左右に auto のマージン


content

DOCTYPE

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

X/HTML

<div id="box01">content</div>

CSS

div#box01 {
  width: 300px;
  height: 200px;
  margin: 0 auto;
}

Taichi Kaminogoya.