Its an XML namespace. It is required when you use XHTML 1.0 or 1.1 doctypes or application/xhtml+xml mimetypes.
You should be using HTML5 doctype, then you don't need it for text/html. Better start from template like this :
<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>domcument title</title>
<link rel="stylesheet" href="/stylesheet.css" type="text/css" />
</head>
<body>
<!-- your html content -->
<script src="/script.js"></script>
</body>
</html>
When you have put your Doctype straight - do and validate you html and your css .
That usually will sove you layout issues.