Editable Iframe sample uses the following script and html to make this possible
<script type='text/javascript>setTimeout(ready,50); function ready(){ if ( document.readyState == "complete" ){ ifrme = document.getElementById ( 'ifrme'); if ( typeof ( ifrme.document ) != "undefined" ){ ifrme.document.designMode = "On"; } if ( typeof ( ifrme.contentDocument ) != "undefined" ){ ifrme.contentDocument.contentEditable = true ; ifrme.contentDocument.designMode = "On"; }else{ setTimeout(ready,50); }</script>