XML in a Nutshell, 3rd Edition

The Document Type Declaration

A valid document includes a reference to the DTD to which it should be compared. This is given in the document’s single document type declaration. A document type declaration looks like this:

<!DOCTYPE person SYSTEM "http://www.cafeconleche.org/dtds/person.dtd">

This says that the root element of the document is person and that the DTD for this document can be found at http://www.cafeconleche.org/dtds/person.dtd.

The document type declaration is included in the prolog of the XML document after the XML declaration but before the root element. (The prolog is everything in the XML document before the root element start-tag.) Example 3-3 demonstrates.

Example 3-3. A valid person document

<?xml version="1.0" standalone="no"?>
<!DOCTYPE person SYSTEM "http://www.cafeconleche.org/dtds/person.dtd">
<person>
  <name>
    <first_name>Alan</first_name>
    <last_name>Turing</last_name>
  </name>
  <profession>computer scientist</profession>
  <profession>mathematician</profession>
  <profession>cryptographer</profession>
</person>

If the document resides at the same base site as the DTD, you can use a relative URL instead of the absolute form. For example:

<!DOCTYPE person SYSTEM "/dtds/person.dtd">

You can even use just the filename if the DTD is in the same directory as the document:

<!DOCTYPE person SYSTEM "person.dtd">

Public IDs

Standard DTDs may actually be stored at multiple URLs. For example, if you’re drawing an SVG picture on your laptop at the beach, you probably want to validate ...