KISS
Don't introduce things that aren't needed
<b>, <i> say use the current font and paragraph attributes, Just those faces. That is it. no CSS or style needed
(I use bold to convey
keywords when giving usage guidance here at MR and underline for strong warning
Gotcha's
OTOH if you are trying to convey 'voice', then <em> or <strong> might be better
<em>, <strong> tend to inherit their qualities from the render engine designers

and need additional style controls (if you care)