<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<style type="text/css">
.td_brd {
border: 10px solid transparent;
}
.td_brd:hover {
border-color:#000;
}
</style>
</head>
<body>
<table>
<tr>
<td>ddf</td>
<td>ddf</td>
<td>ddf</td>
</tr>
<tr>
<td>ddf</td>
<td class="td_brd">ddf</td>
<td>ddf</td>
</tr>
<tr>
<td>ddf</td>
<td>ddf</td>
<td>ddf</td>
</tr>
</table>
</body>
</html>
Какой вы используете border-collapse?