Hi.. I'm trying to study jQuery and I've got a question in some parts of a code.. Here is the code..

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
<html  xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
	<link rel="stylesheet" herf=".css" type="text/css"></link>
	<script src="jquery.js" type="text/javascript"></script>
	<script src=".js" type="text/javascript"></script>
<div id="switcher">
<div class="label">Text Size</div>
<button id="switcher-default">Default</button>
<button id="switcher-large">Bigger</button>
<button id="switcher-small">Smaller</button>
<div class="speech">
<p>Fourscore and seven years ago our fathers brought forth
on this continent a new nation, conceived in liberty,
and dedicated to the proposition that all men are created

So my question is one the statement
"<!DOCTYPE html.. blah blah>" and on the attribute of the <html> tag
"<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">". What are the functions of these statements and are they mandatory that they should be declared?

Thanks in advance for your response..