SGML                  HTML                      XML
        ------------------------------------------------------------------------
             ISO 8879(1986-10) 
             AM1 (1988-07)
                                     RFC 1866 (1995-11)
             TC1 (1996-12)
                                     HTML 3.2 (1997-01)
                                     HTML 4.0 (1997-12)
                                                              XML 1.0 (1998-02)
                                     XHTML PR (1999-08)