JSFiddle - React, Tailwind, and code Playground

HTML

<div class="sail"></div>

CSS

.sail{
    width: 50px;
    height: 100px;
    position:relative;
    overflow:hidden;
}
.sail:after{
    width:200px;
    height:200px;
    content:'';
    position:absolute;
    right:0;
    top:-5px;
    -moz-border-radius:100px;
    border-radius:100px;
    background: #ff0000;
}