XML declaration
XHTML (and other XML) documents often begin with an XML declaration before the DOCTYPE declaration. The XML declaration is not required. The declaration may include the encoding of the document, as shown in this example.
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">The XML declaration may be provided even for XHTML documents
served as text/html.
Because the default encoding for all XML documents is UTF-8 or UTF-16, encoding information in the XML declaration is not required for these encodings, and thus can be omitted as a space-saving optimization.
In addition, although it is technically correct to include the XML declaration in such documents, Appendix C of the XHTML 1.0 specification, “HTML Compatibility Guidelines,” recommends avoiding it, and many authors choose to omit it because of browser-support issues. For example, when Internet Explorer 6 for Windows detects a line of text before the DOCTYPE declaration, it converts to Quirks Mode (see Chapter 9 for details), which can have a damaging effect on how the documents styles are rendered. (This is reportedly fixed in IE 7.) It is required only if your document uses an encoding other than UTF-8 or UTF-16 and if the encoding has not been set on the server.
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