JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

div {
    background:red;
    width:100px;
    height:100px;
   
}
div:after{
      content: '';
      position: absolute;
      display: block;
      border: 40px solid green;
      border-left-width: 15px;
      border-right-width:15px;
      border-left-color: transparent;
}