Web Design in a Nutshell, 3rd Edition

When to Use a DTD

If you create a markup language in XML, it is not mandatory that it have a DTD. In fact, DTDs come with a few disadvantages. A DTD is useful when you have specific markup requirements to apply across a large number of documents. A DTD can ensure that certain data fields are present or delivered in a particular format. You may also want to spend the time preparing a DTD if you need to coordinate content from various sources and authors. Having a DTD makes it easier to find mistakes in your code.

The disadvantages to DTDs are that they require time and effort to develop and are inconvenient to maintain (particularly while the XML language is in flux). DTDs slow down processing times and may be too restrictive on the user’s end. Another problem with DTDs is that they are not compatible with the namespace convention (discussed next). Elements and attributes from another namespace won’t validate under a DTD unless the DTD explicitly includes them. If you are creating just a few XML documents, you may choose not to create a DTD. If you are using namespaces and it is necessary to have documentation of your XML vocabulary, you must use an XML Schema.

Tip

Because XHTML is a markup language that is used on a global scale, it was necessary to define the language and its various versions in DTDs. An XHTML document must include a DOCTYPE declaration to specify which DTD it follows in order to validate.