I’d usen neither. You can use <em> to emphasize a word or phrase or <strong> to give an even stronger emphasis.
Via CSS you can then style your HTML elements accordingly.
Example:
em {
font-weight:bold;
font-style:normal;
color:blue;
}
strong {
font-weight:bold; /* not necessary as browsers have
already set the default font-weight
to bold. It's just for demonstration. */
color:black;
}
Yes, you use CSS to style your HTML elements as you please.
If you have one general style for <strong> elements and one general style for <em> elements, then you don’t need to add a class or ID.
It is sufficient to just have this:
strong {color:black; /* or whatever styles you'd like to add */}
em {font-weight:bold; font-style:normal;}
Edit: Sorry, I didn’t see you deleted your second question. I’ll leave the response in anyway.