the specified namespaceURI using the DOM Level 2 method Element.setAttributeNS.

Per the Namespaces in XML specification, an attribute does not inherit its namespace from the element it is attached to. If an attribute is not explicitly given a namespace, it has no namespace.

The DOM does not handle or enforce namespace validation per se. It is up to the DOM application to do any validation necessary. Note too that the namespace prefix, once it is associated with a particular node, cannot be changed.

• Attr.localName
• Attr.prefix
• Element.namespaceURI