The <em> emphasis element is used to place stress emphasis on text. Where you place emphasis in a sentence changes the meaning of the sentence to where certain parts are put into question or reasserted. It is not meant to be used for general italics formatting, for that we use the <i> element. It is also not meant to be used to express importance in text, we use the <strong> element for that.
Learning is <em>free</em> at DevelopPHP.open in code editor