Valid XML Documents

Most XML browsers will check your document to see whether it is well-formed. Some of them also can check whether it's valid. An XML document is valid if there is a document type definition (DTD) associated with it, and if the document complies with that DTD.

A document's DTD specifies the correct syntax of the document, as we'll see in Chapter 3. DTDs can be stored in a separate file or in the document itself, using a <!DOCTYPE> element. Here's an example in which I add a <!DOCTYPE> element to the greeting XML document we developed earlier:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/css" href="first.css"?>
<!DOCTYPE DOCUMENT [
    <!ELEMENT DOCUMENT (GREETING, MESSAGE)>
    <!ELEMENT GREETING (#PCDATA)>
    <!ELEMENT MESSAGE (#PCDATA)>
]>
<DOCUMENT>
    <GREETING>
        Hello From XML
    </GREETING>
    <MESSAGE>
        Welcome to the wild and woolly world of XML.
    </MESSAGE>
</DOCUMENT>

We'll see more about DTDs in Chapter 3, but this DTD indicates that you can have <GREETING> and <MESSAGE> elements inside a <DOCUMENT> element, that the <DOCUMENT> element is the root element, and that the <GREETING> and <MESSAGE> elements can hold text.

Most XML browsers will check XML documents for well-formedness, but only a few will check for validity. I'll talk more about where to find XML validators in the later section "XML Validators."

We've gotten an overview of XML documents now, including how to display them using style sheets, and what constitutes a well-formed and valid document. However, this is only part of the story. Many XML documents are not designed to be displayed in browsers at all, for example, or even if they are, they're are not designed to be used with modern style sheets (such as browsers that convert XML into industry-specific graphics such as molecular structure, physics equations, or even musical scales). The more powerful use of XML involves parsing an XML document to break it down into its component parts and then handling the resulting data yourself. Next I'll take a look at a few ways of parsing XML data that are available to us.