You can load the iframes after html being rendered by giving empty src in html of iframe and later assigning src by jquery / javascript.
Html
<iframe id="iframe1" ></iframe>
Javascript, iframe could be loaded on some action like button click
document.getElementById('iframe1').src="/default.aspx";
As kern3l said, we can add data attribute in iframe for holding src instead of hard coding.
Html
<iframe id="iframe1" data-frameSrc="/default.aspx"></iframe>
Javascript
ifrmame1 = $('#iframe1')
ifrmam1.src = ifrmam1.data("frameSrc");
You can also make a new frame in jquery and assign src, this will load the page with blank frame.
$('<iframe>', {
src: '/default.aspx',
id: 'myFrame',
frameborder: 0,
scrolling: 'no'
}).appendTo('#parentDivId');
OR
var iframe = document.createElement('iframe');
iframe.frameBorder=0;
iframe.width="300px";
iframe.height="250px";
iframe.id="myFrame";
iframe.setAttribute("src", '/default.aspx');
document.getElementById("parentDivId").appendChild(iframe);