#brown { height: 200em; width: 200px; background-color: brown;}
  #green { height: 50px; width: 50px; margin-top: 50px; background-color: green;}
  #blue { height: 50px; width: 50px; background-color: blue;}
  <div id="brown" class="box"><div id="green" class="box"></div><div id="blue" class="box"></div></div>