<html>
<head>
  <title>Div example</title>
</head>

<body>
<style type="text/css">
.matrix {
  cell-spacing: 0px; 
  border-spacing: 0px;
  border-collapse: collapse; 
  border: 1px solid #cccccc;
}
.matrix td {
  height: 50px; width: 50px; 
  border: 1px solid #cccccc;
  background-color:#00ff00;
}

.matrix td.cell_a {
  background-color:#ff0000;
}

.matrix td.cell_b {
  background-color:#0000ff;
}
</style>

<table summary="" class="matrix">
  <tr>
    <td class="cell_a" ></td>
    <td class="cell_a" ></td>
    <td class="cell_a" ></td>
  </tr>
  <tr>
    <td class="cell_b" ></td>
    <td></td>
    <td></td>
  </tr>
</table>

</body>
</html>