It is possible to learn from both good and bad examples. Usually I write about what you "should" do to optimize your website, but now I will highlight some of the things that you should "not" do when you prepare your web pages. This list is not in any order of importance as all the points should be taken seriously if you want your site to do well in the search engine queries for your main keywords.
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Medianet SEO discussion: The Top Ten SEO Mistakes