A lot of people seem to believe that the <em> tag has replaced <i> and <strong> the <b> html tag. This got very much to do with that both using <i> and <em> gives you by default italic text, while both <b> and <strong> gives you bold text.However, this is not the case. The <i> and <b> tags are not replaced, what they tell is that your text should be in italic or in bold, and they are both purely for styling/ presentation, like the good old html 4 tag <font>.This is not the same as what <em> and <strong> tags tells the reader.
The best way to explain the use of these two tags is to pretend your writing a text by hand; the old fashion pen and paper writing. In this scenario you would underline words and phrases that you want to stress as important, while the most important words and phrases you would probably write in upper case.
<em> is the same as your underlined words and phrases, while <strong> is the text you would write in upper case.
Another way to put it is that <i> and <b> will tell if you voice is dark or light, while <em> and <strong> tells how angry you are.
Actually, people using screen readers can hear the <em> and <strong> text, so they too understand there’s a point you want to stress.
So, start using <em> and <strong> correct, and remember that they got nothing to do with <i> and <b>.