I think Facundo Corradini is right here in calling out our tweet. If you're italicizing text because it should be styled that way (e.g. using italics to display a person's internal thought dialog, as illustrated in our example), then that's an
<i> and not an
<em> is for stress emphasis — as in, a word you would emphasize with your voice, if spoken, to affect meaning.
Plus, I'm always down for long-form articles about the nuances of a handful of HTML elements!