Development files for the GNOME XML library XML is a metalanguage to let you design your own markup language. A regular markup language defines a way to describe information in a certain class of documents (eg HTML). XML lets you define your own customized markup languages for many classes of document. It can do this because it's written in SGML, the international standard metalanguage for markup languages. Install this package if you wish to develop your own programs using the GNOME XML library.