JSFiddle - React, Tailwind, and code Playground

HTML

<div class='concave'>
    <div class='topleftconcave'></div>
    <div class='botleftconcave'></div>
</div>

CSS

div.concave {
    background:blue;
    width:150px;
    height:120px;
    position:relative;
}
div.topleftconcave {
    position:absolute;
    background:white;
    width:25px;
    height:35px;
    border-bottom-right-radius:500px;
}
div.botleftconcave {
    position:absolute;
    background:white;
    width:25px;
    height:35px;
    bottom:0;
    left:0;
    border-top-right-radius:500px;
}