How do I apply <td> (column) widths as a % of a table width in HTML5 using CSS3?

Hi:

I'm in the middle of coming up to speed with HTML5.
I'm having issues applying a css3 style (via class) to a <td>.

I'm basically trying to do a "flow/responsive" layout where the columns
widths are a % of the table.

What I'd like to do is have Col1 and Col2 be a fixed % of the table width,
while Col3 and Col4 use the remaing width as needed.

My Css3 styles look like:
---------------------------------

div.Wrapper{
        background-color: yellow;
       width: 96%;
}
table{
        border: 3px solid black;
        background-color: aquamarine;
        font-size: .75em;
        width: 98%;
}

thead {
        border: 3px solid black;
        font-weight: bold;
        text-decoration: underline;
        text-align: center;
        color: yellow;
}

td {
        border: 3px solid black;
        background-color: orange;
}

td.Col1{
    width: 39.583%;
    font-height: 30px;
    color: aquamarine;
}

td.Col2 {
    width: 5.20833%;
    font-height: 16px;
    color: green;
}




My html looks like:
---------------------------
<div class='Wrapper'>
<table>
   <thead>
      <tr>
         <td class='col1'>Col 1</td>
         <td class='col2'>Col 2</td>
         <td class='col3'>Col 3</td>
         <td class='col4'>Col 4</td>
      </tr>
   </thead>

   <tbody>
    <tr>
         <td class='col1'>Col 1 Value</td>
         <td class='col2'>Col 2 Value</td>
         <td class='col3'>Col 3 Value</td>
         <td class='col4'>Col 4 Value</td>
      </tr>
    <tr>
         <td class='col1'>Col 1 Value</td>
         <td class='col2'>Col 2 Value</td>
         <td class='col3'>Col 3 Value</td>
         <td class='col4'>Col 4 Value</td>
      </tr>
   </tbody>

</table>
</div>