insert html into window

how do I insert html into a window? This doesn't seem to work:

You have four problems here:

1) this is a Rails mailing list -- find a JavaScript list for this question
2) "doesn't seem to work" -- http://catb.org/~esr/faqs/smart-questions.html

var testwindow = window.open ("", "mywindow",
"height=800,width=400,location=no,scrollbars=yes");
var el = testwindow.document.createElement('div');

3) creating an element doesn't insert it into the DOM

el.innerHTML = content;

4) content is apparently undefined.

HTH,