Proposal: creation methods also at element nodes

Dear all!

I propose to have the creation methods "createElement", "createElementNS",
"createAttribute", and "createAttributeNS" also at element nodes (and not only
at document nodes).

The advantage is that depending on the element for which a sub-element or
attribute is created different classes may be instantiated. Implementations
that instantiate always the same classes may delegate the creation to their
owner document.

I use such specialized element and attribute classes to include processing
logic in the DOM.

A consequence would be that such specialized nodes may not be moved freely
in the DOM. A HIERARCHY_REQUEST_ERROR must be raised if they are to be added
to an improper element.

Thanks for your attention,
--Stefan