The XML FAQ

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 Name Start Character is permitted in XML Names (the colon) for use with namespaces (enabling DTDs to distinguish element source, ownership, or application). Despite its classification, a colon may only appear in mid-name, not at the start or the end.