JSFiddle - React, Tailwind, and code Playground

HTML

<div id="msg"></div>

CSS

#msg {
    width:100px;
    height:40px;
    background:red;
    margin-left:40px;
    position:relative;
    
}
#msg:before {
    content:"";
    position:absolute;
    border-bottom: 20px solid transparent;
    border-right: 20px solid red;
    border-top: 20px solid transparent;
    height: 0px;
    width: 0px;
    margin-left:-20px;
}