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The XML FAQ — Frequently-Asked Questions about the Extensible Markup Language

Section 3: Authors

Q 3.7: If XML is just a subset of SGML, can I use my existing SGML tools?

Yes, if they are up to date

Yes, provided you use up-to-date SGML software which knows about the WebSGML Adaptations TC 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 What XML software is available?).

Unless there are very special reasons, you should probably plan to move your SGML to XML anyway.