I have a php file on my test server, and it outputs the following:

There is some extra white space on the page, highlighted by the red box.
Dev tools view:

The extra whitespace is highlighted in the dev tools.
File that is outputted:

As you can see, all of the file is on one line.
Code:
<?php
header("Content-Type: text/html; charset=utf-8");
if(!isset($_GET['action']))
{
$_GET['action'] = "home";
}
echo("<!DOCTYPE HTML>");
echo("<html>");
echo("<head>");
echo("<title>Bug Tracker - ");
echo($_GET['action']);
echo("</title>");
echo('<link rel="stylesheet" href="common.css" />');
echo("</head>");
echo('<body>');
require("action.php");
echo("</body>");
echo("</html>")
?>
This happens in firefox as well.
<h1>tag usually has a default margin. Try to seth1 { margin:0 }in your CSS.