I thought <em> and <strong> had to be defined within the CSS, the fact they most often have font-style: italic; and font-weight: bold; is coincidental, whereas <i> and <b> are explicit in there meaning.
And who decided on <em>. The artefact 'em' already has a well established usage within the same domain relating to character width -- why not <emph>.
Using the same 'word' with different meanings and different 'words' with same meaning... within the same knowledge domain... has long been a pet peeve of mine. The WWW sphere is the worse case I can think of, I often wonder if it's done deliberately, perhaps subconsciously, to establish a new upper class
BR