#all {
  display: box;
  box-orient: vertical;
  height: 400px;
  width: 600px;
  margin-right: auto;
  margin-left: auto;
}

#main {
  display: box;
  box-orient: horizontal;
}

#nav {
  box-flex: 1;
}

#article {
  box-flex: 6;
}

header, footer {
  display: block;
  text-align: center;
}