Use <b> when you want the text to have a different style without contextual importance, but use <strong> when you want the text to have extra importance from a content or SEO perspective. Use <i> to offset the mood of text, but use <em> to make text emphatic.

RDFa is an extension to HTML5 that helps you markup things like People, Places, Events, Recipes and Reviews. Search Engines and Web Services use this markup to generate better search listings and give you better visibility on the Web, so that people can find your website more easily. http://rdfa.info/