Although this is a old post, I have spent 3 hours to fix my issue and I think this might help someone in future.
Here is my jquery-dialog hack to show html content inside an <iframe> :
let modalProperties = {autoOpen: true, width: 900, height: 600, modal: true, title: 'Modal Title'};
let modalHtmlContent = '<div>My Content First div</div><div>My Content Second div</div>';
// create wrapper iframe
let wrapperIframe = $('<iframe src="" frameborder="0" style="width:100%; height:100%;"></iframe>');
// create jquery dialog by a 'div' with 'iframe' appended
$("<div></div>").append(wrapperIframe).dialog(modalProperties);
// insert html content to iframe 'body'
let wrapperIframeDocument = wrapperIframe[0].contentDocument;
let wrapperIframeBody = $('body', wrapperIframeDocument);
wrapperIframeBody.html(modalHtmlContent);