.box {
  width: 100%;
  height: 200px;
  background-color: red;
  margin: 10px;
 float: left
  
}
.cabecera {
  width: 100%;
  height: 100px;
  background-color: green;
  float: left;
  margin: 10px;
}

.contenedor {
  width: 800px;
  height: 800px;
  float: left;
  background-color: lightblue;
}