 XML does not define these by default, but specifications based on it can do so, and XHTML does.In addition, while the HTML 4, XHTML and XML specifications allow specifying numeric character references in either hexadecimal or decimal, SGML and older versions of HTML (and many old implementations) only support decimal references.Thus, to represent curly quotes in XML and SGML, it is safest to use the decimal numeric character references.