EPUB 3 is a more difficult to standard to read, but as far as I can tell it supports most of HTML5 (with some limitations on MathML). As such <i> should work there too (see
this). Note that HTML5 subtly changes the definition of elements like <i> and <b>. Also note that <i class="somestyle"> is valid (in case I gave the wrong impression above), but does mean that the text may not be rendered in italics - if the style says differently). I just think it is a strange thing to do, to say here's some italic text that may not be italic.