Try this:
body { text-align: center; margin: 0; padding: 0 }
#container { margin: 10px auto 0; padding: 10px 0; width: 400px; border: solid 2px #000 }
p { text-align: center; margin-right: auto; margin-left: auto; width: 75% }
It works great in the following situation (in both IE and Firefox):
<html>
<head>
<meta http-equiv="content-type" content="text/html;charset=iso-8859-1" />
<title>Test</title>
<style type="text/css" title="currentStyle">@import url(test.css);</style>
</head>
<body>
<div id="container">
<p>This paragraph should only be 75% of the parent container. The parent container should be 400 pixels wide and should be centered in the middle of the page.</p>
</div>
</body>
</html>
This, of course, uses an external stylesheet, but the same results can be had with an internal one.