bootstrap4.css
.progress-bar-striped {
  background-image: linear-gradient(45deg, rgba(255, 255, 255, 0.15) 25%, transparent 25%, transparent 50%, rgba(255, 255, 255, 0.15) 50%, rgba(255, 255, 255, 0.15) 75%, transparent 75%, transparent);
  background-size: 1rem 1rem;
}
■プログレスバー:progress::プログレスバーのストライプをアニメーションさせる

  <div class="progress mb-3">
    <div class="progress-bar progress-bar-striped progress-bar-animated" role="progressbar" aria-valuenow="25" aria-valuemin="0" aria-valuemax="100" style="width: 25%"></div>
  </div>
  <div class="progress mb-3">
    <div class="progress-bar progress-bar-striped progress-bar-animated bg-success" role="progressbar" aria-valuenow="50" aria-valuemin="0" aria-valuemax="100" style="width: 50%"></div>
  </div>
  <div class="progress mb-3">
    <div class="progress-bar progress-bar-striped progress-bar-animated bg-info" role="progressbar" aria-valuenow="75" aria-valuemin="0" aria-valuemax="100" style="width: 75%"></div>
  </div>
  <div class="progress mb-3">
    <div class="progress-bar progress-bar-striped progress-bar-animated bg-danger" role="progressbar" aria-valuenow="100" aria-valuemin="0" aria-valuemax="100" style="width: 100%"></div>
  </div>
  
■プログレスバー:progress::プログレスバーをストライプにする

  <div class="progress mb-3">
    <div class="progress-bar progress-bar-striped" role="progressbar" style="width: 10%" aria-valuenow="10" aria-valuemin="0" aria-valuemax="100"></div>
  </div>
  <div class="progress mb-3">
    <div class="progress-bar progress-bar-striped bg-success" role="progressbar" style="width: 25%" aria-valuenow="25" aria-valuemin="0" aria-valuemax="100"></div>
  </div>
  <div class="progress mb-3">
    <div class="progress-bar progress-bar-striped bg-info" role="progressbar" style="width: 50%" aria-valuenow="50" aria-valuemin="0" aria-valuemax="100"></div>
  </div>
  <div class="progress mb-3">
    <div class="progress-bar progress-bar-striped bg-warning" role="progressbar" style="width: 75%" aria-valuenow="75" aria-valuemin="0" aria-valuemax="100"></div>
  </div>
  <div class="progress mb-3">
    <div class="progress-bar progress-bar-striped bg-danger" role="progressbar" style="width: 100%" aria-valuenow="100" aria-valuemin="0" aria-valuemax="100"></div>
  </div>