Updated October 8th, 2020.
⌘-I toggles italics on and off. This works almost everywhere on a Mac– Mail, Pages, Text Edit, you name it. Even Microsoft Word. Highlight some text, hit ⌘-I, and the text becomes italicized– unless it was already italicized. In that case, the italicizing is removed. It’s a toggle.
Microsoft was the most notable exception to this standard keyboard shortcut. Rather than using standard keyboard shortcuts that everyone already knew, early versions of Word used Command-Shift-I for italics in Word, along with Command-Shift-B for bold and Command-Shift-U for underline. Eventually Microsoft switched to the shortcuts everyone else uses– Command-I, Command-B, Command-U. Except the Command-Shift versions still work, so use them if you’d like.
Of course you can hit ⌘-I before typing something. If you do that, whatever you type after that is italicized, until you either hit ⌘-I again or click to type somewhere else.
This is very handy.