css3----画半圆弧线

.circle1 {
    width: 100px;
    height: 200px;
    border: 1px solid black;
    border-radius: 100% 0 0 100%/50%;
    border-right: none;
}
.circle2 {
    width: 200px;
    height: 100px;
    border: 1px solid black;
    border-radius: 50% 50% 0 0/100% 100% 0 0;
    border-bottom: none;
}
.circle3 {
    width: 100px;
    height: 200px;
    border: 1px solid black;
    border-radius: 0 100% 100% 0/50%;
    border-left: none;
}
.circle4 {
    width: 200px;
    height: 100px;
    border: 1px solid black;
    border-radius: 0 0 50% 50%/0 0 100% 100% ;
    border-top: none;
}

<ul>
    <li><div class="circle1"></div></li>
    <li><div class="circle2"></div></li>
    <li><div class="circle3"></div></li>
    <li><div class="circle4"></div></li>
</ul>