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copied!<p>Actually <strong>in SGML</strong> (which XML is based on) the URL (=SYSTEM identifier) is in the same way optional as the PUBLIC identifier. If the keyword <code>PUBLIC</code> exists, the document should be always validated by using a built-in DTD (from a catalog) that matches the given formal public identifier (FPI), <code>"-//W3C//DTD GRAMMAR 1.0//EN"</code> in your case. In SGML the PUBLIC identifier could be followed by a system identifier which should be taken as a hint about the DTD that is used. <strong>In XML</strong> this has changed so that the PUBLIC identifier exists, it <strong>must</strong> be followed by a system identifier; but this doesn't change the logic or purpose of these identifiers.</p> <p>The public identifier indicates the specification this document follows so practically it tells the grammar that is used in the whole document, and so in a way it serves the same purpose as namespaces in XML nowadays do. The form of the public identifiers usually follows a common structure and therefore contain a language code. This code only implies in what (natural/human) language the referred specification was written and does not mean that the document itself would contain use this language. In your case, the DOCTYPE does <strong>not</strong> mean that your document should use, refer to or be about English language.</p> <p>Instead of <code>PUBLIC</code> the DOCTYPE declaration can also contain keyword <code>SYSTEM</code> which means that the DTD should be retrieved as a system specific fashion. <code>SYSTEM</code> is followed by only a URL or a file path.</p>

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