Open Content Model like in XDR schema

Hi,

I have been looking at XML-Data and XDR and was wondering why the
concept of an open data model is not directly supported in XML Schema.  For
example, it would be
simple to have the validator simply ignore extra model elements if they
appeared in places
that did not disrupt the structure of the model e.g. at the bottom. This
behaviour is similar
to how browsers deal will unrecognized elements in html and is simple to
implement and
has proved to be very successful.

Thanks.