body {
  font-size: 62.5%; /* font-size 1em = 10px */
}
p {
  font-size: 1.2em; /* 1.2em = 12px */
}

http://sixrevisions.com/css/10-random-css-tricks-you-might-want-to-know-about/

If you would like to use relative units (em) for your font sizes, declaring 62.5% for the font-size property of the body will make it easier to convert px to em. By doing it this way, converting to em is a matter of dividing the px value by 10 (e.g. 24px = 2.4em).

Set body font-size to 62.5% for Easier em Conversion

css, body

CSS