XML.com FAQs > C. Authors of SGML (including writers of HTML: Web page owners)
Question:  C.8 If XML is just a subset of SGML, can I use XML files directly with existing SGML tools?

Yes, provided you use up-to-date SGML software which knows about the WebSGML Adaptations to ISO 8879 (the features needed to support XML, such as the variant form for EMPTY elements; some aspects of the SGML Declaration such as NAMECASE GENERAL NO; multiple attribute token list declarations, etc).

An alternative is to use an SGML DTD to let you create a fully-normalised SGML file, but one which does not use empty elements; and then remove the DocType Declaration so it becomes a well-formed DTDless XML file.

Most SGML tools now handle XML files well, and provide an option switch between the two standards. (see the pointers in the question on software).

This FAQ is from The XML FAQ, maintained by Peter Flynn