JSFiddle - React, Tailwind, and code Playground

HTML

<head>
</head>
<body>
    <div id = "content" contenteditable = "true" oninput = "reformat()"></div>
</body>

CSS

div
{
    height: 80%;
    width: 80%;
    overflow: auto;
    border: 1px solid black;
}

JavaScript

var content = document.getElementById("content");
function reformat()
{
    content.innerHTML = content.innerHTML.replace(/dogs/g, "<b>cat</b>");
}