;
    background: red;
    margin: 10px;
    width: 50px;
    height: 50px;
  }
  #b {
    clear: left;
    width: 50px;
    height: 50px;
    background: green;
  }
  </style>
</head>
<body>
  <div id="left"></div>
  <div id="a">
    <div id="b"></div>
  </div>
</body>
</html>
