What else has changed between SGML and XML?
»The principal changes are in what you can do in writing a Document Type Definition (DTD). To simplify the syntax and make it easier to write processing software, a large number of SGML markup declaration options have been suppressed (see the list of omitted features). An extra delimiter is permitted in Names (the colon) for use in experiments with namespaces (enabling DTDs to distinguish element source, ownership, or application). A colon may only appear in mid-name, though, not at the start or the end. Work is ongoing to define how these can be declared and referenced using element and attribute markup.
