9.3.8 Unsupported features

The current parser is rather limited. While it is able to deal with many serious documents, it omits several less-used features of SGML and XML. Known missing SGML features include

• NOTATION on entities
  Though notation is parsed, notation attributes on external entity declarations are not represented in the output.
• NOTATION attributes
  SGML notations may have attributes, declared using <!ATTLIST #NOT name attrib>. Those data attributes are provided when you declare an external CDATA, NDATA, or SDATA entity. XML does not support external CDATA, NDATA, or SDATA entities, nor any of the other uses to which data attributes are put in SGML.

• SGML declaration
  The `SGML declaration' is fixed, though most of the parameters are handled through indirections in the implementation.

• The RANK feature
  It is regarded as obsolete.
• The LINK feature
  It is regarded as too complicated.
• The CONCUR feature
  Concurrent markup allows a document to be tagged according to more than one DTD at the same time. It is not supported.
• The Catalog files
  Catalog files are not supported.

In the XML mode, the parser recognizes SGML constructs that are not allowed in XML. Also various extensions of XML over SGML are not yet realized. In particular, XInclude is not implemented.