Convert <b> to <strong>, <i> to <em>

<i>Hello <b>World!</b></i>
				

<em>Hello <strong>World!</strong></em>
				

While many HTML4 elements have been brought into HTML5 essentially unchanged, several historically presentational ones have been given semantic meanings.

Let’s look at <i> and <b> and compare them to the semantic stalwarts <em> and <strong>. In summary:

<i> — was italic, now for text in an “alternate voice”, such as transliterated foreign words, technical terms, and typographically italicized text (W3C:Markup, WHATWG)
<b> — was bold, now for “stylistically offset” text, such as keywords and typographically emboldened text (W3C:Markup, WHATWG)
<em> — was emphasis, now for stress emphasis, i.e., something you’d pronounce differently (W3C:Markup, WHATWG)
<strong> — was for stronger emphasis, now for strong importance, basically the same thing (stronger emphasis or importance is now indicated by nesting) (W3C:Markup, WHATWG)

<i> and <b> were HTML4 font style elements and are still used presentationally where appropriate to follow typographic conventions. They now have semantic meaning, however, and their style can be changed via CSS, meaning they’re not only presentational — <b>, for example, doesn’t have to be bold. Because of this, it’s recommended to use classes to indicate meaning to make it easy to change the style later.