JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

body{padding:10px;}
div{
    background-color:#dcdcdc;
    width:400px;
    height:30px;
    position:relative;
    border:solid 1px #D4D4D4;
}
div:after{
    position:absolute;
    display:block;
    content:"";
    top:100%;
    right:0;
    border-bottom:solid 15px transparent;
    border-left:solid 15px #dcdcdc;
    z-index:-1;
}
div:before{
    position:absolute;
    display:block;
    content:"";
    top:100%;
    left:0;
    border-bottom:solid 15px transparent;
    border-right:solid 15px #dcdcdc;
    z-index:-1;
}