The XML FAQ

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.

However, for some static SGML applications (eg large document archives) with well-established and stable software, a good case can be made for ‘not fixing it if it ain't bust’, and deferring a move to XML until an appropriate time comes for a revision of the service or features.