Page layout breaks when Editing page

I've created a site for a client who wants to use Contribute to edit it. The site wasn't built with Dreamweaver (hand-coded). Pages look fine when they're loaded in the Contribute window, but when he goes to Edit the page, the layout breaks. it's a table-less CSS-based layout with some includes, but it looks fine when initially loaded. After editing & publishing, the page generally looks fine again. This would only be a minor annoyance, but sometimes there are blocks of text that, when Editing, suddenly seem to have a near-zero line spacing....so he can't see what he's trying to edit. Is there a setting I'm missing somewhere?
thanks,
benjy