Gentlemen,

Let me remind you that this is the "Search Engine Optimization Forum" and not a section about "Best practices of web design in general".

Coding according to standards, should be promoted, of course, but many have experienced that most search engines don't put too much emphasize on validation, unless there is some serious problem involved which could confuse bots.

The OP lost his good ranking after implementing CSS.
Why is that? Is CSS related at all?
Is his CSS layout so wrong that bots can't follow it completely?

I still believe it is a kind of major content change which CSS produced (as viewed with Lynx), supplemented with REAL small content modifications.
Add some bad coding and it should be enough for big problems.

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Impossible? You just underestimate the time.