1st problem HTML

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.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>
<link href="style.css" type="text/css" rel="stylesheet" />
</head>
<body>
<div id="container">
<div id="titlu"></div>

<div id="navigation">
<ul class="nav_menu">
<li><a href="#">Home</a></li>
<li><a href="#">Login</a></li>
<li><a href="#">Inregistrare</a></li>
<li><a href="#">Resetare parola</a></li>
<li><a href="#">Despre noi</a></li>
</ul>
</div><!--navigation-->


</div><!--container-->


<div id="content_stanga">
<div id="ce_este"></div>
<div id="sus"></div>
<div id="centru">
<div id="centru_div">
text de aratat si poze!
</div>
</div>
<div id="jos"></div>
</div><!--content stanga-->

<div id="content_dreapta">
<div id="cum_pot"></div>
<div id="sus"></div>
<div id="centru">
<div id="centru_div">
explicatiuni
</div>
</div>
<div id="jos"></div>
</div><!--content dreapta-->

</body>
</html>