What courses or sites did you use to learn Search Engine Optimization (SEO)?

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Search Engine Optimization (SEO) is important to build websites and/or blogs that get noticied by search engines. Ofcourse, search engines are a great way to increase traffic. The advanced algorithms that search engines use, especially Google, requires websites to use some formatting to gain attention. I want to learn more about Search Engine Optimization (SEO). What courses or websites helped you learn more about SEO?