I've set up some HTML to display a notification bar. I've also added some basic styles to add border, margin, and padding. If I add an empty notification bar using the div class of alert, you'll see that the notification bar renders with margin, background color, and padding and all of this.
Using the empty pseudo-class we can target the alert notification bar by saying "alert and empty." Inside of that CSS block just say "display none," and you'll see that the empty notification bar disappears.
Instead of targeting the alert class twice, we can chain multiple pseudo-classes together. I'll comment this block. I'll comment this block. I'll come back to the original block. I can say "not" and then inside of the not selector, say "empty." This way, any element with the class of alert that is not empty will receive these following styles.