The border attribute on the table element is obsolete. Use CSS instead

Question

Newbie project. Not allowed to use CSS, only HTML. The table has to have visible borders. When I check to validate I get an Error, see title above. Searched and tried without success. How do I pass validation with only HTML for the following table with borders or something similar to border looking.

<table border="1">

 <thead>
 <tr>
   <th>1</th>
   <th>2</th>
   <th>3</th>
  </tr>
 </thead>

 <tbody>

  <tr>
   <td>4</td>
   <td>5</td>
   <td>6</td>
  </tr>
    
  <tr>
   <td>7</td>
   <td>8</td>
   <td>9</td>
  </tr>
        </tbody>
    </table>

I tried and expected my html to validate. The result was an error as stated in the title.