You could add CSS to the document....
<!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>
<title>Hello</title>
<style type="text/css">
body { margin:0; }
</style>
</head>
<body>
<div style="width: 100px; height: 100px; background: red;">
<div>Hello</div>
</div>
</body>
Or you could add the CSS as an inline style
<!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>
<title>Hello</title>
</head>
<body style="margin:0;">
<div style="width: 100px; height: 100px; background: red;">
<div>Hello</div>
</div>
</body>
</html>
All browsers have a default margin around the top and left edge of the window. There's nothing wrong with your page. You merely need to tell the browser to remove the default margins.
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