What's the difference between and , and ?

Answer 1

You should generally try to avoid and . They were introduced for layouting the page (changing the way how it looks) in early HMTL versions prior to the creation of CSS, like the meanwhile removed font tag, and were mainly kept for backward compatibility and because some forums allow inline HTML and that's an easy way to change the look of text (like BBCode using [i], you can use and so on).

Since the creation of CSS, layouting is actually nothing that should be done in HTML anymore, that's why CSS has been created in the first place (HTML == Structure, CSS == Layout). These tags may as well vanish in the future, after all you can just use CSS and span tags to make text bold/italic if you need a "meaningless" font variation. HTML 5 still allows them but declares that marking text that way has no meaning.

and on the other hand only says that something is "emphasized" or "strongly emphasized", it leaves it completely open to the browser how to render it. Most browsers will render em as italic and strong as bold as the standard suggests by default, but they are not forced to do that (they may use different colors, font sizes, fonts, whatever). You can use CSS to change the behavior the way you desire. You can make em bold if you like and strong bold and red, for example.