JSFiddle - React, Tailwind, and code Playground

HTML

<div class="roundingDiv">
<table class="borderedTable">
<tbody>
  <tr>
    <th class="removeLine">a</th>
    <th>b</th>
    <th>c</th>
    <th>d</th>
  </tr>
  <tr>
    <td class="removeLine">e</td>
    <td>f</td>
    <td>g</td>
    <td>h</td>
  </tr>
</tbody>
</table>
</div>

CSS

.roundingDiv { border:1px solid black; border-radius:50px; overflow:hidden; }
.borderedTable { border-collapse:collapse; width:100%; border:0; }
.borderedTable th, .borderedTable td { border:1px solid black; }
.borderedTable tr:first-child th { border-top:0; }
.borderedTable tr:last-child td { border-bottom:0; }
.borderedTable tr th:first-child, .borderedTable tr td:first-child { border-left:0; }
.borderedTable tr th:last-child, .borderedTable tr td:last-child { border-right:0; }
th.removeLine { border-bottom:0; }
td.removeLine { border-top:0; }