SOURCE

<div class="circle"></div>

.circle {
    width: 100px;
    height: 100px;
    background-color: red;
    position: fixed;
    top: 40%;
    left: 40%;
    border-radius: 50px 50px 0 0;
}

.circle::after {
    display: block;
    content: '';
    width: 64px;
    height: 64px;
    position: absolute;
    left: -64px;
    bottom: 0;
    background: radial-gradient(circle at 0 0, transparent 64px, red 65px)
}

.circle::before {
    display: block;
    content: '';
    width: 64px;
    height: 64px;
    position: absolute;
    right: -64px;
    bottom: 0;
    background: radial-gradient(circle at 100% 0, transparent 64px, red 65px)
}