JSFiddle - React, Tailwind, and code Playground

HTML

<div class="speechbubble"></div>

CSS

.speechbubble{
    height:100px;
    width:100px;
    border:1px solid green;
    position:relative;
    background-color:white;
    border-radius:5px;
}

.speechbubble:after{
    height:10px;
    width:10px;
    border: 1px solid green;
    border-left:1px solid white;
    border-bottom:1px solid white;
    position:absolute;
    content:"";
    top:50px;
    right:-6px;
    transform:rotate(45deg);
    background-color:white;
}