I have some great news : I got formal authorization from Yahoo! to add YUI-based CSS Layouts to BlueGriffon. Let me thank Yahoo! (and in particular Nate Koechley and Christian Heilmann) and then Jake Myers, the guy behind the famous http://layouts.ironmyers.com. The first visible result is the new "New page wizard" that is not checked in yet. See below a very early screenshot of that wizard. Warning, the UI is not at all finalized.

CSS-based Layouts