Dividing the desired line height (18px) by the element font size (12px) gives us the em value for line height. In this example, the line height is 1 and a half times the font size: 1.5em.
Add line height and margin properties to the CSS:
p{
font-size: 0.750em;
line-height: 1.5em;
margin: 1.5em;
}
Now the browser will say to itself, “Oh, line height and margin is set to 1.5em, so that should be 1.5 times the font size. What’s the font size, again? 12px? OK, cool, make line height and margin 1.5 times that, so 18px.”