XML in a Nutshell, 3rd Edition

The XML Declaration

XML documents should (but do not have to) begin with an XML declaration. The XML declaration looks like a processing instruction with the name xml and with version, standalone, and encoding pseudo-attributes. Technically, it’s not a processing instruction, though; it’s just the XML declaration, nothing more, nothing less. Example 2-7 demonstrates.

Example 2-7. A very simple XML document with an XML declaration

<?xml version="1.0" encoding="ASCII" standalone="yes"?>
<person>
  Alan Turing
</person>

XML documents do not have to have an XML declaration. However, if an XML document does have an XML declaration, then that declaration must be the first thing in the document. It must not be preceded by any comments, whitespace, processing instructions, and so forth. The reason is that an XML parser uses the first five characters (<?xml) to make some reasonable guesses about the encoding, such as whether the document uses a single-byte or multibyte character set. The only thing that may precede the XML declaration is an invisible Unicode byte-order mark. We’ll discuss this further in Chapter 5.