JSFiddle - React, Tailwind, and code Playground

HTML

<div class=arrow><div>

CSS

.arrow{

    
}
.arrow:before{
    content:'\a';
    position:absolute;
    top:0;
    left:0;
    width:0;
    height:0;
    border-left:20px solid red;
    border-right:0px solid transparent;
    border-bottom:20px solid transparent;
    border-top:20px solid transparent;
}