In short, Google may no longer be looking at source HTML at all anymore. Since you get a site preview in the results which even highlight the text that caused the ranking in a graphical fashion, I can only assume that Google uses the scraped content to rank sites.
This now makes it impossible to stuff keywords into divs which are not quite visible, or other tricks. Google KNOWS what the page renders like, and therefore can do a much better job of ranking the content like a human would.
Didn't Jill's last email refer to dropped rankings for content in accordians that wasn't immediately visible? This would explain it.
It may soon also affect cheating like making small H1's for the SE's, and larg SPAN's for humans. Google will likely look at text size and contrast and decide from a rendering perspective, what is bigger, what stands out most, and therefore what is most importnat.
I would be very interested in what others think on this topic.
Edited by chrishirst, 17 October 2011 - 05:27 PM.