Why em instead of px?

Question

I heard you should define sizes and distances in your stylesheet with em instead of in pixels. So the question is why should I use em instead of px when defining styles in c

Answer 1

There is a simple solution if you want to use px to specify font size, but still want the usability that em's provide by placing this in your CSS file:

body {
  font-size: 62.5%;
}

Now specify you p (and other) tags like this:

p {
  font-size: 0.8em; /* This is equal to 8px */
  font-size: 1.0em; /* This is equal to 10px */
  font-size: 1.2em; /* This is equal to 12px */
  font-size: 2.0em; /* This is equal to 20px */
}

And so on.