It works. Problem is there is no H1, only H2.
Check out this link http://jsfiddle.net/pPqGe/1545/.
HTML:
<iframe src='jsfiddle.net'></iframe>
JavaScript:
$(document).ready(function() {
$('iframe').contents().find('h2').css('color','red');
$('iframe').contents().find('#header').css('opacity','.2');
});