There are several patents describing how a search engine might analyse the visual appearance and structure of a website, with a special view to give some parts of the page content (the main content) a higher weight or priority then the rest (like footer text and navigation). I would expect that the algorithms used by Google will give a good impression about the page rendering. What conclusions are drawn from this and how it affects the ranking of individual pages for certain search queries, who knows, ... pretty much about this is kept secret and there is a lot of speculation.
Many search engine patents are explored and explained by Bill Slawski on his blog "SEO by the sea":
That there are these patents does of course not tell us, that search engines are working exactly in this or another way, but I think it often gives a good impression about what might be going on behind the scenes.