TO make existing html files to xml you can 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.

   A markup language is a set of words and symbols for describing the identity of pieces of a document. Programs can use this with a stylesheet to create output for screen, print, audio, video, Braille, etc.