The em element marks a word or phrase that should receive extra emphasis or spoken stress. Browsers usually display the marked text in italics, but its meaning is semantic rather than merely visual. section 12.2.6
Emphasize an important word
Use the opening and closing tags around the word that needs added stress: <em>vital</em>. In a sentence, this can appear as: “When mixing pie pastry, it's <em>vital</em> that your butter is as cold as possible.”
Choose em when stress helps get the point across; choose strong when the text is inherently crucial. The elements can be nested, so a stressed word inside important text can appear both italic and bold. section 12.2.6
The em element may look like ordinary italics in a browser, but it communicates emphasis in the HTML. Use the i element instead for alternative text such as titles, technical terms, foreign phrases, or thoughts. section 12.2.6 section 12.6
