Return the value of namespaceUri specified at the last call to reset or the constructor, if non‐zero, else return the URI returned by the prefix stack for the current value of prefix(). Return an empty string if prefix() is an empty string, prefixStack() is zero, or prefix() is not active in the prefix stack. Note that, as per the XML namespace standard, an empty prefix does NOT refer to the default namespace, but rather refers to NO namespace.