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The XML FAQ — Frequently-Asked Questions about the Extensible Markup Language
Section 2: Existing users
Q 2.8: Do I have to switch from SGML or HTML to XML?
Not if you don't want to
No, existing HTML applications software will continue to work with existing files. There is now very little SGML left, but what there is will continue to work with its existing software. But as with any enhanced facility, if you want to view or download and use XML files, you will need to use XML-aware software. Much more has been developed for XML than there ever was for SGML, so almost all current effort is concentrated on XML (and XHTML and HTML5).