CSS/XHTML vs. Regular HTML for Landing Page Quality Score

Although we have not been able to find documentation on this, it seems that Google Adwords tends to favor regular (basic) HTML landing pages vs. modern CSS/XHTML landing pages. We have done tests where the exact same META data and content in a landing page will have a range of Poor (CSS/XHTML version) to Great (basic HTML version) quality scores. Considering this can make or break a campaign, try experimenting a bit. If you have a modern site, you may want to try to build a separate folder for basic HTML landing pages and see how those fare vs. your regular site pages.

A search marketing firm should be able to handle all of this in house for you.