Mozilla seems to allow:
document.designMode = 'on'; // apply designMode to the whole page
document.getElementById('anIframe').contentDocument.designMode = 'on'; // apply to an Iframe
But I don't want to to set designMode on the whole page and I don't want to use frameset unless I really have to.

IE seems to allow...
document.getElementById('aSpanOrADiv').contentEditable = true;
...but I can't get Mozilla to do anything similar. Is there a way to selectively apply designMode/contentEditable to a div or something else permissable in XHTML strict, or are iframes just the way to do this?