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Bootstrap class: .table-borderless

<table class="table table-borderless">
  <thead>
    <tr>
      <th>#</th>
      <th>First Name</th>
      <th>Last Name</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th scope="row">1</th>
      <td>Mark</td>
      <td>Otto</td>
    </tr>
    <tr>
      <th scope="row">2</th>
      <td>Jacob</td>
      <td>Thornton</td>
    </tr>
  </tbody>
</table>

Preview

# First Name Last Name
1 Mark Otto
2 Jacob Thornton

Sass source

/* _tables.scss:64 */
.table-borderless {
  th,
  td,
  thead th,
  tbody + tbody {
    border: 0;
  }
}

Uses variables

/* _tables.scss:64 */