Emma, xowiki uses tidy to correct HTML, since this does not rely on any kind of editor. It is nice that tinymce produces HTML for a configurable document type (xinha loves xhtml). About accessibility: didn't you say that for accessibility, javascript has always be deactivated. With javascript deactivated, both rich text editors just look like a plain textarea.
Only with javascript activated, one is able to get to the table with the access keys, etc. described on that referenced page. so, i am wondering, how you deal in your accessibility tests with rich-text editors....