putting html inside an iframe (using javascript)

View the source of this page: http://mg.to/test/dynaframe.html It appears to do exactly what you want to do.

$(function() {
    var $frame = $('<iframe style="width:200px; height:100px;">');
    $('body').html( $frame );
    setTimeout( function() {
        var doc = $frame[0].contentWindow.document;
        var $body = $('body',doc);
        $body.html('<h1>Test</h1>');
    }, 1 );
});

You can do it without jQuery also:

var iframe = document.getElementById('iframeID');
iframe = iframe.contentWindow || ( iframe.contentDocument.document || iframe.contentDocument);

iframe.document.open();
iframe.document.write('Hello World!');
iframe.document.close();

jQuery's html strips body, html and head tags from the inserted HTML.