Do you mean that I should change the document type on the page itself (not a realistic option), or in the contents_update_ContentValue1.js/contents_insert_ContentValue1.js files?
FCKConfig.DocType = '<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">';
No - I haven't tried changing either of those.