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 ...
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