In http://lists.w3.org/Archives/Public/www-dom-ts/2003Jun/0005.html

David Clover wrote:

Looking at CVS I see at least this one remains, in
namednodemapsetnameditemns10:

  <getNamedItemNS var="entity" obj="entities" namespaceURI="null"
  localName='"ent1"'/>
  ...same for notation...

Curt Arnold writes:

The transforms do not have any special interpretation for null and treat its use
in this fragment as a variable reference.  They do not attempt to diagnose use
of undeclared variables and leave that to the compiler, but in this case, the
resulting code is legal.  

The proper way is to define a variable and set the isNull attribute.  This null
is typed and should be much easier to handle in other language transforms.

<var name="nullNS" type="DOMString" isNull="true"/>
...
  <getNamedItemNS var="entity" obj="entities" namespaceURI="nullNS"
  localName='"ent1"'/>
  ...same for notation...