Either convert them to conform to some new document type (with or without a DTD or Schema) and write a stylesheet to go with them; or edit them to conform to XHTML. It is necessary to convert existing HTML files because XML does not permit end-tag minimisation (missing, etc), unquoted attribute values, and a number of other SGML shortcuts which have been normal in most HTML DTDs. However, many HTML authoring tools already produce almost (but not quite) well-formed XML. You may be able to convert HTML to XHTML using the Dave Raggett’s HTML Tidy program, which can clean up some of the formatting mess left behind by inadequate HTML editors, and even separate out some of the formatting to a stylesheet, but there is usually still some hand-editing to do.