JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
</ul>

CSS

body {
    background: darkgrey;
}
li {
    display: block;
    list-style: none;
    width: 200px;
    height: 50px;
    background: darkgrey;
    position: relative;
    border: 2px solid lightblue;
    margin-top: 5px;
}

li:first-child:after {
    content: '';
    display: block;
    width: 30px;
    height: 30px;
    background: darkgrey;
    border-right: 2px solid lightblue;
    position: absolute;
    top: -17px;
    left: -17px;
    transform: rotate(45deg);
}