HTML Element Style How to - Center div inside anther div


We would like to know how to center div inside anther div.


<!DOCTYPE html>
<style type='text/css'>
div {<!--  w w w.j a va  2 s .  co m-->
  border: 1px solid black;
  height: 100px;

#big {
  width: 80%;
  min-width: 640px;
  padding: 100px;
  margin: auto;
  <div id="big">
    <div id="small"></div>

The code above is rendered as follows: