bootstrap4.css
.rounded-circle {
border-radius: 50% !important;
}
■影shadow 角丸rounded：ユーティリティ：角丸を設定するクラス rounded
<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>