bootstrap4.css
.rounded-top {
  border-top-left-radius: 0.25rem !important;
  border-top-right-radius: 0.25rem !important;
}
■影shadow 角丸rounded:ユーティリティ:角丸を設定するクラス rounded

rounded rounded-top rounded-right rounded-bottom rounded-left rounded-circle 正円 rounded-0
<span class="rounded">rounded</span>
<span class="rounded-top">rounded-top</span>
<span class="rounded-right">rounded-right</span>
<span class="rounded-bottom">rounded-bottom</span>
<span class="rounded-left">rounded-left</span>
<span class="rounded-circle">rounded-circle</span>
<span class="rounded-circle" id="circle">正円</span><!-- 幅と高さを同じ(100px)に設定 -->
<span class="rounded-0">rounded-0</span>