As Michael Ott stated in his blog post “Regarding semantics” semantics is all about meanings, using tags for their given purpose.
Why should one use semantics in his webpages ? Semantics webpages will help your website to be more SEO friendly (see Google Webmaster Guidelines: Design and content guidelines).
Google will pay more attention to a webpage and rank it better when it’s semantic (H1 tags used properly, UL LI used properly, etc. ) rather than a webpage that’s .. just there, without nothing semantic about it.
Semantic mark-up isn’t equal to SEO On-page (See On Page SEO), but it’s a task in the process that you must do in order to be more friendly for search engines.
Optimization for search engines is just one advantage of semantic mark-up. Other advantage is for screen readers.
See semantic web article on Wikipedia
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