JSFiddle - React, Tailwind, and code Playground

HTML

<div class="all">


<div class='concave'>
    <div class='rightconcave'></div>
</div>
</div>

CSS

div.concave {
    background:green;
    width:150px;
    height:150px;
    position:relative;
}

div.rightconcave {
    position:absolute;
    background:white;
    width:150px;
    height:150px;
    bottom:0;
    left:0;
    border-top-left-radius:500px;
}
div.all {background:yellow}