Works

HTML
<div>
	<p></p>
</div>

CSS

p:empty {
	padding: 15px;
	background-color: #333;
	color: #fff;
}

Doesn't Work

Paragraph 1:Sweet icing applicake. Apple pie applicake bonbon cake danish cookie jujubes. Topping croissant tiramisu pie jelly-o pudding.

HTML
<div>
<p><strong>Paragraph 1:</strong>Sweet icing applicake. Apple pie applicake bonbon cake danish cookie jujubes. Topping croissant tiramisu pie jelly-o pudding.</p>
</div>

HTML
<div>
	<p> </p>
</div>

CSS

p:empty {
	padding: 15px;
	background-color: #333;
	color: #fff;
}

This has space, thus it is not considered empty.