Borders on Cells

How can I put a border around certain cells rather than the whole table. Is there a way either in Style Sheets or pure HTML?

Code:

---

<style>
td.a {
  border: 2px solid black;
}
</style>

<table border="0">
<tr>
<td>no border</td>
<td class="a">border</td>
</tr>
</table>

---

or you can use nested tables...