...,
absolute_file_name(dtd(Type),
                   [ extensions([dtd]),
                     access(read)
                   ], DtdFile),
...

Note that DTD objects may be modified while processing errornous documents. For example, loading an SGML document starting with <?xml ...?> switches the DTD to XML mode and encountering unknown elements adds these elements to the DTD object. Re-using a DTD object to parse multiple documents should be restricted to situations where the documents processed are known to be error-free.

The DTD html is handled separately. The Prolog flag html_dialect specifies the default html dialect, which is either html4 or html5 (default).^{3Note that HTML5 has no DTD. The loaded DTD is an informal DTD that includes most of the HTML5 extensions (http://www.cs.tut.fi/~jkorpela/html5-dtd.html). In addition, the parser sets the dialect flag of the DTD object. This is used by the parser to accept HTML extensions.} Next, the corresponding DTD is loaded.