XML.com FAQs > B. Existing users of SGML (including HTML: everyone who browses the Web)
Question:  B.4 Do I have to switch from SGML or HTML to XML?

No, existing SGML and HTML applications software will continue to work with existing files. 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. There is much more being developed for XML than there ever was for SGML, so a lot of users are moving.

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