Open a new tab with custom HTML instead of a URL
April 2021 Edit: This answer is obsolete now. Chrome banned loading data URIs into the top window, and the iframe solution doesn't work for me in Firefox.
Original answer: If the other answer gives you Error: Permission denied to access property "document", see this question about how to handle same-origin policy problems, or this one.
Or, quick and dirty, use a data URI:
var html = '<html><head></head><body>ohai</body></html>';
var uri = "data:text/html," + encodeURIComponent(html);
var newWindow = window.open(uri);
You can do this:
var newWindow = window.open();
and then do
newWindow.document.write("ohai");