Default Namespaces
To declare and use the default namespace, you omit the alias and the colon from the declaration on the element as shown on this book element <BOOK xmlns="https://www.contoso.com/books.dtd">.
Any element that is not fully qualified with a namespace prefix belongs to the default namespace. When multiple namespaces are used within an XML document, defining one namespace as the default namespace provides a cleaner looking document. Only the elements from namespaces other than the default namespace need to be fully qualified. Default namespaces only apply to elements and do not apply to attributes.