<table>
  <tr>
    <td colspan="4" class="header">Заголовок</td>
  </tr>
  <tr>
    <td>1</td>
    <td>2</td>
    <td>3</td>
    <td>4</td>
  </tr>
  <tr>
    <td>5</td>
    <td>6</td>
    <td>7</td>
    <td>8</td>
  </tr>
  <tr>
    <td>9</td>
    <td>10</td>
    <td>11</td>
    <td>12</td>
  </tr>
  <tr>
    <td>13</td>
    <td>14</td>
    <td>15</td>
    <td>16</td>
  </tr>
</table>
.header {
  position: relative;
}

.header:before {
  content: "";
  left: 50%;
  transform: translateX(-50%);
  bottom: 0px;
  width: 94%;
  position: absolute;
  border-bottom: 4px solid #f00;
}
td {
  border: 1px solid #000;
}
td,
.header {
  padding: 16px;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.