XMLs "predecessor" is full-blown SGML, and since XML is just a proper subset of SGML, XML is no improvement. The one thing it brought was DTD-less, canonical angle-bracket markup where documents can be parsed without grammar rules eg DTDs (this was also incorporated into SGML via the WebSGML adaptations, so that XML could remain a proper subset of SGML). But this also means XML is useless for parsing HTML, the most important markup language (XML was started to supersede HTML as XHTML, but that failed).