Is there a particular reason you're not going with a strict DOCTYPE?
The code is technically OK, but I'd put that into a PHP (or any other programming language) include so that you only ever need to edit a single file instead of a hundred or more, should the need arise.
Example (with your code):
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<meta http-equiv="content-type" content="text/html; charset=UTF-8" />
<meta name="keywords" content="example" />
<meta name="description" content="example" />
"stylesheet" type="text/css" href="example.css" />
And in your index.php (or any other language):
// Include header HTML document into index.php