The B and I would have been in HTML since the stone age, if it existed then. And actually, they still are. Although they are purely presentational, they haven't even been deprecated in XHTML 1.1.
To replace <b>, simply use this:
font-weight: bold;
And for <i>, it's:
font-style: italic;
However, it's important to note that if you're using bold or italics to place emphasis on words, you should still use the proper HTML elements, namely em and strong. That way, screenreaders can change the way they read those words, and have the document make more sense.
To decide if you need to read them, try reading your text out loud. If you put a verbal emphasis on the words, use em (commonly rendered as italics). If you need even stronger emphasis (like somebody SHOUTING), use strong (commonly rendered as bold).