Datasheet

default namespace. There is no way to get one "automatically". If you don’t define a default namespace,

and then you write an unprefixed element or attribute, that element or attribute is in no namespace at

all.

Namespace prefixes can be declared on any element in a document, not just the root element. This

includes changing the default namespace. If you declare a prefix that has been declared on an ancestor

element, then the new prefix declaration works for the element where it’s declared and all its child ele-

ments.

You may declare multiple prefixes for the same namespace URI. Doing so is perfectly allowable; how-

ever, remember that namespace equality is based on the namespace URI, not the namespace prefix.

Thus elements that look like they should be in the same namespace can actually be in different name-

spaces. It all depends on which URI the namespace prefixes have been bound to. Also note that certain

namespaces have commonly accepted uses, such as the xsi prefix used in this example. Here are some of

the more common prefixes:

Namespace Prefix Namespace URI Usage

xsi http://www.w3.org/2001/XMLSchema-instance XML Schema Instance

xsd http://www.w3.org/2001/XMLSchema XML Schema

xsl http://www.w3.org/1999/XSL/Transform XSLT

fo http://www.w3.org/1999/XSL/Format XSL Formatting

Objects

xlink http://www.w3.org/1999/xlink XLink

svg http://www.w3.org/2000/svg Scalable Vector

Graphics

ds http://www.w3.org/2000/09/xmldsig# XML Signature

xenc http://www.w3.org/2001/04/xmlenc# XML Encryption

Validity

The second rule for XML documents is validity. It’s a little odd to say "rule" because XML documents

don’t have to be valid, but there are well defined rules that say what it means for a document to be valid.

Validity is the next step up from well-formedness. Validity lets you say things like this: Every book ele-

ment must have a title element followed by an author element, followed by an isbn element, and so on.

Validity says that the document is valid according to the rules of some grammar. (Remember diagram-

ming sentences in high-school English? It’s the same kind of thing we’re talking about here for valid

XML documents.)

Because a document can only be valid according to the rules of a grammar, you need a way to describe

the grammar the XML document must follow. At the moment, there are three major possibilities: DTDs,

the W3C’s XML Schema, and OASIS’s Relax-NG.

Chapter 1

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