This is a browser issue, not a dreamweaver issue... notice that in firefox, this probably works, but does not in IE.
Test out the differences below in a new test.html file or something and between firefox and ie...
<table width="400" border="0" cellspacing="0" cellpadding="0">
<Tr>
<td style="height:10px;background-color:#00FFFF;"></td>
</Tr>
</table>
<br />
<br />
<table width="400" border="0" cellspacing="0" cellpadding="0">
<Tr>
<td style="height:10px;background-color:#00FFFF;"> </td>
</Tr>
</table>
<br />
<br />
<table width="400" border="0" cellspacing="0" cellpadding="0">
<Tr>
<td style="height:10px;background-color:#00FFFF;">_</td>
</Tr>
</table>