JSFiddle - React, Tailwind, and code Playground

HTML

<div class="test">
    
</div>

CSS

.test {
    width: 300px;
    height: 300px;
    background: blue;
    position: relative;
   
}
.test:after {
    display: block;
    content: "";
    position: absolute;
    width: 100%;
    height: 25px;
    border-top: blue 10px solid;
    bottom: -23px;
    left: 0;
    border-radius: 40%;
}