One thing I've found when trying to develop pure CSS skins is that the DOCTYPE declaration can make or break most skins, at least in their rendering. By default, many browsers will enter 'Quirks' mode which is basically an everything-goes mode which will render even the most broken HTML (misplaced/unclosed tags, mixed formatting, etc). I spent 4hrs troubleshooting a sample skin I was working with from freecsstemplates.org before I realized that my Doctype declaration was malformed and the browser was entering quirks mode, thereby ignoring / mis-rendering many of my styles. After I applied the correct doctype declaration in my skin.doctype.xml file, the page began rendering much more predictably. The downside of this (and, in my ever-so-humble opinion) is that Pure CSS designs don't play well with older browsers unless you apply alot of ugly hacks to deal with some of the more common problems (I.E. <6's ability to render PNGs, the 1px padding/spacing issue, etc). Even still, some of the more current styling methods may not render correctly period (I seem to recall one client having signficant troubles viewing a menu created using ALA's Suckerfish menu system when viewed in IE5).
Personally, if I was dealing with a situation where old browser compatibility was a priority then pure CSS may have to take a back seat just to allow me to account for the many styling issues with older browsers. SEO can still be solid, just make sure you keep from nesting too many tables, and make sure you have your nav system towards the beginning of the page. Also, submitting your sitemap never hurts. If you're not happy with the default one provided by DNN or need more levels of content indexed, check out xml-sitemaps.com.
I'm open for any corrections if I may have erroneously misstated something.