Edit in JSFiddle

//Add Summary if not present
$('table:not(:has(summary))').attr('summary', 'This is a table for forum replies');

//Add caption if not present
$('table:not(:has(caption))').prepend('<caption>Reply to forum posts here</caption>');

//Add column scope if not present
$('thead tr > th:not(:has(scope))').attr('scope', 'column');

//Add row scope to if not present
$('tbody tr > th:not(:has(scope))').attr('scope', 'row');

<table>
    <thead>
    <tr>
        <th>This is a header</th>
        <th>This is a header</th>
         <th>This is a header</th>
         <th>This is a header</th>
    </tr>
    </thead>
    <tbody>
    <tr>
        <th>Test</th>
        <td>Test</td>
        <td>Test</td>
        <td>Test</td>
    </tr>
    <tr>
        <th>Test</th>
        <td>Test</td>
        <td>Test</td>
        <td>Test</td>
    </tr>
    </tbody>
</table>
        

table{
    width:100%;
    }
thead tr > th[scope="column"]{
    background:#ccc;
    }
tbody tr > th[scope="row"]{
    background:#89b958;
    }
td{
    border:1px solid #ccc;
    }