The xml declaration is the first line of code and indicates that the content is a HTML document.
version attribute
The version attribute is coded as version="1.0", which indicates the document follows the XML version 1.0 syntax. The document should be coded to be "well-formed", even if it is not an entirely "valid" HTML document.
encoding attribute
The encoding attribute indicates what character encoding has been used to encode the HTML document as a file or byte input stream. Most importantly, among other things, this determines how many bytes of raw data are consumed to compose each character of the decoded document.
The xml declaration appears at the very beginning of an HTML document:
<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="/site-template.xsl"?><!DOCTYPE html><html xmlns="http://www.w3.org/1999/xhtml"> ...</html>
It is recommended that the encoding be specified using the preferred (MIME) name as specified in the IANA's Official Names for Character Sets. Some examples:
<?xml version="1.0" encoding="UTF-8"?><?xml version="1.0" encoding="ISO-8859-1"?><?xml version="1.0" encoding="US-ASCII"?><?xml version="1.0" encoding="EUC-JP"?><?xml version="1.0" encoding="UTF-16"?><?xml version="1.0" encoding="ISO-10646-UCS-2"?><?xml version="1.0" encoding="ISO-10646-UCS-4"?>
The xml declaration does not appear in HTML documents conforming to the 1997 HTML version 4 standard. For 2000 and later XHTML 1.x documents, the xml declaration was optional. For HTML version 5 documents, it's use is highly recommended.