Whats the best Web page structure convention that should be used when created and publishing professional Web pages?

I've created three HTML files on my PC with different formatting structures and had a look at the file sizes and I figured it's a pretty important thing to take into consideration when creating Web pages. So, care to vote please? And also, give me your views, opinions and your knowledge too please . Don't forget your brains, I want them too hehe.

one.html - 366 Bytes
Code HTML4Strict:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
</head>
 
<body>
<div id="mainContainer">
<h1>Hello World!</h1>
</div>
</body>
</html>

two.html - 372 Bytes
Code HTML4Strict:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
	<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
	<title>Untitled Document</title>
</head>
 
<body>
	<div id="mainContainer">
		<h1>Hello World!</h1>
	</div>
</body>
</html>

three.html - 384 Bytes
Code HTML4Strict:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
 
	<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
	<title>Untitled Document</title>
 
</head>
 
<body>
 
	<div id="mainContainer">
 
		<h1>Hello World!</h1>
 
	</div>
 
</body>
</html>

I currently use convention three.

Andrew Cooper