The XML Declaration

To begin an XML document, it is a good idea to include the XML declaration as the very first line of the document. I say "good idea" because, though the XML declaration is optional, it is suggested by the W3C specification.

Essentially, the XML declaration is a processing instruction that notifies the processing agent that the following document has been marked up as an XML document. It will look something like the following:

<?xml version = "1.0"?>

We'll talk more about the gory details of processing instructions later, but we can at least explain how the XML declaration works.

All processing instructions, including the XML declaration, begin with <? and end with ?>. Following the initial <?, you will find the name of the processing instruction, which in this case is "xml".

The XML processing instruction, requires that you specify a version attribute and allows you to specify optional standalone and encoding attributes.

In its full regalia, the XML declaration might look like the following:

<?xml version	= "1.0"? 
standalone	= "yes" 
encoding	= "UTF-8"?>

The version Attribute

As we said before, if you do decide to use the optional XML declaration, you must define the version attribute. As of this writing, the current version of XML is 1.0. Note that if you include the optional attributes, version must be specified first.

The standalone Attribute

The standalone attribute specifies whether the document has any markup declarations that are defined in a separate document. Thus, if standalone is set to "yes", there will be no markup declarations in external DTDs. Setting it to "no" leaves the issue open. The document may or may not access external DTDs.

The encoding Attribute

Data Versus Markup
Introduction to XML For Web Developers | Table of Contents
Elements