Extracted from xml.dom.minidom:
~~~
Node(...):
   ...
   ownerDocument = None
   ...

Element(Node):
    __slots__=('ownerDocument', ...)
    ...
~~~

As Element declares an ownerDocument attribute in __slots__, Node's ownerDocument attribute is hidden:
~~~
class B: b=1;
class D(B): __slots__={'b'}
D().b -> AttributeError
~~~

This leads to a strange behaviour were accessing a base attribute fails with an attribute error.

Should the Node.ownerDocument attribute not be removed?
Or its name removed from the Element.__slots__ list?
Ie have the attribute in the base or the derivative, but not both.

Independent note: <https://docs.python.org/3/reference/datamodel.html#slots> says:
> When inheriting from a class without __slots__ [Node], the __dict__ attribute of that class will always be accessible, so a __slots__ definition in the subclass [Element] is meaningless.

So as for as I understand Element.__slots__ does not reduce the Element() footprint (it was introduced for that).

It looks like the __slots__ declaration on Node was missed when the other slots changes were made (by MvL in 3931f043b79a).  Note that the Node base class (xml.dom.Node) has a __slots__ declaration, but it was added after MvL's patch (by Florent in 49c5511b234a).

Changing this now may have backward compatibility issues, though.