Kobo readers would display bold instead of italic when <em> was used in the past. I can't remember now if it was for just specific fonts or even if it still happens or was fixed by now. But that got me in the habit of using <i> and <b> just for the reason DiapDealer said... it works on everything, including the then buggy Kobo firmware. I also figured <i> and <b> are shorter (yes, I'm lazy, less typing is always good
), and as long they both did the same thing and both generally supported, I didn't see a reason to change to <em> and <strong>.