I think it's easier to add "constraints" to existing WYSIWYG apps, than to create something from scratch. Like, take MS Word and enforce using only styles (no inline formatting), no more than 1 consecutive space and line break, etc. In the end, you'll get a well formatted XML.

We're working on a way of allowing designers to deploy arbitrarily structured, editable content with a truly wysiwyg editor: http://www.decalcms.com/tour/ what do you think?

I've seen and even worked with some examples of this approach earlier. It works well when you have definitive templates that all the content would fit but you return to the step 1 when you need a flexible white box for the user to fill.

There's actually a way to just include arbitrary HTML (but it's not shown in the quick initial demo).

If you put your email in at the end you get access to a full demo that shows you all the features of the full CMS. Thanks for checking it out!