JSFiddle - React, Tailwind, and code Playground

HTML

<div id="pent"></div>

CSS

#pent{
    height: 50px;
    width: 100px;
    position: relative;
    border: 1px solid black;
    border-bottom: 0;
}
#pent:before{
    content: '';
    position: absolute;
    bottom: 0; 
    left: -1px;
    width:45px; 
    height:45px;
    -webkit-transform-origin: 0 100%;
    -moz-transform-origin: 0 100%;
    -ms-transform-origin: 0 100%;
    transform-origin: 0 100%;
    -webkit-transform: scaleX(1.57) rotate(45deg);
    -moz-transform: scaleX(1.57) rotate(45deg);
    -ms-transform: scaleX(1.57) rotate(45deg);
    transform: scaleX(1.57) rotate(45deg);
    border: 1px solid black;
    border-top: 0;
    border-left: 0;
}