table中td为空的如何加边框

<style>
table{
    border-collapse:collapse;
}
table td{
    border:1px solid red;
    empty-cells:show;
}
</style>

 

<table>
<tr><td>2</td><td>2</td></tr>
<tr><td></td><td>2</td></tr>
<tr><td>2</td><td>2</td></tr>
<tr><td>&nbsp;</td><td>2</td></tr>
<tr><td>2</td><td>2</td></tr>
</table>