This is killing me.
Application layout:
<html>
<head>
...
</head>
<body topmargin="0"><%= yield %></body>
</html>
Home:
<div id="content">...</div>
Yet in the rendered page, across all browsers, there’s a line of preformatted whitespace that pushes the content off the top of the window.

Where is this coming from?
Resolved
Invalid HTML markup (li directly within a div) caused all browsers to add a gap at the top. Strange!
You must have an open html tag somewhere, just run your html through a validator and see if it catches any.
http://validator.w3.org/#validate_by_input