1-border-radius 画圆

  .circle{
            width: 100px;
            height: 100px;
            background-color: aqua;
            border-radius: 50px;
        }

        .topCircle{
            width: 100px;
            height: 50px;
            background-color: aqua;
            border-radius: 50px 50px 0px 0px;
        }

 <h2>圆形</h2>
    <div class="circle">

    </div>

    <h2>上半圆</h2>
    <div class="topCircle">

    </div>