I never had the chance to run a test yet, but someone else did.
Maybe you would like to see his results:
Google SEO Test - Google Prefers Valid HTML & CSS | Hobo SEO UK

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"Being an expert isn't telling other people what you know. It's understanding what questions to ask, and flexibly applying your knowledge to the specific situation at hand. Being an expert means providing sensible, highly contextual direction." Jeff Atwood
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