You want to allow users to add rich-text markup to a text area. You want users to be able to use a WYSIWYG editor in the browser.
Adding HTML markup to a text area was not part of the original W3C specification. But a general-purpose "annotation" element is being considered by the W3C. Please see the following link for examples of how a rich-text editor is added to different XForms tools:
To be done...
Although there were frequent discussions about using rich text editors with XForms, the initial article about using FCKeditor with XForms was posted on the IBM Developerworks web site in October 2007 in an article titled Extending XForms to Enable Rich Text Editing by Steve K Speicher and Andy Smith. These instructions with just a few modifications should be attributed to them.