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This module is part of a collection dedicated to learning XML.
I recommend that you open another copy of this document in a separate browser window and use the following links to easily find and view the figures and listings whileyou are reading about them.
I recommend that you also study the other lessons in my extensive collection of online programmingtutorials. You will find a consolidated index at www.DickBaldwin.com .
In previous lessons, I have discussed tags, elements, content, and attributes in detail. The time has come to take up the following topics:
What is a DTD?
Figure 1 contains a quotation from the XML FAQ that describes a DTD.
| Figure 1 . What is a DTD? |
|---|
"A DTD is usually a file (or several files to be usedtogether) which contains a formal definition of a particular
type of document. This sets out what names can be used forelements, where they may occur, and how they all fit together.
For example, if you want a document type to describe<LIST>s
which contain<ITEM>s, part of your DTD would contain
something like<!ELEMENT item (#pcdata)><!ELEMENT list (item)+>This defines items containing text, and lists containing
items.It's a formal language which lets processors automatically
parse a document and identify where every element comes andhow they relate to each other, so that stylesheets, navigators,
browsers, search engines, databases, printing routines, andother applications can be used." |
DTDs are complicated
I included the above quotation to emphasize one very important point -- DTDs are complicated. The creation of a DTD of any significanceis a very complex task.
The good news!
The good news is that many of you will never need to worry about having to create a DTD for two reasons:
A validating XHTML editor
For example, I wrote the original version of this HTML document using a validating XHTML editor named Amaya . Even though the editor uses a DTD to confirm that my document is a valid XHTML document (and warns me if it isn't) , it wasn't necessary for me to write the DTD. The people who wrote the editor also wrote the DTD.
Three Parts
It is reasonable to think of an XML document as consisting of three parts, some of which are optional. I'm going to refer to the parts as files just so Iwill have something to call them (but they don't have to be separate physical files) .
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