"rel=nofollow" is not a recognised attribute for the <iframe> tag according to W3C specs.
 
 Use some javascript and document.write to place the iframe code on the page. You'll still need to make sure that the url is recognised as being a url, so I'd suggest first passing the html through the javascript escape() function, then copying and pasting that into the javascript code:
 
 <script type="text/javascript">
document.write( unescape('your%20escaped%20html%20code') );
</script>

or, place it in an external .js file, in a directory blocked by robots.txt, and write it to an element using getElementById() in the page.