I have no clue how the source editor works. Fortunately, there's an "html" button there, and I know HTML. The HTML for some of the markup is not obvious, but you can always use the buttons to insert the markup, view the HTML, and then copy-and-paste the HTML into the document you're working on in your favorite editor.

I wouldn't edit an important document in a form on a page anyway -- my fingers know Emacs, and control-w means "delete selection" in emacs and "delete this tab and my entire form" in Firefox, so I often accidentally delete tabs when editing.