Defining font size and line height

In theory, defining font sizes should be easy enough. You use the font-size property, and
then you can set the value to an absolute size, a relative size, a length, or a percentage. For
instance, you might set the following:
h1 {
font-size: 20px;
}
p {
font-size: 12px;
}

Alternatively, you might go for something like this:
h1 {
font-size: 150%;
}
p {
font-size: 90%;
}