JSFiddle - React, Tailwind, and code Playground

HTML

<div class='parent'>
    <div class='child'>
        Child
    </div>
    See?
</div>

CSS

.parent{
    overflow:hidden;
    border-radius:10px;
    border:5px solid black;
    background:blue;
}
.child{
    background:red;
    border-top-left-radius: 5px;
    width:40px;
}