By default TR is an inline element. Here is a KLUDGE to fix your problem:
<style type='text/css'>
th, tr { display:inline-block }
table {width:352px; margin:2em auto; }
th {border: double #090; border-width: 4px 0 0 0}
tr {border: dotted 2px #f00; border-width: 0 3px 3px 3px;}
th, td {width:88px; text-align:center}
</style>
</head>
<body>
<table>
<tr>
<th>tom</th>
<th>dick</th>
<th>harry</th>
</tr>
<tr>
<td>tom</td>
<td>dick</td>
<td>harry</td>
</tr>
<tr>
<td>tom</td>
<td>dick</td>
<td>harry</td>
</tr>
</table>