Page layout algorithm improvement: Google has started analyzing the actual layout of pages, and is now penalizing pages that don’t have a lot of content on top. This is an official Google announcement:
If you click on a website and the part of the website you see first either doesn’t have a lot of visible content above-the-fold or dedicates a large fraction of the site’s initial screen real estate to ads, that’s not a very good user experience. Such sites may not rank as highly going forward.
It used to be that search engine crawlers were oblivious to CSS and layout, and just analyzed markup. Not so much anymore.